diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
new file mode 100644
index 00000000..547e44d4
--- /dev/null
+++ b/.github/FUNDING.yml
@@ -0,0 +1,5 @@
+# These are supported funding model platforms
+
+github: [sudheerj]
+custom: https://buymeacoffee.com/sudheerj
+
diff --git a/.github/workflows/main.yaml b/.github/workflows/main.yaml
deleted file mode 100644
index 8b41896e..00000000
--- a/.github/workflows/main.yaml
+++ /dev/null
@@ -1,34 +0,0 @@
-name: Documentation
-
-on:
-  push:
-    branches: [ master ]
-
-env:
-  FILE_NAME: react-interview-questions
-
-jobs:
-  convert_via_pandoc:
-    runs-on: ubuntu-18.04    
-    steps:      
-      - uses: actions/checkout@v2    
-
-      - name: Create output directory
-        run: |
-          mkdir output  # create output dir          
-      
-      - name: Create PDF
-        uses: docker://pandoc/latex:2.10
-        with:
-          args: --pdf-engine=xelatex --output=output/${{env.FILE_NAME}}.pdf README.md
-
-      - name: Create epub
-        uses: docker://pandoc/latex:2.10
-        with:
-          args: --output=output/${{env.FILE_NAME}}.epub README.md
-      
-      - name: Upload
-        uses: actions/upload-artifact@master
-        with:
-          name: output
-          path: output
\ No newline at end of file
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 00000000..beea88e6
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2017-Present Sudheer Jonna
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
\ No newline at end of file
diff --git a/README.md b/README.md
index 25c9cdb3..e2e32b9e 100644
--- a/README.md
+++ b/README.md
@@ -1,521 +1,798 @@
 # React Interview Questions & Answers
 
-> Click :star:if you like the project. Pull Request are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates.
+> Click :star: if you like the project. Pull Requests are highly appreciated. Follow me [@SudheerJonna](https://twitter.com/SudheerJonna) for technical updates.
 
 ---
+
+<div>
+<p align="center">
+  <a href="https://www.greatfrontend.com/questions/react-interview-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react&fpr=sudheerj&gnrs=sudheerj">
+    <img src="./images/collab/greatfrontend-react.gif" alt="GreatFrontEnd React Interview Questions" width="100%">
+  </a>
+</p>
+</div>
+
+> Practice 280+ JavaScript coding interview questions in-browser. Built by ex-FAANG interviewers. No AI-generated fluff. No fake reviews. [Try GreatFrontEnd →](https://www.greatfrontend.com/questions/react-interview-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react&fpr=sudheerj&gnrs=sudheerj) 💡
+
+---
+
+<div>
+<p align="center">
+  <a href="https://www.frontendlead.com/coding-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react" rel="dofollow">
+    <img src="./images/collab/frontendlead-banner.png" alt="React interview" width="100%">
+  </a>
+</p>
+</div>
+
+> 🚀 Ace React interview questions with solutions from FAANG+ companies! [Try FrontendLead →](https://www.frontendlead.com/coding-questions?utm_source=github&utm_medium=referral&utm_campaign=sudheerj-react) 🚀
+
+---
+
 <div>
-Learn to code and get hired with <a href="https://zerotomastery.io/?utm_source=github&utm_medium=sponsor&utm_campaign=reactjs-interview-questions">Zero To Mastery:</a>
-<ol>
-<li>This <a href="https://links.zerotomastery.io/react_sudheer">React course</a> is good if you’re struggling to learn React beyond the basics</li>
-<li>This <a href="http://links.zerotomastery.io/mci_sudheer">coding interview bootcamp</a> is helpful if you’re serious about getting hired as a developer</li>
-</ol>
+<p align="center">
+  <a href="https://zerotomastery.io/?utm_source=github&utm_medium=sponsor&utm_campaign=reactjs-interview-questions">
+    <img src="https://process.fs.teachablecdn.com/ADNupMnWyR7kCWRvm76Laz/resize=height:70/https://www.filepicker.io/api/file/AKYtjj5SSGyJuyZrkAB2" alt="ZTM Logo" width="100" height="50">
+  </a>
+    <ol>
+      <li>I recommend this <a href="https://links.zerotomastery.io/react_sudheer">React course</a> if you’re serious about learning React and want to go beyond the basics.</li>
+      <li>Want to ace your coding interview and get hired at your dream company? <a href="http://links.zerotomastery.io/mci_sudheer">Take this coding interview bootcamp</a>.</li>
+    </ol>
+  </p>
 </div>
 
 ---
 
-**Note:** This repository is specific to ReactJS. Please check [Javascript Interview questions](https://github.com/sudheerj/javascript-interview-questions) for core javascript questions.
+**Note:** This repository is specific to ReactJS. Please check [JavaScript Interview Questions](https://github.com/sudheerj/javascript-interview-questions) for core JavaScript questions and [Data Structures and Algorithms](https://github.com/sudheerj/datastructures-algorithms) for DSA-related questions or problems.
 
-## Downloading PDF/Epub formats
+### Table of Contents
 
-You can download the PDF and Epub version of this repository from the latest run on the [actions tab](https://github.com/sudheerj/reactjs-interview-questions/actions).
+<details open>
+<summary>
+Hide/Show table of contents
+</summary>
+
+| No. | Questions                                                                                                                                                                                                                        |
+| --- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+|     | **Core React**                                                                                                                                                                                                                   |
+| 1   | [What is React?](#what-is-react)                                                                                                                                                                                                 |
+| 2   | [What is the history behind React’s evolution?](#what-is-the-history-behind-react-evolution)                                                                                                                                      |
+| 3   | [What are the major features of React?](#what-are-the-major-features-of-react)                                                                                                                                                   |
+| 4   | [What is JSX?](#what-is-jsx)                                                                                                                                                                                                     |
+| 5   | [What is the difference between an Element and a Component?](#what-is-the-difference-between-an-element-and-a-component)                                                                                                              |
+| 6   | [How do you create components in React?](#how-to-create-components-in-react)                                                                                                                                                     |
+| 7   | [When should you use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component)                                                                                                    |
+| 8   | [What are Pure Components?](#what-are-pure-components)                                                                                                                                                                           |
+| 9   | [What is state in React?](#what-is-state-in-react)                                                                                                                                                                               |
+| 10  | [What are props in React?](#what-are-props-in-react)                                                                                                                                                                             |
+| 11  | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props)                                                                                                                               |
+| 12  | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling)                                                                                                   |
+| 13  | [What are synthetic events in React?](#what-are-synthetic-events-in-react)                                                                                                                                                       |
+| 14  | [What are inline conditional expressions?](#what-are-inline-conditional-expressions)                                                                                                                                             |
+| 15  | [What is the "key" prop and what is its benefit when used in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements)                                                                    |
+| 16  | [What is the Virtual DOM?](#what-is-virtual-dom)                                                                                                                                                                                 |
+| 17  | [How does the Virtual DOM work?](#how-virtual-dom-works)                                                                                                                                                                         |
+| 18  | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom)                                                                                                         |
+| 19  | [What is React Fiber?](#what-is-react-fiber)                                                                                                                                                                                     |
+| 20  | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber)                                                                                                                                                   |
+| 21  | [What are controlled components?](#what-are-controlled-components)                                                                                                                                                               |
+| 22  | [What are uncontrolled components?](#what-are-uncontrolled-components)                                                                                                                                                           |
+| 23  | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement)                                                                                                 |
+| 24  | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react)                                                                                                                                                         |
+| 25  | [What are Higher-Order Components?](#what-are-higher-order-components)                                                                                                                                                           |
+| 26  | [What is the children prop?](#what-is-children-prop)                                                                                                                                                                             |
+| 27  | [How do you write comments in React?](#how-to-write-comments-in-react)                                                                                                                                                           |
+| 28  | [What is reconciliation?](#what-is-reconciliation)                                                                                                                                                                               |
+| 29  | [Does the lazy function support named exports?](#does-the-lazy-function-support-named-exports)                                                                                                                                   |
+| 30  | [Why does React use className instead of the class attribute?](#why-react-uses-classname-over-class-attribute)                                                                                                                   |
+| 31  | [What are Fragments?](#what-are-fragments)                                                                                                                                                                                       |
+| 32  | [Why are Fragments better than container divs?](#why-fragments-are-better-than-container-divs)                                                                                                                                   |
+| 33  | [What are portals in React?](#what-are-portals-in-react)                                                                                                                                                                         |
+| 34  | [What are stateless components?](#what-are-stateless-components)                                                                                                                                                                 |
+| 35  | [What are stateful components?](#what-are-stateful-components)                                                                                                                                                                   |
+| 36  | [How do you apply validation to props in React?](#how-to-apply-validation-on-props-in-react)                                                                                                                                     |
+| 37  | [What are the advantages of React?](#what-are-the-advantages-of-react)                                                                                                                                                           |
+| 38  | [What are the limitations of React?](#what-are-the-limitations-of-react)                                                                                                                                                         |
+| 39  | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking)                                                                                                               |
+| 40  | [What is the use of the react-dom package?](#what-is-the-use-of-react-dom-package)                                                                                                                                               |
+| 41  | [What is ReactDOMServer?](#what-is-reactdomserver)                                                                                                                                                                               |
+| 42  | [How do you use innerHTML in React?](#how-to-use-innerhtml-in-react)                                                                                                                                                             |
+| 43  | [How do you apply styles in React?](#how-to-use-styles-in-react)                                                                                                                                                                 |
+| 44  | [How are events different in React?](#how-events-are-different-in-react)                                                                                                                                                         |
+| 45  | [What is the impact of using indexes as keys?](#what-is-the-impact-of-indexes-as-keys)                                                                                                                                          |
+| 46  | [How do you conditionally render components?](#how-do-you-conditionally-render-components)                                                                                                                                       |
+| 47  | [Why do we need to be careful when spreading props on DOM elements?](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements)                                                                                             |
+| 48  | [How do you memoize a component?](#how-do-you-memoize-a-component)                                                                                                                                                               |
+| 49  | [How do you implement Server-Side Rendering (SSR)?](#how-you-implement-server-side-rendering-or-ssr)                                                                                                                              |
+| 50  | [How do you enable production mode in React?](#how-to-enable-production-mode-in-react)                                                                                                                                           |
+| 51  | [Do Hooks replace render props and higher-order components?](#do-hooks-replace-render-props-and-higher-order-components)                                                                                                         |
+| 52  | [What is a switching component?](#what-is-a-switching-component)                                                                                                                                                                 |
+| 53  | [What are React Mixins?](#what-are-react-mixins)                                                                                                                                                                                 |
+| 54  | [What are the pointer events supported in React?](#what-are-the-pointer-events-supported-in-react)                                                                                                                               |
+| 55  | [Why should component names start with a capital letter?](#why-should-component-names-start-with-capital-letter)                                                                                                                  |
+| 56  | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16)                                                                                                                           |
+| 57  | [How do you loop inside JSX?](#how-to-loop-inside-jsx)                                                                                                                                                                           |
+| 58  | [How do you access props within attribute quotes?](#how-do-you-access-props-in-attribute-quotes)                                                                                                                                 |
+| 59  | [What is a React PropType array with shape?](#what-is-react-proptype-array-with-shape)                                                                                                                                           |
+| 60  | [How do you conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes)                                                                                                                                 |
+| 61  | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom)                                                                                                                         |
+| 62  | [Why is ReactDOM separated from React?](#why-reactdom-is-separated-from-react)                                                                                                                                                   |
+| 63  | [How do you use the React label element?](#how-to-use-react-label-element)                                                                                                                                                       |
+| 64  | [How do you combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects)                                                                                                                               |
+| 65  | [How do you re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized)                                                                                                             |
+| 66  | [How do you pretty-print JSON with React?](#how-to-pretty-print-json-with-react)                                                                                                                                                 |
+| 67  | [Why can’t you update props in React?](#why-you-cant-update-props-in-react)                                                                                                                                                      |
+| 68  | [How do you focus an input element on page load?](#how-to-focus-an-input-element-on-page-load)                                                                                                                                   |
+| 69  | [How can you find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser)                                                                                              |
+| 70  | [How do you add Google Analytics for React Router?](#how-to-add-google-analytics-for-react-router)                                                                                                                               |
+| 71  | [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react)                                                                                                       |
+| 72  | [How do you import and export components using React and ES6?](#how-to-import-and-export-components-using-react-and-es6)                                                                                                         |
+| 73  | [What are the exceptions to React component naming?](#what-are-the-exceptions-on-react-component-naming)                                                                                                                         |
+| 74  | [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react)                                                                                                                            |
+| 75  | [What are common folder structures for React?](#what-are-the-common-folder-structures-for-react)                                                                                                                                 |
+| 76  | [What are popular packages for animation?](#what-are-the-popular-packages-for-animation)                                                                                                                                         |
+| 77  | [What are the benefits of style modules?](#what-is-the-benefit-of-styles-modules)                                                                                                                                                |
+| 78  | [What are popular React-specific linters?](#what-are-the-popular-react-specific-linters)                                                                                                                                         |
+|     | **React Router**                                                                                                                                                                                                                 |
+| 79  | [What is React Router?](#what-is-react-router)                                                                                                                                                                                   |
+| 80  | [How is React Router different from the history library?](#how-react-router-is-different-from-history-library)                                                                                                                   |
+| 81  | [What are the <Router> components of React Router v6?](#what-are-the-router-components-of-react-router-v6)                                                                                                                       |
+| 82  | [What is the purpose of the push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history)                                                                                                   |
+| 83  | [How do you programmatically navigate using React Router v4?](#how-do-you-programmatically-navigate-using-react-router-v4)                                                                                                       |
+| 84  | [How do you get query parameters in React Router v4?](#how-to-get-query-parameters-in-react-router-v4)                                                                                                                           |
+| 85  | [Why do you get a "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning)                                                                                                |
+| 86  | [How do you pass params to the history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4)                                                                                            |
+| 87  | [How do you implement a default or NotFound page?](#how-to-implement-default-or-notfound-page)                                                                                                                                   |
+| 88  | [How do you get history in React Router v4?](#how-to-get-history-on-react-router-v4)                                                                                                                                             |
+| 89  | [How do you perform an automatic redirect after login?](#how-to-perform-automatic-redirect-after-login) 
+|     | **React Internationalization**                                                                                                                                                                                                   |
+| 90  | [What is React Intl?](#what-is-react-intl)                                                                                                                                                                                       |
+| 91  | [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl)                                                                                                                                           |
+| 92  | [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl)                                                                                                                         |
+| 93  | [How do you use FormattedMessage as a placeholder with React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl)                                                                                                |
+| 94  | [How do you access the current locale with React Intl?](#how-to-access-current-locale-with-react-intl)                                                                                                                           |
+| 95  | [How do you format a date using React Intl?](#how-to-format-date-using-react-intl)                                                                                                                                               |
+|     | **React Testing**                                                                                                                                                                                                                |
+| 96  | [What is the Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing)                                                                                                                                     |
+| 97  | [What is the TestRenderer package in React?](#what-is-testrenderer-package-in-react)                                                                                                                                             |
+| 98  | [What is the purpose of the ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package)                                                                                                                             |
+| 99  | [What is Jest?](#what-is-jest)                                                                                                                                                                                                   |
+| 100 | [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine)                                                                                                                                   |
+| 101 | [Can you give a simple example of a Jest test case?](#give-a-simple-example-of-jest-test-case)                                                                                                                                   |
+|     | **React Redux**                                                                                                                                                                                                                  |
+| 102 | [What is Flux?](#what-is-flux)                                                                                                                                                                                                   |
+| 103 | [What is Redux?](#what-is-redux)                                                                                                                                                                                                 |
+| 104 | [What are the core principles of Redux?](#what-are-the-core-principles-of-redux)                                                                                                                                                 |
+| 105 | [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux)                                                                                                                           |
+| 106 | [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops)                                                                             |
+| 107 | [Can you dispatch an action in a reducer?](#can-i-dispatch-an-action-in-reducer)                                                                                                                                                 |
+| 108 | [How do you access the Redux store outside a component?](#how-to-access-redux-store-outside-a-component)                                                                                                                         |
+| 109 | [What are the drawbacks of the MVW pattern?](#what-are-the-drawbacks-of-mvw-pattern)                                                                                                                                            |
+| 110 | [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs)                                                                                                                         |
+| 111 | [How do you reset state in Redux?](#how-to-reset-state-in-redux)                                                                                                                                                                 |
+| 112 | [What is the difference between React Context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux)                                                                                                   |
+| 113 | [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers)                                                                                                                                 |
+| 114 | [How do you make an AJAX request in Redux?](#how-to-make-ajax-request-in-redux)                                                                                                                                                  |
+| 115 | [Should you keep all component states in the Redux store?](#should-i-keep-all-components-state-in-redux-store)                                                                                                                   |
+| 116 | [What is the proper way to access the Redux store?](#what-is-the-proper-way-to-access-redux-store)                                                                                                                               |
+| 117 | [What is the difference between a component and a container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux)                                                                               |
+| 118 | [What is the purpose of constants in Redux?](#what-is-the-purpose-of-the-constants-in-redux)                                                                                                                                     |
+| 119 | [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops)                                                                                                           |
+| 120 | [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops)                                                 |
+| 121 | [How do you structure Redux top-level directories?](#how-to-structure-redux-top-level-directories)                                                                                                                               |
+| 122 | [What is Redux Saga?](#what-is-redux-saga)                                                                                                                                                                                       |
+| 123 | [What is the mental model of Redux Saga?](#what-is-the-mental-model-of-redux-saga)                                                                                                                                               |
+| 124 | [What are the differences between call and put in Redux Saga?](#what-are-the-differences-between-call-and-put-in-redux-saga)                                                                                                     |
+| 125 | [What is Redux Thunk?](#what-is-redux-thunk)                                                                                                                                                                                     |
+| 126 | [What are the differences between Redux Saga and Redux Thunk?](#what-are-the-differences-between-redux-saga-and-redux-thunk)                                                                                                     |
+| 127 | [What is Redux DevTools?](#what-is-redux-devtools)                                                                                                                                                                               |
+| 128 | [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools)                                                                                                                                             |
+| 129 | [What are Redux selectors and why should you use them?](#what-are-redux-selectors-and-why-use-them)                                                                                                                              |
+| 130 | [What is Redux Form?](#what-is-redux-form)                                                                                                                                                                                       |
+| 131 | [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form)                                                                                                                                           |
+| 132 | [How do you add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux)                                                                                                                                       |
+| 133 | [How do you set the initial state in Redux?](#how-to-set-initial-state-in-redux)                                                                                                                                                 |
+| 134 | [How is Relay different from Redux?](#how-relay-is-different-from-redux)                                                                                                                                                         |
+| 135 | [What is an action in Redux?](#what-is-an-action-in-redux)                                                                                                                                                                       |
+|     | **React Native**                                                                                                                                                                                                                 |
+| 136 | [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react)                                                                                                                 |
+| 137 | [How do you test React Native apps?](#how-to-test-react-native-apps)                                                                                                                                                             |
+| 138 | [How do you log in React Native?](#how-to-do-logging-in-react-native)                                                                                                                                                            |
+| 139 | [How do you debug React Native apps?](#how-to-debug-your-react-native)                                                                                                                                                           |
+|     | **React Supported Libraries and Integration**                                                                                                                                                                                    |
+| 140 | [What is Reselect and how does it work?](#what-is-reselect-and-how-it-works)                                                                                                                                                     |
+| 141 | [What is Flow?](#what-is-flow)                                                                                                                                                                                                   |
+| 142 | [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes)                                                                                                                         |
+| 143 | [How do you use Font Awesome icons in React?](#how-to-use-font-awesome-icons-in-react)                                                                                                                                           |
+| 144 | [What is React DevTools?](#what-is-react-dev-tools)                                                                                                                                                                              |
+| 145 | [Why does DevTools not load in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files)                                                                                                                  |
+| 146 | [How do you use Polymer in React?](#how-to-use-polymer-in-react)                                                                                                                                                                 |
+| 147 | [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs)                                                                                                                                    |
+| 148 | [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular)                                                                                                                           |
+| 149 | [Why is the React tab not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools)                                                                                                                                 |
+| 150 | [What are styled-components?](#what-are-styled-components)                                                                                                                                                                       |
+| 151 | [Can you give an example of styled-components?](#give-an-example-of-styled-components)                                                                                                                                           |
+| 152 | [What is Relay?](#what-is-relay)                                                                                                                                                                                                 |
+|     | **Miscellaneous**                                                                                                                                                                                                                |
+| 153 | [What are the main features of the Reselect library?](#what-are-the-main-features-of-reselect-library)                                                                                                                           |
+| 154 | [Can you give an example of Reselect usage?](#give-an-example-of-reselect-usage)                                                                                                                                                 |
+| 155 | [Can Redux only be used with React?](#can-redux-only-be-used-with-react)                                                                                                                                                         |
+| 156 | [Do you need a specific build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux)                                                                                                                     |
+| 157 | [How do Redux Form initial values get updated from state?](#how-redux-form-initialvalues-get-updated-from-state)                                                                                                                 |
+| 158 | [How do React PropTypes allow different types for one prop?](#how-react-proptypes-allow-different-types-for-one-prop)                                                                                                             |
+| 159 | [Can you import an SVG file as a React component?](#can-i-import-an-svg-file-as-react-component)                                                                                                                                  |
+| 160 | [What is render hijacking in React?](#what-is-render-hijacking-in-react)                                                                                                                                                         |
+| 161 | [How do you pass numbers to a React component?](#how-to-pass-numbers-to-react-component)                                                                                                                                         |
+| 162 | [Do you need to keep all state in Redux? Should you ever use React’s internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state)                                                          |
+| 163 | [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react)                                                                                                                 |
+| 164 | [What is the React.memo function?](#what-is-react-memo-function)                                                                                                                                                                 |
+| 165 | [What is the React.lazy function?](#what-is-react-lazy-function)                                                                                                                                                                 |
+| 166 | [How do you prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate)                                                                                                                     |
+| 167 | [How do you render arrays, strings, and numbers in React v16?](#how-do-you-render-array-strings-and-numbers-in-react-16-version)                                                                                                  |
+| 168 | [What are Hooks?](#what-are-hooks)                                                                                                                                                                                               |
+| 169 | [What rules must be followed for Hooks?](#what-rules-need-to-be-followed-for-hooks)                                                                                                                                             |
+| 170 | [How do you ensure Hooks follow the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project)                                                                                                             |
+| 171 | [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux)                                                                                                                             |
+| 172 | [What are the benefits of React Router v4?](#what-are-the-benefits-of-react-router-v4)                                                                                                                                           |
+| 173 | [Can you describe the componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature)                                                                                       |
+| 174 | [In which scenarios do error boundaries not catch errors?](#in-which-scenarios-do-error-boundaries-not-catch-errors)                                                                                                             |
+| 175 | [What is the behavior of uncaught errors in React v16?](#what-is-the-behavior-of-uncaught-errors-in-react-16)                                                                                                                    |
+| 176 | [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries)                                                                                                                         |
+| 177 | [What is the benefit of a component stack trace from an error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary)                                                                                       |
+| 178 | [What are default props?](#what-are-default-props)                                                                                                                                                                               |
+| 179 | [What is the purpose of the displayName class property?](#what-is-the-purpose-of-displayname-class-property)                                                                                                                     |
+| 180 | [What is the browser support for React applications?](#what-is-the-browser-support-for-react-applications)                                                                                                                       |
+| 181 | [What is code-splitting?](#what-is-code-splitting)                                                                                                                                                                               |
+| 182 | [What are keyed Fragments?](#what-are-keyed-fragments)                                                                                                                                                                           |
+| 183 | [Does React support all HTML attributes?](#does-react-support-all-html-attributes)                                                                                                                                               |
+| 184 | [When do component props default to true?](#when-component-props-defaults-to-true)                                                                                                                                               |
+| 185 | [What is Next.js and what are its major features?](#what-is-nextjs-and-major-features-of-it)                                                                                                                                     |
+| 186 | [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component)                                                                                                                             |
+| 187 | [How do you prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times)                                                                                                   |
+| 188 | [How does JSX prevent injection attacks?](#how-jsx-prevents-injection-attacks)                                                                                                                                                   |
+| 189 | [How do you update rendered elements?](#how-do-you-update-rendered-elements)                                                                                                                                                     |
+| 190 | [How do you indicate that props are read-only?](#how-do-you-say-that-props-are-read-only)                                                                                                                                        |
+| 191 | [What are the conditions for safely using an index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key)                                                                                                         |
+| 192 | [Do keys need to be globally unique?](#is-it-keys-should-be-globally-unique)                                                                                                                                                     |
+| 193 | [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling)                                                                                                                                   |
+| 194 | [What are the advantages of Formik over the Redux Form library?](#what-are-the-advantages-of-formik-over-redux-form-library)                                                                                                     |
+| 195 | [Why are you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance)                                                                                                                                       |
+| 196 | [Can you use web components in a React application?](#can-i-use-web-components-in-react-application)                                                                                                                             |
+| 197 | [What is a dynamic import?](#what-is-dynamic-import)                                                                                                                                                                             |
+| 198 | [What are loadable components?](#what-are-loadable-components)                                                                                                                                                                   |
+| 199 | [What is a Suspense component?](#what-is-suspense-component)                                                                                                                                                                     |
+| 200 | [What is route-based code splitting?](#what-is-route-based-code-splitting)                                                                                                                                                       |
+| 201 | [What is the purpose of the default value in Context?](#what-is-the-purpose-of-default-value-in-context)                                                                                                                         |
+| 202 | [What is the diffing algorithm?](#what-is-diffing-algorithm)                                                                                                                                                                     |
+| 203 | [What rules are covered by the diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm)                                                                                                                             |
+| 204 | [When do you need to use refs?](#when-do-you-need-to-use-refs)                                                                                                                                                                   |
+| 205 | [Must a prop be named "render" for render props?](#is-it-prop-must-be-named-as-render-for-render-props)                                                                                                                          |
+| 206 | [What are the problems with using render props with Pure Components?](#what-are-the-problems-of-using-render-props-with-pure-components)                                                                                         |
+| 207 | [What is the windowing technique?](#what-is-windowing-technique)                                                                                                                                                                 |
+| 208 | [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx)                                                                                                                                                   |
+| 209 | [What is the typical use case for portals?](#what-is-the-typical-use-case-of-portals)                                                                                                                                           |
+| 210 | [How do you set a default value for an uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component)                                                                                                         |
+| 211 | [What is your favorite React stack?](#what-is-your-favorite-react-stack)                                                                                                                                                         |
+| 212 | [What is the difference between the real DOM and the Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom)                                                                                                     |
+| 213 | [How do you add Bootstrap to a React application?](#how-to-add-bootstrap-to-a-react-application)                                                                                                                                 |
+| 214 | [Can you list the top websites or applications using React as a front-end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework)                                                         |
+| 215 | [Is it recommended to use the CSS-in-JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react)                                                                                                             |
+| 216 | [Do you need to rewrite all class components with Hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks)                                                                                                              |
+| 217 | [How do you fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks)                                                                                                                                                   |
+| 218 | [Do Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes)                                                                                                                                           |
+| 219 | [What is the stable release for Hooks support?](#what-is-the-stable-release-for-hooks-support)                                                                                                                                   |
+| 220 | [Why do we use array destructuring (square bracket notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate)                                                                               |
+| 221 | [What sources were used for introducing Hooks?](#what-are-the-sources-used-for-introducing-hooks)                                                                                                                                |
+| 222 | [How do you access the imperative API of web components?](#how-do-you-access-imperative-api-of-web-components)                                                                                                                   |
+| 223 | [What is Formik?](#what-is-formik)                                                                                                                                                                                               |
+| 224 | [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux)                                                                     |
+| 225 | [Do browsers understand JSX code?](#do-browsers-understand-jsx-code)                                                                                                                                                             |
+| 226 | [Can you describe data flow in React?](#describe-about-data-flow-in-react)                                                                                                                                                       |
+| 227 | [What is MobX?](#what-is-mobx)                                                                                                                                                                                                   |
+| 228 | [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx)                                                                                                                             |
+| 229 | [Should you learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs)                                                                                                                                     |
+| 230 | [What is concurrent rendering?](#what-is-concurrent-rendering)                                                                                                                                                                   |
+| 231 | [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode)                                                                                                 |
+| 232 | [Can you use JavaScript URLs in React v16.9?](#can-i-use-javascript-urls-in-react169)                                                                                                                                            |
+| 233 | [What is the purpose of the ESLint plugin for Hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks)                                                                                                                           |
+| 234 | [What is the difference between imperative and declarative programming in React?](#what-is-the-difference-between-imperative-and-declarative-in-react)                                                                             |
+| 235 | [What are the benefits of using TypeScript with ReactJS?](#what-are-the-benefits-of-using-typescript-with-reactjs)                                                                                                               |
+| 236 | [How do you ensure a user remains authenticated on page refresh while using Context API state management?](#how-do-you-make-sure-that-user-remains-authenticated-on-page-refresh-while-using-context-api-state-management)         |
+| 237 | [What are the benefits of the new JSX transform?](#what-are-the-benefits-of-new-jsx-transform)                                                                                                                                   |
+| 238 | [How is the new JSX transform different from the old transform?](#how-is-the-new-jsx-transform-different-from-old-transform)                                                                                                     |
+| 239 | [What are React Server Components?](#what-are-react-server-components)                                                                                                                                                           |
+| 240 | [What is prop drilling?](#what-is-prop-drilling)                                                                                                                                                                                 |
+| 241 | [What is the difference between the useState and useRef Hooks?](#what-is-the-difference-between-usestate-and-useref-hook)                                                                                                       |
+| 242 | [What is a wrapper component?](#what-is-a-wrapper-component)                                                                                                                                                                     |
+| 243 | [What are the differences between the useEffect and useLayoutEffect Hooks?](#what-are-the-differences-between-useeffect-and-uselayouteffect-hooks)                                                                               |
+| 244 | [What are the differences between functional and class components?](#what-are-the-differences-between-functional-and-class-components)                                                                                           |
+| 245 | [What is Strict Mode in React?](#what-is-strict-mode-in-react)                                                                                                                                                                   |
+| 246 | [What is the benefit of Strict Mode?](#what-is-the-benefit-of-strict-mode)                                                                                                                                                       |
+| 247 | [Why does Strict Mode render twice in React?](#why-does-strict-mode-render-twice-in-react)                                                                                                                                       |
+| 248 | [What are the rules of JSX?](#what-are-the-rules-of-jsx)                                                                                                                                                                         |
+| 249 | [What is the reason multiple JSX tags must be wrapped?](#what-is-the-reason-behind-multiple-jsx-tags-to-be-wrapped)                                                                                                              |
+| 250 | [How do you prevent mutating array variables?](#how-do-you-prevent-mutating-array-variables)                                                                                                                                     |
+| 251 | [What are capture phase events?](#what-are-capture-phase-events)                                                                                                                                                                 |
+| 252 | [How does React update the screen in an application?](#how-does-react-updates-screen-in-an-application)                                                                                                                          |
+| 253 | [How does React batch multiple state updates?](#how-does-react-batch-multiple-state-updates)                                                                                                                                     |
+| 254 | [Is it possible to prevent automatic batching?](#is-it-possible-to-prevent-automatic-batching)                                                                                                                                   |
+| 255 | [What is React hydration?](#what-is-react-hydration)                                                                                                                                                                             |
+| 256 | [How do you update objects inside state?](#how-do-you-update-objects-inside-state)                                                                                                                                               |
+| 257 | [How do you update nested objects inside state?](#How-do-you-update-nested-objects-inside-state)                                                                                                                                 |
+| 258 | [How do you update arrays inside state?](#how-do-you-update-arrays-inside-state)                                                                                                                                                 |
+| 259 | [How do you use the Immer library for state updates?](#how-do-you-use-immer-library-for-state-updates)                                                                                                                           |
+| 260 | [What are the benefits of preventing direct state mutations?](#what-are-the-benefits-of-preventing-the-direct-state-mutations)                                                                                                 |
+| 261 | [What are the preferred and non-preferred array operations for updating state?](#what-are-the-preferred-and-non-preferred-array-operations-for-updating-the-state)                                                             |
+| 262 | [What will happen when defining nested function components?](#what-will-happen-by-defining-nested-function-components)                                                                                                         |
+| 263 | [Can I use keys for non-list items?](#can-i-use-keys-for-non-list-items)                                                                                                                                                         |
+| 264 | [What are the guidelines to follow for writing reducers?](#what-are-the-guidelines-to-be-followed-for-writing-reducers)                                                                                                       |
+|     | **Hooks**                                                                                                                                                                                                                 |
+| 265 | [What is useReducer hook? Can you describe its usage?](#what-is-use-reducer-hook-can-you-describe-its-usage)                                                                                                               |
+| 266 | [How do you compare useState and useReducer?](#how-do-you-compare-use-state-and-use-reducer)                                                                                                                                     |
+| 267 | [How does Context work with the useContext hook?](#how-does-context-works-using-usecontext-hook)                                                                                                                                 |
+| 268 | [What are the use cases of the useContext hook?](#what-are-the-use-cases-of-usecontext-hook)                                                                                                                                     |
+| 269 | [When should you use client and server components?](#when-to-use-client-and-server-components)                                                                                                                                   |
+| 270 | [What are the differences between the Page Router and App Router in Next.js?](#what-are-the-differences-between-page-router-and-app-router-in-nextjs)          |                                                                         |                                                                                                                                                                   |
+|                                                                                                                                       |
+</details>
 
 ### Table of Contents
 
-| No. | Questions |
-| --- | --------- |
-|   | **Core React** |
-|1  | [What is React?](#what-is-react) |
-|2  | [What are the major features of React?](#what-are-the-major-features-of-react) |
-|3  | [What is JSX?](#what-is-jsx) |
-|4  | [What is the difference between Element and Component?](#what-is-the-difference-between-element-and-component) |
-|5  | [How to create components in React?](#how-to-create-components-in-react) |
-|6  | [When to use a Class Component over a Function Component?](#when-to-use-a-class-component-over-a-function-component) |
-|7  | [What are Pure Components?](#what-are-pure-components) |
-|8  | [What is state in React?](#what-is-state-in-react) |
-|9  | [What are props in React?](#what-are-props-in-react) |
-|10 | [What is the difference between state and props?](#what-is-the-difference-between-state-and-props) |
-|11 | [Why should we not update the state directly?](#why-should-we-not-update-the-state-directly) |
-|12 | [What is the purpose of callback function as an argument of setState()?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate)
-|13 | [What is the difference between HTML and React event handling?](#what-is-the-difference-between-html-and-react-event-handling) |
-|14 | [How to bind methods or event handlers in JSX callbacks?](#how-to-bind-methods-or-event-handlers-in-jsx-callbacks) |
-|15 | [How to pass a parameter to an event handler or callback?](#how-to-pass-a-parameter-to-an-event-handler-or-callback) |
-|16 | [What are synthetic events in React?](#what-are-synthetic-events-in-react) |
-|17 | [What are inline conditional expressions?](#what-are-inline-conditional-expressions) |
-|18 | [What is "key" prop and what is the benefit of using it in arrays of elements?](#what-is-key-prop-and-what-is-the-benefit-of-using-it-in-arrays-of-elements) |
-|19 | [What is the use of refs?](#what-is-the-use-of-refs) |
-|20 | [How to create refs?](#how-to-create-refs)
-|21 | [What are forward refs?](#what-are-forward-refs) |
-|22 | [Which is preferred option with in callback refs and findDOMNode()?](#which-is-preferred-option-with-in-callback-refs-and-finddomnode) |
-|23 | [Why are String Refs legacy?](#why-are-string-refs-legacy) |
-|24 | [What is Virtual DOM?](#what-is-virtual-dom) |
-|25 | [How Virtual DOM works?](#how-virtual-dom-works) |
-|26 | [What is the difference between Shadow DOM and Virtual DOM?](#what-is-the-difference-between-shadow-dom-and-virtual-dom) |
-|27 | [What is React Fiber?](#what-is-react-fiber) |
-|28 | [What is the main goal of React Fiber?](#what-is-the-main-goal-of-react-fiber) |
-|29 | [What are controlled components?](#what-are-controlled-components) |
-|30 | [What are uncontrolled components?](#what-are-uncontrolled-components) |
-|31 | [What is the difference between createElement and cloneElement?](#what-is-the-difference-between-createelement-and-cloneelement) |
-|32 | [What is Lifting State Up in React?](#what-is-lifting-state-up-in-react) |
-|33 | [What are the different phases of component lifecycle?](#what-are-the-different-phases-of-component-lifecycle) |
-|34 | [What are the lifecycle methods of React?](#what-are-the-lifecycle-methods-of-react) |
-|35 | [What are Higher-Order components?](#what-are-higher-order-components) |
-|36 | [How to create props proxy for HOC component?](#how-to-create-props-proxy-for-hoc-component) |
-|37 | [What is context?](#what-is-context) |
-|38 | [What is children prop?](#what-is-children-prop) |
-|39 | [How to write comments in React?](#how-to-write-comments-in-react) |
-|40 | [What is the purpose of using super constructor with props argument?](#what-is-the-purpose-of-using-super-constructor-with-props-argument) |
-|41 | [What is reconciliation?](#what-is-reconciliation) |
-|42 | [How to set state with a dynamic key name?](#how-to-set-state-with-a-dynamic-key-name) |
-|43 | [What would be the common mistake of function being called every time the component renders?](#what-would-be-the-common-mistake-of-function-being-called-every-time-the-component-renders) |
-|44 | [Is lazy function supports named exports?](#is-lazy-function-supports-named-exports) |
-|45 | [Why React uses className over class attribute?](#why-react-uses-classname-over-class-attribute) |
-|46 | [What are fragments?](#what-are-fragments) |
-|47 | [Why fragments are better than container divs?](#why-fragments-are-better-than-container-divs) |
-|48 | [What are portals in React?](#what-are-portals-in-react) |
-|49 | [What are stateless components?](#what-are-stateless-components) |
-|50 | [What are stateful components?](#what-are-stateful-components) |
-|51 | [How to apply validation on props in React?](#how-to-apply-validation-on-props-in-react) |
-|52 | [What are the advantages of React?](#what-are-the-advantages-of-react) |
-|53 | [What are the limitations of React?](#what-are-the-limitations-of-react) |
-|54 | [What are error boundaries in React v16](#what-are-error-boundaries-in-react-v16) |
-|55 | [How are error boundaries handled in React v15?](#how-are-error-boundaries-handled-in-react-v15) |
-|56 | [What are the recommended ways for static type checking?](#what-are-the-recommended-ways-for-static-type-checking) |
-|57 | [What is the use of react-dom package?](#what-is-the-use-of-react-dom-package) |
-|58 | [What is the purpose of render method of react-dom?](#what-is-the-purpose-of-render-method-of-react-dom) |
-|59 | [What is ReactDOMServer?](#what-is-reactdomserver) |
-|60 | [How to use InnerHtml in React?](#how-to-use-innerhtml-in-react) |
-|61 | [How to use styles in React?](#how-to-use-styles-in-react) |
-|62 | [How events are different in React?](#how-events-are-different-in-react) |
-|63 | [What will happen if you use setState in constructor?](#what-will-happen-if-you-use-setstate-in-constructor) |
-|64 | [What is the impact of indexes as keys?](#what-is-the-impact-of-indexes-as-keys) |
-|65 | [Is it good to use setState() in componentWillMount() method?](#is-it-good-to-use-setstate-in-componentwillmount-method) |
-|66 | [What will happen if you use props in initial state?](#what-will-happen-if-you-use-props-in-initial-state) |
-|67 | [How do you conditionally render components?](#how-do-you-conditionally-render-components)
-|68 | [Why we need to be careful when spreading props on DOM elements??](#why-we-need-to-be-careful-when-spreading-props-on-dom-elements) |
-|69 | [How you use decorators in React?](#how-you-use-decorators-in-react) |
-|70 | [How do you memoize a component?](#how-do-you-memoize-a-component) |
-|71 | [How you implement Server-Side Rendering or SSR?](#how-you-implement-server-side-rendering-or-ssr) |
-|72 | [How to enable production mode in React?](#how-to-enable-production-mode-in-react) |
-|73 | [What is CRA and its benefits?](#what-is-cra-and-its-benefits) |
-|74 | [What is the lifecycle methods order in mounting?](#what-is-the-lifecycle-methods-order-in-mounting) |
-|75 | [What are the lifecycle methods going to be deprecated in React v16?](#what-are-the-lifecycle-methods-going-to-be-deprecated-in-react-v16) |
-|76 | [What is the purpose of getDerivedStateFromProps() lifecycle method?](#what-is-the-purpose-of-getderivedstatefromprops-lifecycle-method) |
-|77 | [What is the purpose of getSnapshotBeforeUpdate() lifecycle method?](#what-is-the-purpose-of-getsnapshotbeforeupdate-lifecycle-method) |
-|78 | [Do Hooks replace render props and higher order components?](#do-hooks-replace-render-props-and-higher-order-components) |
-|79 | [What is the recommended way for naming components?](#what-is-the-recommended-way-for-naming-components) |
-|80 | [What is the recommended ordering of methods in component class?](#what-is-the-recommended-ordering-of-methods-in-component-class) |
-|81 | [What is a switching component?](#what-is-a-switching-component) |
-|82 | [Why we need to pass a function to setState()?](#why-we-need-to-pass-a-function-to-setstate) |
-|83 | [What is strict mode in React?](#what-is-strict-mode-in-react) |
-|84 | [What are React Mixins?](#what-are-react-mixins) |
-|85 | [Why is isMounted() an anti-pattern and what is the proper solution?](#why-is-ismounted-an-anti-pattern-and-what-is-the-proper-solution) |
-|86 | [What are the Pointer Events supported in React?](#what-are-the-pointer-events-supported-in-react) |
-|87 | [Why should component names start with capital letter?](#why-should-component-names-start-with-capital-letter) |
-|88 | [Are custom DOM attributes supported in React v16?](#are-custom-dom-attributes-supported-in-react-v16) |
-|89 | [What is the difference between constructor and getInitialState?](#what-is-the-difference-between-constructor-and-getinitialstate) |
-|90 | [Can you force a component to re-render without calling setState?](#can-you-force-a-component-to-re-render-without-calling-setstate) |
-|91 | [What is the difference between super() and super(props) in React using ES6 classes?](#what-is-the-difference-between-super-and-superprops-in-react-using-es6-classes) |
-|92 | [How to loop inside JSX?](#how-to-loop-inside-jsx) |
-|93 | [How do you access props in attribute quotes?](#how-do-you-access-props-in-attribute-quotes) |
-|94 | [What is React PropType array with shape?](#what-is-react-proptype-array-with-shape) |
-|95 | [How to conditionally apply class attributes?](#how-to-conditionally-apply-class-attributes) |
-|96 | [What is the difference between React and ReactDOM?](#what-is-the-difference-between-react-and-reactdom) |
-|97 | [Why ReactDOM is separated from React?](#why-reactdom-is-separated-from-react) |
-|98 | [How to use React label element?](#how-to-use-react-label-element) |
-|99 | [How to combine multiple inline style objects?](#how-to-combine-multiple-inline-style-objects) |
-|100| [How to re-render the view when the browser is resized?](#how-to-re-render-the-view-when-the-browser-is-resized)
-|101| [What is the difference between setState and replaceState methods?](#what-is-the-difference-between-setstate-and-replacestate-methods) |
-|102| [How to listen to state changes?](#how-to-listen-to-state-changes) |
-|103| [What is the recommended approach of removing an array element in react state?](#what-is-the-recommended-approach-of-removing-an-array-element-in-react-state) |
-|104| [Is it possible to use React without rendering HTML?](#is-it-possible-to-use-react-without-rendering-html) |
-|105| [How to pretty print JSON with React?](#how-to-pretty-print-json-with-react) |
-|106| [Why you can't update props in React?](#why-you-cant-update-props-in-react) |
-|107| [How to focus an input element on page load?](#how-to-focus-an-input-element-on-page-load) |
-|108| [What are the possible ways of updating objects in state?](#what-are-the-possible-ways-of-updating-objects-in-state) |
-|110| [How can we find the version of React at runtime in the browser?](#how-can-we-find-the-version-of-react-at-runtime-in-the-browser) |
-|111| [What are the approaches to include polyfills in your create-react-app?](#what-are-the-approaches-to-include-polyfills-in-your-create-react-app) |
-|112| [How to use https instead of http in create-react-app?](#how-to-use-https-instead-of-http-in-create-react-app) |
-|113| [How to avoid using relative path imports in create-react-app?](#how-to-avoid-using-relative-path-imports-in-create-react-app) |
-|114| [How to add Google Analytics for react-router?](#how-to-add-google-analytics-for-react-router) |
-|115| [How to update a component every second?](#how-to-update-a-component-every-second) |
-|116| [How do you apply vendor prefixes to inline styles in React?](#how-do-you-apply-vendor-prefixes-to-inline-styles-in-react) |
-|117| [How to import and export components using react and ES6?](#how-to-import-and-export-components-using-react-and-es6) |
-|118| [What are the exceptions on React component naming?](#what-are-the-exceptions-on-react-component-naming) |
-|119| [Why is a component constructor called only once?](#why-is-a-component-constructor-called-only-once) |
-|120| [How to define constants in React?](#how-to-define-constants-in-react) |
-|121| [How to programmatically trigger click event in React?](#how-to-programmatically-trigger-click-event-in-react) |
-|122| [Is it possible to use async/await in plain React?](#is-it-possible-to-use-asyncawait-in-plain-react) |
-|123| [What are the common folder structures for React?](#what-are-the-common-folder-structures-for-react) |
-|124| [What are the popular packages for animation?](#what-are-the-popular-packages-for-animation) |
-|125| [What is the benefit of styles modules?](#what-is-the-benefit-of-styles-modules) |
-|126| [What are the popular React-specific linters?](#what-are-the-popular-react-specific-linters) |
-|127| [How to make AJAX call and In which component lifecycle methods should I make an AJAX call?](#how-to-make-ajax-call-and-in-which-component-lifecycle-methods-should-i-make-an-ajax-call) |
-|128| [What are render props?](#what-are-render-props) |
-|   | **React Router** |
-|129| [What is React Router?](#what-is-react-router) |
-|130| [How React Router is different from history library?](#how-react-router-is-different-from-history-library) |
-|131| [What are the \<Router> components of React Router v4?](#what-are-the-router-components-of-react-router-v4) |
-|132| [What is the purpose of push and replace methods of history?](#what-is-the-purpose-of-push-and-replace-methods-of-history) |
-|133| [How do you programmatically navigate using React router v4?](#how-do-you-programmatically-navigate-using-react-router-v4) |
-|134| [How to get query parameters in React Router v4](#how-to-get-query-parameters-in-react-router-v4) |
-|135| [Why you get "Router may have only one child element" warning?](#why-you-get-router-may-have-only-one-child-element-warning) |
-|136| [How to pass params to history.push method in React Router v4?](#how-to-pass-params-to-historypush-method-in-react-router-v4) |
-|137| [How to implement default or NotFound page?](#how-to-implement-default-or-notfound-page) |
-|138| [How to get history on React Router v4?](#how-to-get-history-on-react-router-v4) |
-|139| [How to perform automatic redirect after login?](#how-to-perform-automatic-redirect-after-login) |
-|   | **React Internationalization** |
-|140| [What is React-Intl?](#what-is-react-intl) |
-|141| [What are the main features of React Intl?](#what-are-the-main-features-of-react-intl) |
-|142| [What are the two ways of formatting in React Intl?](#what-are-the-two-ways-of-formatting-in-react-intl) |
-|143| [How to use FormattedMessage as placeholder using React Intl?](#how-to-use-formattedmessage-as-placeholder-using-react-intl) |
-|144| [How to access current locale with React Intl](#how-to-access-current-locale-with-react-intl) |
-|145| [How to format date using React Intl?](#how-to-format-date-using-react-intl) |
-|   | **React Testing** |
-|146| [What is Shallow Renderer in React testing?](#what-is-shallow-renderer-in-react-testing) |
-|147| [What is TestRenderer package in React?](#what-is-testrenderer-package-in-react) |
-|148| [What is the purpose of ReactTestUtils package?](#what-is-the-purpose-of-reacttestutils-package) |
-|149| [What is Jest?](#what-is-jest) |
-|150| [What are the advantages of Jest over Jasmine?](#what-are-the-advantages-of-jest-over-jasmine) |
-|151| [Give a simple example of Jest test case](#give-a-simple-example-of-jest-test-case) |
-|   | **React Redux** |
-|152| [What is Flux?](#what-is-flux) |
-|153| [What is Redux?](#what-is-redux) |
-|154| [What are the core principles of Redux?](#what-are-the-core-principles-of-redux) |
-|155| [What are the downsides of Redux compared to Flux?](#what-are-the-downsides-of-redux-compared-to-flux) |
-|156| [What is the difference between mapStateToProps() and mapDispatchToProps()?](#what-is-the-difference-between-mapstatetoprops-and-mapdispatchtoprops) |
-|157| [Can I dispatch an action in reducer?](#can-i-dispatch-an-action-in-reducer) |
-|158| [How to access Redux store outside a component?](#how-to-access-redux-store-outside-a-component) |
-|159| [What are the drawbacks of MVW pattern](#what-are-the-drawbacks-of-mvw-pattern) |
-|160| [Are there any similarities between Redux and RxJS?](#are-there-any-similarities-between-redux-and-rxjs) |
-|161| [How to dispatch an action on load?](#how-to-dispatch-an-action-on-load) |
-|162| [How to use connect from React Redux?](#how-to-use-connect-from-react-redux) |
-|163| [How to reset state in Redux?](#how-to-reset-state-in-redux) |
-|164| [Whats the purpose of at symbol in the redux connect decorator?](#whats-the-purpose-of-at-symbol-in-the-redux-connect-decorator) |
-|165| [What is the difference between React context and React Redux?](#what-is-the-difference-between-react-context-and-react-redux) |
-|166| [Why are Redux state functions called reducers?](#why-are-redux-state-functions-called-reducers) |
-|167| [How to make AJAX request in Redux?](#how-to-make-ajax-request-in-redux) |
-|168| [Should I keep all component's state in Redux store?](#should-i-keep-all-components-state-in-redux-store) |
-|169| [What is the proper way to access Redux store?](#what-is-the-proper-way-to-access-redux-store) |
-|170| [What is the difference between component and container in React Redux?](#what-is-the-difference-between-component-and-container-in-react-redux) |
-|171| [What is the purpose of the constants in Redux? ](#what-is-the-purpose-of-the-constants-in-redux) |
-|172| [What are the different ways to write mapDispatchToProps()?](#what-are-the-different-ways-to-write-mapdispatchtoprops) |
-|173| [What is the use of the ownProps parameter in mapStateToProps() and mapDispatchToProps()?](#what-is-the-use-of-the-ownprops-parameter-in-mapstatetoprops-and-mapdispatchtoprops) |
-|174| [How to structure Redux top level directories?](#how-to-structure-redux-top-level-directories) |
-|175| [What is redux-saga?](#what-is-redux-saga) |
-|176| [What is the mental model of redux-saga?](#what-is-the-mental-model-of-redux-saga) |
-|177| [What are the differences between call and put in redux-saga](#what-are-the-differences-between-call-and-put-in-redux-saga) |
-|178| [What is Redux Thunk?](#what-is-redux-thunk) |
-|179| [What are the differences between redux-saga and redux-thunk](#what-are-the-differences-between-redux-saga-and-redux-thunk) |
-|180| [What is Redux DevTools?](#what-is-redux-devtools) |
-|181| [What are the features of Redux DevTools?](#what-are-the-features-of-redux-devtools) |
-|182| [What are Redux selectors and Why to use them?](#what-are-redux-selectors-and-why-to-use-them) |
-|183| [What is Redux Form?](#what-is-redux-form) |
-|184| [What are the main features of Redux Form?](#what-are-the-main-features-of-redux-form) |
-|185| [How to add multiple middlewares to Redux?](#how-to-add-multiple-middlewares-to-redux) |
-|186| [How to set initial state in Redux?](#how-to-set-initial-state-in-redux) |
-|187| [How Relay is different from Redux?](#how-relay-is-different-from-redux) |
-|188| [What is an action in Redux?](#what-is-an-action-in-redux) |
-|   | **React Native** |
-|188| [What is the difference between React Native and React?](#what-is-the-difference-between-react-native-and-react) |
-|189| [How to test React Native apps?](#how-to-test-react-native-apps) |
-|190| [How to do logging in React Native?](#how-to-do-logging-in-react-native) |
-|191| [How to debug your React Native?](#how-to-debug-your-react-native) |
-|   | **React supported libraries and Integration** |
-|192| [What is reselect and how it works?](#what-is-reselect-and-how-it-works) |
-|193| [What is Flow?](#what-is-flow) |
-|194| [What is the difference between Flow and PropTypes?](#what-is-the-difference-between-flow-and-proptypes) |
-|195| [How to use font-awesome icons in React?](#how-to-use-font-awesome-icons-in-react) |
-|196| [What is React Dev Tools?](#what-is-react-dev-tools) |
-|197| [Why is DevTools not loading in Chrome for local files?](#why-is-devtools-not-loading-in-chrome-for-local-files) |
-|198| [How to use Polymer in React?](#how-to-use-polymer-in-react) |
-|199| [What are the advantages of React over Vue.js?](#what-are-the-advantages-of-react-over-vuejs) |
-|200| [What is the difference between React and Angular?](#what-is-the-difference-between-react-and-angular) |
-|201| [Why React tab is not showing up in DevTools?](#why-react-tab-is-not-showing-up-in-devtools) |
-|202| [What are styled components?](#what-are-styled-components) |
-|203| [Give an example of Styled Components?](#give-an-example-of-styled-components) |
-|204| [What is Relay?](#what-is-relay) |
-|205| [How to use TypeScript in create-react-app application?](#how-to-use-typescript-in-create-react-app-application) |
-|   | **Miscellaneous** |
-|206| [What are the main features of reselect library?](#what-are-the-main-features-of-reselect-library) |
-|207| [Give an example of reselect usage?](#give-an-example-of-reselect-usage) |
-|209| [Does the statics object work with ES6 classes in React?](#does-the-statics-object-work-with-es6-classes-in-react) |
-|210| [Can Redux only be used with React?](#can-redux-only-be-used-with-react) |
-|211| [Do you need to have a particular build tool to use Redux?](#do-you-need-to-have-a-particular-build-tool-to-use-redux) |
-|212| [How Redux Form initialValues get updated from state?](#how-redux-form-initialvalues-get-updated-from-state) |
-|213| [How React PropTypes allow different type for one prop?](#how-react-proptypes-allow-different-types-for-one-prop) |
-|214| [Can I import an SVG file as react component?](#can-i-import-an-svg-file-as-react-component) |
-|215| [Why are inline ref callbacks or functions not recommended?](#why-are-inline-ref-callbacks-or-functions-not-recommended)|
-|216| [What is render hijacking in React?](#what-is-render-hijacking-in-react)|
-|217| [What are HOC factory implementations?](#what-are-hoc-factory-implementations)|
-|218| [How to pass numbers to React component?](#how-to-pass-numbers-to-react-component)|
-|219| [Do I need to keep all my state into Redux? Should I ever use react internal state?](#do-i-need-to-keep-all-my-state-into-redux-should-i-ever-use-react-internal-state)|
-|220| [What is the purpose of registerServiceWorker in React?](#what-is-the-purpose-of-registerserviceworker-in-react)|
-|221| [What is React memo function?](#what-is-react-memo-function)|
-|222| [What is React lazy function?](#what-is-react-lazy-function)|
-|223| [How to prevent unnecessary updates using setState?](#how-to-prevent-unnecessary-updates-using-setstate)|
-|224| [How do you render Array, Strings and Numbers in React 16 Version?](#how-do-you-render-array-strings-and-numbers-in-react-16-version)|
-|225| [How to use class field declarations syntax in React classes?](#how-to-use-class-field-declarations-syntax-in-react-classes)|
-|226| [What are hooks?](#what-are-hooks)|
-|227| [What rules need to be followed for hooks?](#what-rules-need-to-be-followed-for-hooks)|
-|228| [How to ensure hooks followed the rules in your project?](#how-to-ensure-hooks-followed-the-rules-in-your-project)|
-|229| [What are the differences between Flux and Redux?](#what-are-the-differences-between-flux-and-redux)|
-|230| [What are the benefits of React Router V4?](#what-are-the-benefits-of-react-router-v4)|
-|231| [Can you describe about componentDidCatch lifecycle method signature?](#can-you-describe-about-componentdidcatch-lifecycle-method-signature)|
-|232| [In which scenarios error boundaries do not catch errors?](#in-which-scenarios-error-boundaries-do-not-catch-errors)|
-|233| [Why do you not need error boundaries for event handlers?](#why-do-you-not-need-error-boundaries-for-event-handlers)|
-|234| [What is the difference between try catch block and error boundaries?](#what-is-the-difference-between-try-catch-block-and-error-boundaries)|
-|235| [What is the behavior of uncaught errors in react 16?](#what-is-the-behavior-of-uncaught-errors-in-react-16)|
-|236| [What is the proper placement for error boundaries?](#what-is-the-proper-placement-for-error-boundaries)|
-|237| [What is the benefit of component stack trace from error boundary?](#what-is-the-benefit-of-component-stack-trace-from-error-boundary)|
-|238| [What is the required method to be defined for a class component?](#what-is-the-required-method-to-be-defined-for-a-class-component)|
-|239| [What are the possible return types of render method?](#what-are-the-possible-return-types-of-render-method)|
-|240| [What is the main purpose of constructor?](#what-is-the-main-purpose-of-constructor)|
-|241| [Is it mandatory to define constructor for React component?](#is-it-mandatory-to-define-constructor-for-react-component)|
-|242| [What are default props?](#what-are-default-props)|
-|243| [Why should not call setState in componentWillUnmount?](#why-should-not-call-setstate-in-componentwillunmount)|
-|244| [What is the purpose of getDerivedStateFromError?](#what-is-the-purpose-of-getderivedstatefromerror)|
-|245| [What is the methods order when component re-rendered?](#what-is-the-methods-order-when-component-re-rendered)|
-|246| [What are the methods invoked during error handling?](#what-are-the-methods-invoked-during-error-handling)|
-|247| [What is the purpose of displayName class property?](#what-is-the-purpose-of-displayname-class-property)|
-|248| [What is the browser support for react applications?](#what-is-the-browser-support-for-react-applications)|
-|249| [What is the purpose of unmountComponentAtNode method?](#what-is-the-purpose-of-unmountcomponentatnode-method)|
-|250| [What is code-splitting?](#what-is-code-splitting)|
-|251| [What is the benefit of strict mode?](#what-is-the-benefit-of-strict-mode)|
-|252| [What are Keyed Fragments?](#what-are-keyed-fragments)|
-|253| [Does React support all HTML attributes?](#does-react-support-all-html-attributes)|
-|254| [What are the limitations with HOCs?](#what-are-the-limitations-with-hocs)|
-|255| [How to debug forwardRefs in DevTools?](#how-to-debug-forwardrefs-in-devtools)|
-|256| [When component props defaults to true?](#when-component-props-defaults-to-true)|
-|257| [What is NextJS and major features of it?](#what-is-nextjs-and-major-features-of-it)|
-|258| [How do you pass an event handler to a component?](#how-do-you-pass-an-event-handler-to-a-component)|
-|259| [Is it good to use arrow functions in render methods?](#is-it-good-to-use-arrow-functions-in-render-methods)|
-|260| [How to prevent a function from being called multiple times?](#how-to-prevent-a-function-from-being-called-multiple-times)|
-|261| [How JSX prevents Injection Attacks?](#how-jsx-prevents-injection-attacks)|
-|262| [How do you update rendered elements?](#how-do-you-update-rendered-elements)|
-|263| [How do you say that props are read only?](#how-do-you-say-that-props-are-read-only)|
-|264| [How do you say that state updates are merged?](#how-do-you-say-that-state-updates-are-merged)|
-|265| [How do you pass arguments to an event handler?](#how-do-you-pass-arguments-to-an-event-handler)|
-|266| [How to prevent component from rendering?](#how-to-prevent-component-from-rendering)|
-|267| [What are the conditions to safely use the index as a key?](#what-are-the-conditions-to-safely-use-the-index-as-a-key)|
-|268| [Is it keys should be globally unique?](#is-it-keys-should-be-globally-unique)|
-|269| [What is the popular choice for form handling?](#what-is-the-popular-choice-for-form-handling)|
-|270| [What are the advantages of formik over redux form library?](#what-are-the-advantages-of-formik-over-redux-form-library)|
-|271| [Why do you not required to use inheritance?](#why-do-you-not-required-to-use-inheritance)|
-|272| [Can I use web components in react application?](#can-i-use-web-components-in-react-application)|
-|273| [What is dynamic import?](#what-is-dynamic-import)|
-|274| [What are loadable components?](#what-are-loadable-components)|
-|275| [What is suspense component?](#what-is-suspense-component)|
-|276| [What is route based code splitting?](#what-is-route-based-code-splitting)|
-|277| [Give an example on How to use context?](#give-an-example-on-how-to-use-context)|
-|278| [What is the purpose of default value in context?](#what-is-the-purpose-of-default-value-in-context)|
-|279| [How do you use contextType?](#how-do-you-use-contexttype)|
-|280| [What is a consumer?](#what-is-a-consumer)|
-|281| [How do you solve performance corner cases while using context?](#how-do-you-solve-performance-corner-cases-while-using-context)|
-|282| [What is the purpose of forward ref in HOCs?](#what-is-the-purpose-of-forward-ref-in-hocs)|
-|283| [Is it ref argument available for all functions or class components?](#is-it-ref-argument-available-for-all-functions-or-class-components)|
-|284| [Why do you need additional care for component libraries while using forward refs?](#why-do-you-need-additional-care-for-component-libraries-while-using-forward-refs)|
-|285| [How to create react class components without ES6?](#how-to-create-react-class-components-without-es6)|
-|286| [Is it possible to use react without JSX?](#is-it-possible-to-use-react-without-jsx)|
-|287| [What is diffing algorithm?](#what-is-diffing-algorithm)|
-|288| [What are the rules covered by diffing algorithm?](#what-are-the-rules-covered-by-diffing-algorithm)|
-|289| [When do you need to use refs?](#when-do-you-need-to-use-refs)|
-|290| [Is it prop must be named as render for render props?](#is-it-prop-must-be-named-as-render-for-render-props)|
-|291| [What are the problems of using render props with pure components?](#what-are-the-problems-of-using-render-props-with-pure-components)|
-|292| [How do you create HOC using render props?](#how-do-you-create-hoc-using-render-props)|
-|293| [What is windowing technique?](#what-is-windowing-technique)|
-|294| [How do you print falsy values in JSX?](#how-do-you-print-falsy-values-in-jsx)|
-|295| [What is the typical use case of portals?](#what-is-the-typical-use-case-of-portals)|
-|296| [How do you set default value for uncontrolled component?](#how-do-you-set-default-value-for-uncontrolled-component)|
-|297| [What is your favorite React stack?](#what-is-your-favorite-react-stack)|
-|298| [What is the difference between Real DOM and Virtual DOM?](#what-is-the-difference-between-real-dom-and-virtual-dom)|
-|299| [How to add Bootstrap to a react application?](#how-to-add-bootstrap-to-a-react-application)|
-|300| [Can you list down top websites or applications using react as front end framework?](#can-you-list-down-top-websites-or-applications-using-react-as-front-end-framework)|
-|301| [Is it recommended to use CSS In JS technique in React?](#is-it-recommended-to-use-css-in-js-technique-in-react)|
-|302| [Do I need to rewrite all my class components with hooks?](#do-i-need-to-rewrite-all-my-class-components-with-hooks)|
-|303| [How to fetch data with React Hooks?](#how-to-fetch-data-with-react-hooks)|
-|304| [Is Hooks cover all use cases for classes?](#is-hooks-cover-all-use-cases-for-classes)|
-|305| [What is the stable release for hooks support?](#what-is-the-stable-release-for-hooks-support)|
-|306| [Why do we use array destructuring (square brackets notation) in useState?](#why-do-we-use-array-destructuring-square-brackets-notation-in-usestate)|
-|307| [What are the sources used for introducing hooks?](#what-are-the-sources-used-for-introducing-hooks)|
-|308| [How do you access imperative API of web components?](#how-do-you-access-imperative-api-of-web-components)|
-|309| [What is formik?](#what-is-formik)|
-|310| [What are typical middleware choices for handling asynchronous calls in Redux?](#what-are-typical-middleware-choices-for-handling-asynchronous-calls-in-redux)|
-|311| [Do browsers understand JSX code?](#do-browsers-understand-jsx-code)|
-|312| [Describe about data flow in react?](#describe-about-data-flow-in-react)|
-|313| [What is react scripts?](#what-is-react-scripts)|
-|314| [What are the features of create react app?](#what-are-the-features-of-create-react-app)|
-|315| [What is the purpose of renderToNodeStream method?](#what-is-the-purpose-of-rendertonodestream-method)|
-|316| [What is MobX?](#what-is-mobx)|
-|317| [What are the differences between Redux and MobX?](#what-are-the-differences-between-redux-and-mobx)|
-|318| [Should I learn ES6 before learning ReactJS?](#should-i-learn-es6-before-learning-reactjs)|
-|319| [What is Concurrent Rendering?](#what-is-concurrent-rendering)|
-|320| [What is the difference between async mode and concurrent mode?](#what-is-the-difference-between-async-mode-and-concurrent-mode)|
-|321| [Can I use javascript urls in react16.9?](#can-i-use-javascript-urls-in-react169)|
-|322| [What is the purpose of eslint plugin for hooks?](#what-is-the-purpose-of-eslint-plugin-for-hooks)|
-|323| [What is the difference between Imperative and Declarative in React?](#what-is-the-difference-between-imperative-and-declarative-in-react)|
-|324| [What are the benefits of using typescript with reactjs?](#what-are-the-benefits-of-using-typescript-with-reactjs)|
-|325| [How do you make sure that user remains authenticated on page refresh while using Context API State Management?](#how-do-you-make-sure-that-user-remains-authenticated-on-page-refresh-while-using-context-api-state-management)|
-|326| [What are the benefits of new JSX transform?](#what-are-the-benefits-of-new-jsx-transform)|
-|327| [How does new JSX transform different from old transform?](#how-does-new-jsx-transform-different-from-old-transform)|
-|328| [How do you get redux scaffolding using create-react-app?](#how-do-you-get-redux-scaffolding-using-create-react-app)|
-|329| [What are React Server components?](#what-are-react-server-components)|
-|330| [What is prop drilling?](#what-is-prop-drilling)|
-|331| [What is state mutation and how to prevent it?](#what-is-state-mutation-and-how-to-prevent-it)|
-|332| [What is the difference between useState and useRef hook?](#what-is-the-difference-between-usestate-and-useref-hook)|
+<details open>
+<summary>
+Hide/Show table of contents
+</summary>
+
+| No. | Questions                                                                                                                                                                                  |
+| --- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
+|     | **Old Q&A**                                                                                                                                                                                |
+| 1   | [Why should we not update the state directly?](#why-should-we-not-update-the-state-directly)                                                                                               |
+| 2   | [What is the purpose of callback function as an argument of setState()?](#what-is-the-purpose-of-callback-function-as-an-argument-of-setstate)                                             |
+| 3   | [How to bind methods or event handlers in JSX callbacks?](#how-to-bind-methods-or-event-handlers-in-jsx-callbacks)                                                                         |
+| 4   | [How to pass a parameter to an event handler or callback?](#how-to-pass-a-parameter-to-an-event-handler-or-callback)                                                                       |
+| 5   | [What is the use of refs?](#what-is-the-use-of-refs)                                                                                                                                       |
+| 6   | [How to create refs?](#how-to-create-refs)                                                                                                                                                 |
+| 7   | [What are forward refs?](#what-are-forward-refs)                                                                                                                                           |
+| 8   | [Which is preferred option with in callback refs and findDOMNode()?](#which-is-preferred-option-with-in-callback-refs-and-finddomnode)                                                     |
+| 9   | [Why are String Refs legacy?](#why-are-string-refs-legacy)                                                                                                                                 |
+| 10  | [What are the different phases of component lifecycle?](#what-are-the-different-phases-of-component-lifecycle)                                                                             |
+| 11  | [What are the lifecycle methods of React?](#what-are-the-lifecycle-methods-of-react)                                                                                                       |
+| 12  | [How to create props proxy for HOC component?](#how-to-create-props-proxy-for-hoc-component)                                                                                               |
+| 13  | [What is context?](#what-is-context)                                                                                                                                                       |
+| 14  | [What is the purpose of using super constructor with props argument?](#what-is-the-purpose-of-using-super-constructor-with-props-argument)                                                 |
+| 15  | [How to set state with a dynamic key name?](#how-to-set-state-with-a-dynamic-key-name)                                                                                                     |
+| 16  | [What would be the common mistake of function being called every time the component renders?](#what-would-be-the-common-mistake-of-function-being-called-every-time-the-component-renders) |
+| 17  | [What are error boundaries in React v16](#what-are-error-boundaries-in-react-v16)                                                                                                          |
+| 18  | [How are error boundaries handled in React v15?](#how-are-error-boundaries-handled-in-react-v15)                                                                                           |
+| 19  | [What is the purpose of render method of react-dom?](#what-is-the-purpose-of-render-method-of-react-dom)                                                                                   |
+| 20  | [What will happen if you use setState in constructor?](#what-will-happen-if-you-use-setstate-in-constructor)                                                                               |
+| 21  | [Is it good to use setState() in componentWillMount() method?](#is-it-good-to-use-setstate-in-componentwillmount-method)                                                                   |
+| 22  | [What will happen if you use props in initial state?](#what-will-happen-if-you-use-props-in-initial-state)                                                                                 |
+| 23  | [How you use decorators in React?](#how-you-use-decorators-in-react)                                                                                                                       |
+| 24  | [What is CRA and its benefits?](#what-is-cra-and-its-benefits)                                                                                                                             |
+| 25  | [What is the lifecycle methods order in mounting?](#what-is-the-lifecycle-methods-order-in-mounting)                                                                                       |
+| 26  | [What are the lifecycle methods going to be deprecated in React v16?](#what-are-the-lifecycle-methods-going-to-be-deprecated-in-react-v16)                                                 |
+| 27  | [What is the purpose of getDerivedStateFromProps() lifecycle method?](#what-is-the-purpose-of-getderivedstatefromprops-lifecycle-method)                                                   |
+| 28  | [What is the purpose of getSnapshotBeforeUpdate() lifecycle method?](#what-is-the-purpose-of-getsnapshotbeforeupdate-lifecycle-method)                                                     |
+| 29  | [What is the recommended way for naming components?](#what-is-the-recommended-way-for-naming-components)                                                                                   |
+| 30  | [What is the recommended ordering of methods in component class?](#what-is-the-recommended-ordering-of-methods-in-component-class)                                                         |
+| 31  | [Why we need to pass a function to setState()?](#why-we-need-to-pass-a-function-to-setstate)                                                                                               |
+| 32  | [Why is isMounted() an anti-pattern and what is the proper solution?](#why-is-ismounted-an-anti-pattern-and-what-is-the-proper-solution)                                                   |
+| 33  | [What is the difference between constructor and getInitialState?](#what-is-the-difference-between-constructor-and-getinitialstate)                                                         |
+| 34  | [Can you force a component to re-render without calling setState?](#can-you-force-a-component-to-re-render-without-calling-setstate)                                                       |
+| 35  | [What is the difference between super() and super(props) in React using ES6 classes?](#what-is-the-difference-between-super-and-superprops-in-react-using-es6-classes)                     |
+| 36  | [What is the difference between setState and replaceState methods?](#what-is-the-difference-between-setstate-and-replacestate-methods)                                                     |
+| 37  | [How to listen to state changes?](#how-to-listen-to-state-changes)                                                                                                                         |
+| 38  | [What is the recommended approach of removing an array element in react state?](#what-is-the-recommended-approach-of-removing-an-array-element-in-react-state)                             |
+| 39  | [Is it possible to use React without rendering HTML?](#is-it-possible-to-use-react-without-rendering-html)                                                                                 |
+| 40  | [What are the possible ways of updating objects in state?](#what-are-the-possible-ways-of-updating-objects-in-state)                                                                       |
+| 41  | [What are the approaches to include polyfills in your create-react-app?](#what-are-the-approaches-to-include-polyfills-in-your-create-react-app)                                           |
+| 42  | [How to use https instead of http in create-react-app?](#how-to-use-https-instead-of-http-in-create-react-app)                                                                             |
+| 43  | [How to avoid using relative path imports in create-react-app?](#how-to-avoid-using-relative-path-imports-in-create-react-app)                                                             |
+| 44  | [How to update a component every second?](#how-to-update-a-component-every-second)                                                                                                         |
+| 45  | [Why is a component constructor called only once?](#why-is-a-component-constructor-called-only-once)                                                                                       |
+| 46  | [How to define constants in React?](#how-to-define-constants-in-react)                                                                                                                     |
+| 47  | [How to programmatically trigger click event in React?](#how-to-programmatically-trigger-click-event-in-react)                                                                             |
+| 48  | [How to make AJAX call and In which component lifecycle methods should I make an AJAX call?](#how-to-make-ajax-call-and-in-which-component-lifecycle-methods-should-i-make-an-ajax-call)   |
+| 49  | [What are render props?](#what-are-render-props)                                                                                                                                           |
+| 50  | [How to dispatch an action on load?](#how-to-dispatch-an-action-on-load)                                                                                                                   |
+| 51  | [How to use connect from React Redux?](#how-to-use-connect-from-react-redux)                                                                                                               |
+| 52  | [Whats the purpose of at symbol in the redux connect decorator?](#whats-the-purpose-of-at-symbol-in-the-redux-connect-decorator)                                                           |
+| 53  | [How to use TypeScript in create-react-app application?](#how-to-use-typescript-in-create-react-app-application)                                                                           |
+| 54  | [Does the statics object work with ES6 classes in React?](#does-the-statics-object-work-with-es6-classes-in-react)                                                                         |
+| 55  | [Why are inline ref callbacks or functions not recommended?](#why-are-inline-ref-callbacks-or-functions-not-recommended)                                                                   |
+| 56  | [What are HOC factory implementations?](#what-are-hoc-factory-implementations)                                                                                                             |
+| 57  | [How to use class field declarations syntax in React classes?](#how-to-use-class-field-declarations-syntax-in-react-classes)                                                               |
+| 58  | [Why do you not need error boundaries for event handlers?](#why-do-you-not-need-error-boundaries-for-event-handlers)                                                                       |
+| 59  | [What is the difference between try catch block and error boundaries?](#what-is-the-difference-between-try-catch-block-and-error-boundaries)                                               |
+| 60  | [What is the required method to be defined for a class component?](#what-is-the-required-method-to-be-defined-for-a-class-component)                                                       |
+| 61  | [What are the possible return types of render method?](#what-are-the-possible-return-types-of-render-method)                                                                               |
+| 62  | [What is the main purpose of constructor?](#what-is-the-main-purpose-of-constructor)                                                                                                       |
+| 63  | [Is it mandatory to define constructor for React component?](#is-it-mandatory-to-define-constructor-for-react-component)                                                                   |
+| 64  | [Why should not call setState in componentWillUnmount?](#why-should-not-call-setstate-in-componentwillunmount)                                                                             |
+| 65  | [What is the purpose of getDerivedStateFromError?](#what-is-the-purpose-of-getderivedstatefromerror)                                                                                       |
+| 66  | [What is the methods order when component re-rendered?](#what-is-the-methods-order-when-component-re-rendered)                                                                             |
+| 67  | [What are the methods invoked during error handling?](#what-are-the-methods-invoked-during-error-handling)                                                                                 |
+| 68  | [What is the purpose of unmountComponentAtNode method?](#what-is-the-purpose-of-unmountcomponentatnode-method)                                                                             |
+| 69  | [What are the limitations with HOCs?](#what-are-the-limitations-with-hocs)                                                                                                                 |
+| 70  | [How to debug forwardRefs in DevTools?](#how-to-debug-forwardrefs-in-devtools)                                                                                                             |
+| 71  | [Is it good to use arrow functions in render methods?](#is-it-good-to-use-arrow-functions-in-render-methods)                                                                               |
+| 72  | [How do you say that state updates are merged?](#how-do-you-say-that-state-updates-are-merged)                                                                                             |
+| 73  | [How do you pass arguments to an event handler?](#how-do-you-pass-arguments-to-an-event-handler)                                                                                           |
+| 74  | [How to prevent component from rendering?](#how-to-prevent-component-from-rendering)                                                                                                       |
+| 75  | [Give an example on How to use context?](#give-an-example-on-how-to-use-context)                                                                                                           |
+| 76  | [How do you use contextType?](#how-do-you-use-contexttype)                                                                                                                                 |
+| 77  | [What is a consumer?](#what-is-a-consumer)                                                                                                                                                 |
+| 78  | [How do you solve performance corner cases while using context?](#how-do-you-solve-performance-corner-cases-while-using-context)                                                           |
+| 79  | [What is the purpose of forward ref in HOCs?](#what-is-the-purpose-of-forward-ref-in-hocs)                                                                                                 |
+| 80  | [Is it ref argument available for all functions or class components?](#is-it-ref-argument-available-for-all-functions-or-class-components)                                                 |
+| 81  | [Why do you need additional care for component libraries while using forward refs?](#why-do-you-need-additional-care-for-component-libraries-while-using-forward-refs)                     |
+| 82  | [How to create react class components without ES6?](#how-to-create-react-class-components-without-es6)                                                                                     |
+| 83  | [Is it possible to use react without JSX?](#is-it-possible-to-use-react-without-jsx)                                                                                                       |
+| 84  | [How do you create HOC using render props?](#how-do-you-create-hoc-using-render-props)                                                                                                     |
+| 85  | [What is react scripts?](#what-is-react-scripts)                                                                                                                                           |
+| 86  | [What are the features of create react app?](#what-are-the-features-of-create-react-app)                                                                                                   |
+| 87  | [What is the purpose of renderToNodeStream method?](#what-is-the-purpose-of-rendertonodestream-method)                                                                                     |
+| 88  | [How do you get redux scaffolding using create-react-app?](#how-do-you-get-redux-scaffolding-using-create-react-app)                                                                       |
+| 89  | [What is state mutation and how to prevent it?](#what-is-state-mutation-and-how-to-prevent-it)                                                                                             |
+
+</details>
 
 ## Core React
 
+1.  ### What is React?
 
-    
-1. ### What is React?
+    React (aka React.js or ReactJS) is an **open-source front-end JavaScript library** for building user interfaces based on components. It's used for handling the view layer in web and mobile applications, and allows developers to create reusable UI components and manage the state of those components efficiently.
 
-    React is an **open-source front-end JavaScript library** that is used for building user interfaces, especially for single-page applications. It is used for handling view layer for web and mobile apps. React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer working for Facebook. React was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012.
+    React was created by [Jordan Walke](https://github.com/jordwalke), a software engineer at Facebook (now Meta). It was first deployed on Facebook's News Feed in 2011 and on Instagram in 2012. The library was open-sourced in May 2013 and has since become one of the most popular JavaScript libraries for building modern user interfaces.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
+   2.  ### What is the history behind React evolution?
+       The history of ReactJS started in 2010 with the creation of **XHP**. XHP is a PHP extension which improved the syntax of the language such that XML document fragments become valid PHP expressions and the primary purpose was used to create custom and reusable HTML elements.
+
+       The main principle of this extension was to make front-end code easier to understand and to help avoid cross-site scripting attacks. The project was successful to prevent the malicious content submitted by the scrubbing user.
+
+       But there was a different problem with XHP in which dynamic web applications require many roundtrips to the server, and XHP did not solve this problem. Also, the whole UI was re-rendered for small change in the application. Later, the initial prototype of React is created with the name **FaxJ** by Jordan inspired from XHP. Finally after sometime React has been introduced as a new library into JavaScript world.
+
+       <details>
+           <summary><b>See deep-dive answer</b></summary>
+           The evolution of React has a fascinating history that spans over a decade:
     
-2. ### What are the major features of React?
+       **2010-2011: The Origins**
+       - The journey began with **XHP**, a PHP extension created at Facebook that allowed HTML components to be used in PHP code
+       - XHP improved front-end code readability and helped prevent cross-site scripting (XSS) attacks
+       - However, XHP had limitations with dynamic web applications, requiring frequent server roundtrips and complete UI re-renders for small changes
 
-    The major features of React are:
+       **2011-2012: Early Development**
+       - Jordan Walke created the first prototype called **FaxJS** (later renamed to React), inspired by XHP's component model
+       - The key innovation was bringing XHP's component model to JavaScript with performance improvements
+       - React introduced the Virtual DOM concept to solve the performance issues of full page re-renders
+       - First deployed internally on Facebook's News Feed in 2011 and Instagram in 2012
 
-    * It uses **VirtualDOM** instead of RealDOM considering that RealDOM manipulations are expensive.
-    * Supports **server-side rendering**.
-    * Follows **Unidirectional** data flow or data binding.
-    * Uses **reusable/composable** UI components to develop the view.
+       **2013: Public Release**
+       - React was officially open-sourced at JSConf US in May 2013
+       - Initial public reception was mixed, with some developers skeptical about the JSX syntax and the approach of mixing markup with JavaScript
 
+       **2014-2015: Growing Adoption**
+       - React Native was announced in 2015, extending React's paradigm to mobile app development
+       - The ecosystem began to grow with tools like Redux for state management
+       - Companies beyond Facebook began adopting React for production applications
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-3. ### What is JSX?
+       **2016-2018: Maturation**
+       - React 16 ("Fiber") was released in 2017 with a complete rewrite of the core architecture
+       - Introduction of new features like Error Boundaries, Portals, and improved server-side rendering
+       - React 16.3 introduced the Context API for easier state management
 
-    *JSX* is a XML-like syntax extension to ECMAScript (the acronym stands for *JavaScript XML*). Basically it just provides syntactic sugar for the `React.createElement()` function, giving us expressiveness of JavaScript along with HTML like template syntax.
+       **2019-Present: Modern React**
+       - React Hooks were introduced in React 16.8 (February 2019), revolutionizing state management in functional components
+       - React 17 (October 2020) focused on making React upgrades easier
+       - React 18 (March 2022) introduced concurrent rendering and automatic batching
+       - React continues to evolve with Server Components, the new React compiler (React Forget), and other performance improvements
+       </details>
 
-    In the example below text inside `<h1>` tag is returned as JavaScript function to the render function.
+       **Note:** JSX, React's syntax extension, was indeed inspired by XHP's approach of embedding XML-like syntax in code.
 
-    ```jsx harmony
-    class App extends React.Component {
-      render() {
-        return(
-          <div>
-            <h1>{'Welcome to React world!'}</h1>
-          </div>
-        )
-      }
-    }
-    ```
+       **[⬆ Back to Top](#table-of-contents)**
 
+3.  ### What are the major features of React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-4. ### What is the difference between Element and Component?
+    React offers a powerful set of features that have made it one of the most popular JavaScript libraries for building user interfaces:
 
-    An *Element* is a plain object describing what you want to appear on the screen in terms of the DOM nodes or other components. *Elements* can contain other *Elements* in their props. Creating a React element is cheap. Once an element is created, it is never mutated.
+    **Core Features:**
 
-    The object representation of React Element would be as follows:
+    - **Component-Based Architecture**: React applications are built using components - independent, reusable pieces of code that return HTML via a render function. This modular approach enables better code organization, reusability, and maintenance.
 
-    ```javascript
-    const element = React.createElement(
-      'div',
-      {id: 'login-btn'},
-      'Login'
-    )
-    ```
+    - **Virtual DOM**: React creates an in-memory data structure cache, computes the resulting differences, and efficiently updates only the changed parts in the browser DOM. This approach significantly improves performance compared to direct DOM manipulation.
 
-    The above `React.createElement()` function returns an object:
+    - **JSX (JavaScript XML)**: A syntax extension that allows writing HTML-like code in JavaScript. JSX makes the code more readable and expressive while providing the full power of JavaScript.
 
-    ```
-    {
-      type: 'div',
-      props: {
-        children: 'Login',
-        id: 'login-btn'
-      }
-    }
-    ```
+    - **Unidirectional Data Flow**: React follows a one-way data binding model where data flows from parent to child components. This makes the code more predictable and easier to debug.
 
-    And finally it renders to the DOM using `ReactDOM.render()`:
+    - **Declarative UI**: React allows you to describe what your UI should look like for a given state, and it handles the DOM updates when the underlying data changes.
 
-    ```html
-    <div id='login-btn'>Login</div>
+    **Advanced Features:**
+
+    - **React Hooks**: Introduced in React 16.8, hooks allow using state and other React features in functional components without writing classes.
+
+    - **Context API**: Provides a way to share values between components without explicitly passing props through every level of the component tree.
+
+    - **Error Boundaries**: Components that catch JavaScript errors anywhere in their child component tree and display fallback UI instead of crashing.
+
+    - **Server-Side Rendering (SSR)**: Enables rendering React components on the server before sending HTML to the client, improving performance and SEO.
+
+    - **Concurrent Mode**: A set of new features (in development) that help React apps stay responsive and gracefully adjust to the user's device capabilities and network speed.
+
+    - **React Server Components**: A new feature that allows components to be rendered entirely on the server, reducing bundle size and improving performance.
+
+    - **Suspense**: A feature that lets your components "wait" for something before rendering, supporting code-splitting and data fetching with cleaner code.
+
+    These features collectively make React powerful for building everything from small widgets to complex, large-scale web applications.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+4.  ### What is JSX?
+
+    _JSX_ stands for _JavaScript XML_ and it is an XML-like syntax extension to ECMAScript. Basically it just provides the syntactic sugar for the `React.createElement(type, props, ...children)` function, giving us expressiveness of JavaScript along with HTML like template syntax.
+
+    In the example below, the text inside `<h1>` tag is returned as JavaScript function to the render function.
+
+    ```jsx harmony
+    export default function App() {
+      return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
+    }
     ```
 
-    Whereas a **component** can be declared in several different ways. It can be a class with a `render()` method or it can be defined as a function. In either case, it takes props as an input, and returns a JSX tree as the output:
+    If you don't use JSX syntax then the respective JavaScript code should be written as below,
 
     ```javascript
-    const Button = ({ onLogin }) =>
-      <div id={'login-btn'} onClick={onLogin}>Login</div>
+    import { createElement } from "react";
+
+    export default function App() {
+      return createElement(
+        "h1",
+        { className: "greeting" },
+        "Hello, this is a JSX Code!"
+      );
+    }
     ```
 
-    Then JSX gets transpiled to a `React.createElement()` function tree:
+     <details><summary><b>See Class</b></summary>
+     <p>
 
-    ```javascript
-    const Button = ({ onLogin }) => React.createElement(
-      'div',
-      { id: 'login-btn', onClick: onLogin },
-      'Login'
-    )
+    ```jsx harmony
+    class App extends React.Component {
+      render() {
+        return <h1 className="greeting">{"Hello, this is a JSX Code!"}</h1>;
+      }
+    }
     ```
 
+     </p>
+     </details>
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-5. ### How to create components in React?
+    **Note:** JSX is stricter than HTML
 
-    There are two possible ways to create a component.
+    **[⬆ Back to Top](#table-of-contents)**
 
-    1. **Function Components:** This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the first parameter and return React elements:
+5.  ### What is the difference between an Element and a Component?
 
-        ```jsx harmony
-        function Greeting({ message }) {
-          return <h1>{`Hello, ${message}`}</h1>
+      **Element:**
+      - A React **Element** is a plain JavaScript object that describes what you want to see on the UI. It represents a DOM node or a component at a specific point in time. 
+      - Elements are immutable: once created, you cannot change their properties. Instead, you create new elements to reflect updates.
+      - Elements can be nested within other elements through their `props`.
+      - Creating an element is a fast, lightweight operation—it does **not** create any actual DOM nodes or render anything to the screen directly.
 
-        }
+        **Example (without JSX):**
+        ```js
+        const element = React.createElement("button", { id: "login-btn" }, "Login");
         ```
 
-    2. **Class Components:** You can also use ES6 class to define a component. The above function component can be written as:
+        **Equivalent JSX syntax:**
+        ```jsx
+        <button id="login-btn">Login</button>
+        ```
 
-        ```jsx harmony
-        class Greeting extends React.Component {
-          render() {
-            return <h1>{`Hello, ${this.props.message}`}</h1>
+        **The object returned by `React.createElement`:**
+        ```js
+        {
+          type: 'button',
+          props: {
+            id: 'login-btn',
+            children: 'Login'
           }
         }
         ```
+        Elements are then passed to the React DOM renderer (e.g., `ReactDOM.render()`), which translates them to actual DOM nodes.
 
+        ---
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-6. ### When to use a Class Component over a Function Component?
+      **Component:**
+      - A **Component** is a function or class that returns an element (or a tree of elements) to describe part of the UI. Components can accept inputs (called **props**) and manage their own state (in case of class or function components with hooks).
+      - Components allow you to split the UI into independent, reusable pieces, each isolated and composable.
+      - You can define a component using a function or a class:
 
-    If the component needs *state or lifecycle methods* then use class component otherwise use function component.
-    *However, from React 16.8 with the addition of Hooks, you could use state , lifecycle  methods and other features that were only available in class component right in your function component.*
-    *So, it is always recommended to use Function components, unless you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries *
+        **Example (Function Component with JSX):**
+        ```jsx
+        const Button = ({ handleLogin }) => (
+          <button id="login-btn" onClick={handleLogin}>
+            Login
+          </button>
+        );
+        ```
 
+        When JSX is compiled, it's transformed into a tree of `React.createElement` calls:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-7. ### What are Pure Components?
+        ```js
+        const Button = ({ handleLogin }) =>
+          React.createElement(
+            "button",
+            { id: "login-btn", onClick: handleLogin },
+            "Login"
+          );
+        ```
 
-    *`React.PureComponent`* is exactly the same as *`React.Component`* except that it handles the `shouldComponentUpdate()` method for you. When props or state changes, *PureComponent* will do a shallow comparison on both props and state. *Component* on the other hand won't compare current props and state to next out of the box. Thus, the component will re-render by default whenever `shouldComponentUpdate` is called.
+        ---
 
+      **In summary:**
+      - **Elements** are the smallest building blocks in React—objects that describe what you want to see.
+      - **Components** are functions or classes that return elements and encapsulate logic, structure, and behavior for parts of your UI.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-8. ### What is state in React?
+       > Think of **elements** as the instructions for creating UI, and **components** as reusable blueprints that combine logic and structure to generate those instructions.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+6.  ### How to create components in React?
+
+    Components are the building blocks of creating User Interfaces(UI) in React. There are two possible ways to create a component.
+
+    1. **Function Components:** This is the simplest way to create a component. Those are pure JavaScript functions that accept props object as the one and only one parameter and return React elements to render the output:
+
+       ```jsx harmony
+       function Greeting({ message }) {
+         return <h1>{`Hello, ${message}`}</h1>;
+       }
+       ```
+
+    2. **Class Components:** You can also use ES6 class to define a component. The above function component can be written as a class component:
+
+       ```jsx harmony
+       class Greeting extends React.Component {
+         render() {
+           return <h1>{`Hello, ${this.props.message}`}</h1>;
+         }
+       }
+       ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+7.  ### When to use a Class Component over a Function Component?
+
+    After the addition of Hooks(i.e. React 16.8 onwards) it is always recommended to use Function components over Class components in React. Because you could use state, lifecycle methods and other features that were only available in class component present in function component too.
+
+    But even there are two reasons to use Class components over Function components.
+
+    1. If you need a React functionality whose Function component equivalent is not present yet, like Error Boundaries.
+    2. In older versions, If the component needs _state or lifecycle methods_ then you need to use class component.
+
+    So the summary to this question is as follows:
+
+    **Use Function Components:**
+
+    - If you don't need state or lifecycle methods, and your component is purely presentational.
+    - For simplicity, readability, and modern code practices, especially with the use of React Hooks for state and side effects.
+
+    **Use Class Components:**
+
+    - If you need to manage state or use lifecycle methods.
+    - In scenarios where backward compatibility or integration with older code is necessary.
+
+    **Note:** You can also use reusable [react error boundary](https://github.com/bvaughn/react-error-boundary) third-party component without writing any class. i.e, No need to use class components for Error boundaries.
+
+    The usage of Error boundaries from the above library is quite straight forward.
+
+    > **_Note when using react-error-boundary:_** ErrorBoundary is a client component. You can only pass props to it that are serializable or use it in files that have a `"use client";` directive.
+
+    ```jsx
+    "use client";
+
+    import { ErrorBoundary } from "react-error-boundary";
+
+    <ErrorBoundary fallback={<div>Something went wrong</div>}>
+      <ExampleApplication />
+    </ErrorBoundary>;
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+8.  ### What are Pure Components?
+
+    Pure components are the components which render the same output for the same state and props. In function components, you can achieve these pure components through memoized `React.memo()` API wrapping around the component. This API prevents unnecessary re-renders by comparing the previous props and new props using shallow comparison. So it will be helpful for performance optimizations.
+
+    But at the same time, it won't compare the previous state with the current state because function component itself prevents the unnecessary rendering by default when you set the same state again.
+
+    The syntactic representation of memoized components looks like below,
+
+    ```jsx
+    const MemoizedComponent = memo(SomeComponent, arePropsEqual?);
+    ```
+
+    Below is the example of how child component(i.e., EmployeeProfile) prevents re-renders for the same props passed by parent component(i.e.,EmployeeRegForm).
+
+    ```jsx
+    import { memo, useState } from "react";
+
+    const EmployeeProfile = memo(function EmployeeProfile({ name, email }) {
+      return (
+        <>
+          <p>Name:{name}</p>
+          <p>Email: {email}</p>
+        </>
+      );
+    });
+    export default function EmployeeRegForm() {
+      const [name, setName] = useState("");
+      const [email, setEmail] = useState("");
+      return (
+        <>
+          <label>
+            Name:{" "}
+            <input value={name} onChange={(e) => setName(e.target.value)} />
+          </label>
+          <label>
+            Email:{" "}
+            <input value={email} onChange={(e) => setEmail(e.target.value)} />
+          </label>
+          <hr />
+          <EmployeeProfile name={name} />
+        </>
+      );
+    }
+    ```
+
+    In the above code, the email prop has not been passed to child component. So there won't be any re-renders for email prop change.
+
+    In class components, the components extending _`React.PureComponent`_ instead of _`React.Component`_ become the pure components. When props or state changes, _PureComponent_ will do a shallow comparison on both props and state by invoking `shouldComponentUpdate()` lifecycle method.
+
+    **Note:** `React.memo()` is a higher-order component.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+9.  ### What is state in React?
+
+    _State_ of a component is an object that holds some information that may change over the lifetime of the component. The important point is whenever the state object changes, the component re-renders. It is always recommended to make our state as simple as possible and minimize the number of stateful components.
+
+    ![state](images/state.jpg)
+
+    Let's take an example of **User** component with `message` state. Here, **useState** hook has been used to add state to the User component and it returns an array with current state and function to update it.
+
+    ```jsx harmony
+    import { useState } from "react";
+
+    function User() {
+      const [message, setMessage] = useState("Welcome to React world");
 
-    *State* of a component is an object that holds some information that may change over the lifetime of the component. We should always try to make our state as simple as possible and minimize the number of stateful components.
+      return (
+        <div>
+          <h1>{message}</h1>
+        </div>
+      );
+    }
+    ```
 
-    Let's create a user component with message state,
+    Whenever React calls your component or access `useState` hook, it gives you a snapshot of the state for that particular render.
 
+    <details><summary><b>See Class</b></summary>
+    <p>
 
     ```jsx harmony
+    import React from "react";
     class User extends React.Component {
       constructor(props) {
-        super(props)
+        super(props);
 
         this.state = {
-          message: 'Welcome to React world'
-        }
+          message: "Welcome to React world",
+        };
       }
 
       render() {
@@ -523,21 +800,21 @@ You can download the PDF and Epub version of this repository from the latest run
           <div>
             <h1>{this.state.message}</h1>
           </div>
-        )
+        );
       }
     }
     ```
 
-    ![state](images/state.jpg)
+    </p>
+    </details>
 
     State is similar to props, but it is private and fully controlled by the component ,i.e., it is not accessible to any other component till the owner component decides to pass it.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-9. ### What are props in React?
+10. ### What are props in React?
 
-    *Props* are inputs to components. They are single values or objects containing a set of values that are passed to components on creation using a naming convention similar to HTML-tag attributes. They are data passed down from a parent component to a child component.
+    _Props_ are inputs to components. They are single values or objects containing a set of values that are passed to components on creation similar to HTML-tag attributes. Here, the data is passed down from a parent component to a child component.
 
     The primary purpose of props in React is to provide following component functionality:
 
@@ -548,519 +825,434 @@ You can download the PDF and Epub version of this repository from the latest run
     For example, let us create an element with `reactProp` property:
 
     ```jsx harmony
-    <Element reactProp={'1'} />
-    ```
-
-    This `reactProp` (or whatever you came up with) name then becomes a property attached to React's native props object which originally already exists on all components created using React library.
-
+    <Element reactProp={"1"} />
     ```
-    props.reactProp
-    ```
-    
 
-    **Example: Props in Class Based Component**
-
-    ```jsx
-    import React from 'react'
-    import ReactDOM from 'react-dom'
-
-    class ChildComponent extends React.Component {
-        render() {
-            return (
-                <div>
-                    <p>{this.props.name}</p>
-                    <p>{this.props.age}</p>
-                </div>
-            )
-        }
-    }
+    This `reactProp` (or whatever you came up with) attribute name then becomes a property attached to React's native props object which originally already exists on all components created using React library.
 
-    class ParentComponent extends React.Component {
-        render() {
-            return (
-                <div>
-                    <ChildComponent name='John' age='30' />
-                    <ChildComponent name='Mary' age='25' />
-                </div>
-            )
-        }
-    }
+    ```jsx harmony
+    props.reactProp;
     ```
 
-    **Example: Props in Functional Component**
+    For example, the usage of props in function component looks like below:
 
     ```jsx
-    import React from 'react'
-    import ReactDOM from 'react-dom'
+    import React from "react";
+    import ReactDOM from "react-dom";
 
     const ChildComponent = (props) => {
-        return (
-            <div>
-                <p>{props.name}</p>
-                <p>{props.age}</p>
-            </div>
-        )
-    }
+      return (
+        <div>
+          <p>{props.name}</p>
+          <p>{props.age}</p>
+          <p>{props.gender}</p>
+        </div>
+      );
+    };
 
     const ParentComponent = () => {
-        return (
-            <div>
-                <ChildComponent name='John' age='30' />
-                <ChildComponent name='Mary' age='25' />
-            </div>
-        )
-    }
+      return (
+        <div>
+          <ChildComponent name="John" age="30" gender="male" />
+          <ChildComponent name="Mary" age="25" geneder="female" />
+        </div>
+      );
+    };
     ```
 
+The properties from props object can be accessed directly using destructing feature from ES6 (ECMAScript 2015). It is also possible to fallback to default value when the prop value is not specified. The above child component can be simplified like below.
 
+```jsx harmony
+const ChildComponent = ({ name, age, gender = "male" }) => {
+  return (
+    <div>
+      <p>{name}</p>
+      <p>{age}</p>
+      <p>{gender}</p>
+    </div>
+  );
+};
+```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-10. ### What is the difference between state and props?
+**Note:** The default value won't be used if you pass `null` or `0` value. i.e, default value is only used if the prop value is missed or `undefined` value has been passed.
 
-    Both *props* and *state* are plain JavaScript objects. While both of them hold information that influences the output of render, they are different in their functionality with respect to component. Props get passed to the component similar to function parameters whereas state is managed within the component similar to variables declared within a function.
+  <details><summary><b>See Class</b></summary>
+     The Props accessed in Class Based Component as below
 
+```jsx
+import React from "react";
+import ReactDOM from "react-dom";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-11. ### Why should we not update the state directly?
+class ChildComponent extends React.Component {
+  render() {
+    return (
+      <div>
+        <p>{this.props.name}</p>
+        <p>{this.props.age}</p>
+        <p>{this.props.gender}</p>
+      </div>
+    );
+  }
+}
 
-    If you try to update the state directly then it won't re-render the component.
+class ParentComponent extends React.Component {
+  render() {
+    return (
+      <div>
+        <ChildComponent name="John" age="30" gender="male" />
+        <ChildComponent name="Mary" age="25" gender="female" />
+      </div>
+    );
+  }
+}
+```
 
-    ```javascript
-    //Wrong
-    this.state.message = 'Hello world'
-    ```
+  </details>
 
-    Instead use `setState()` method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.
+**[⬆ Back to Top](#table-of-contents)**
 
-    ```javascript
-    //Correct
-    this.setState({ message: 'Hello World' })
-    ```
+11. ### What is the difference between state and props?
 
-    **Note:** You can directly assign to the state object either in *constructor* or using latest javascript's class field declaration syntax.
+    In React, both **state** and **props** are plain JavaScript objects, but they serve different purposes and have distinct behaviors:
 
+    ### State
+    - **Definition:**  
+      State is a data structure that is managed within a component. It represents information that can change over the lifetime of the component.
+    - **Mutability:**  
+      State is mutable, meaning it can be changed using the setter function (`setState` in class components or the updater function from `useState` in functional components).
+    - **Scope:**  
+      State is local to the component where it is defined. Only that component can modify its own state.
+    - **Usage:**  
+      State is typically used for data that needs to change in response to user actions, network responses, or other dynamic events.
+    - **Re-rendering:**  
+      Updating the state triggers a re-render of the component and its descendants.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-12. ### What is the purpose of callback function as an argument of `setState()`?
+    ### Props
+    - **Definition:**  
+      Props (short for “properties”) are inputs to a component, provided by its parent component.
+    - **Mutability:**  
+      Props are read-only. A component cannot modify its own props; they are immutable from the component’s perspective.
+    - **Scope:**  
+      Props are used to pass data and event handlers down the component tree, enabling parent components to configure or communicate with their children.
+    - **Usage:**  
+      Props are commonly used to make components reusable and configurable. They allow the same component to be rendered with different data or behavior.
+    - **Analogy:**  
+      Think of props as arguments to a function, whereas state is like variables declared inside the function.
 
-    The callback function is invoked when setState finished and the component gets rendered. Since `setState()` is **asynchronous** the callback function is used for any post action.
+    ### Summary Table
 
-    **Note:** It is recommended to use lifecycle method rather than this callback function.
+    | Feature   | State                               | Props                             |
+    |-----------|-------------------------------------|-----------------------------------|
+    | Managed by| The component itself                | Parent component                  |
+    | Mutable   | Yes                                 | No (read-only)                    |
+    | Scope     | Local to the component              | Passed from parent to child       |
+    | Usage     | Manage dynamic data and UI changes  | Configure and customize component |
+    | Update    | Using setState/useState             | Cannot be updated by the component|
 
-    ```javascript
-    setState({ name: 'John' }, () => console.log('The name has updated and component re-rendered'))
-    ```
+    ---
 
+    **[⬆ Back to Top](#table-of-contents)**
+
+12. ### What is the difference between HTML and React event handling?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-13. ### What is the difference between HTML and React event handling?
     Below are some of the main differences between HTML and React event handling,
 
-    1. In HTML, the event name usually represents in *lowercase* as a convention:
+    1. In HTML, the event name usually represents in _lowercase_ as a convention:
 
-        ```html
-        <button onclick='activateLasers()'>
-        ```
+       ```html
+       <button onclick="activateLasers()"></button>
+       ```
 
-       Whereas in React it follows *camelCase* convention:
+       Whereas in React it follows _camelCase_ convention:
 
-        ```jsx harmony
-        <button onClick={activateLasers}>
-        ```
+       ```jsx harmony
+       <button onClick={activateLasers}>
+       ```
 
     2. In HTML, you can return `false` to prevent default behavior:
 
-        ```html
-        <a href='#' onclick='console.log("The link was clicked."); return false;' />
-        ```
+       ```html
+       <a
+         href="#"
+         onclick='console.log("The link was clicked."); return false;'
+       />
+       ```
 
        Whereas in React you must call `preventDefault()` explicitly:
 
-        ```javascript
-        function handleClick(event) {
-          event.preventDefault()
-          console.log('The link was clicked.')
-        }
-        ```
+       ```javascript
+       function handleClick(event) {
+         event.preventDefault();
+         console.log("The link was clicked.");
+       }
+       ```
 
     3. In HTML, you need to invoke the function by appending `()`
        Whereas in react you should not append `()` with the function name. (refer "activateLasers" function in the first point for example)
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-14. ### How to bind methods or event handlers in JSX callbacks?
+13. ### What are synthetic events in React?
 
-    There are 3 possible ways to achieve this:
+    `SyntheticEvent` is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including `stopPropagation()` and `preventDefault()`, except the events work identically across all browsers. The native events can be accessed directly from synthetic events using `nativeEvent` attribute.
 
-    1.	**Binding in Constructor:** In JavaScript classes, the methods are not bound by default. The same thing applies for React event handlers defined as class methods. Normally we bind them in constructor.
+    Let's take an example of `BookStore` title search component with the ability to get all native event properties
 
-        ```javascript
-        class Foo extends Component {
-          constructor(props) {
-            super(props);
-            this.handleClick = this.handleClick.bind(this);
-          }
-          handleClick() {
-            console.log('Click happened');
-          }
-          render() {
-            return <button onClick={this.handleClick}>Click Me</button>;
-          }
-        }
+    ```js
+    function BookStore() {
+      function handleTitleChange(e) {
+        console.log("The new title is:", e.target.value);
+        console.log('Synthetic event:', e); // React SyntheticEvent
+        console.log('Native event:', e.nativeEvent); // Browser native event
+        e.stopPropagation();
+        e.preventDefault();
+      }
 
-        ```
+      return <input name="title" onChange={handleTitleChange} />;
+    }
+    ```
+    
+    List of common synthetic events are:
 
-    2. **Public class fields syntax:** If you don't like to use bind approach then *public class fields syntax* can be used to correctly bind callbacks.
+    *   `onClick`
+    *   `onChange`
+    *   `onSubmit`
+    *   `onKeyDown`, `onKeyUp`
+    *   `onFocus`, `onBlur`
+    *   `onMouseEnter`, `onMouseLeave`
+    *   `onTouchStart`, `onTouchEnd`
 
-        ```jsx harmony
-        handleClick = () => {
-          console.log('this is:', this)
-        }
-        ```
+    **[⬆ Back to Top](#table-of-contents)**
 
-        ```jsx harmony
-        <button onClick={this.handleClick}>
-          {'Click me'}
-        </button>
-        ```
+14. ### What are inline conditional expressions?
 
-    3. **Arrow functions in callbacks:** You can use *arrow functions* directly in the callbacks.
-
-        ```jsx harmony
-        handleClick() {
-            console.log('Click happened');
-        }
-        render() {
-            return <button onClick={() => this.handleClick()}>Click Me</button>;
-        }
-        ```
-
-    **Note:** If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with `.bind()` or *public class fields syntax* approach considering performance.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-15. ### How to pass a parameter to an event handler or callback?
-
-    You can use an *arrow function* to wrap around an *event handler* and pass parameters:
-
-    ```jsx harmony
-    <button onClick={() => this.handleClick(id)} />
-    ```
-
-    This is an equivalent to calling `.bind`:
-
-    ```jsx harmony
-    <button onClick={this.handleClick.bind(this, id)} />
-    ```
-    Apart from these two approaches, you can also pass arguments to a function which is defined as arrow function
-    ```jsx harmony
-    <button onClick={this.handleClick(id)} />
-    handleClick = (id) => () => {
-        console.log("Hello, your ticket number is", id)
-    };
-    ```
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-16. ### What are synthetic events in React?
-
-    `SyntheticEvent` is a cross-browser wrapper around the browser's native event. Its API is same as the browser's native event, including `stopPropagation()` and `preventDefault()`, except the events work identically across all browsers.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-17. ### What are inline conditional expressions?
-
-    You can use either *if statements* or *ternary expressions* which are available from JS to conditionally render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator `&&`.
+    You can use either _if statements_ or _ternary expressions_ which are available in JS(and JSX in React) to conditionally execute or render expressions. Apart from these approaches, you can also embed any expressions in JSX by wrapping them in curly braces and then followed by JS logical operator `&&`. It is helpful to render elements conditionally within a single line and commonly used for concise logic, especially in JSX rendering.
 
     ```jsx harmony
-    <h1>Hello!</h1>
+    <h1>Hello!</h1>;
     {
-        messages.length > 0 && !isLogin?
-          <h2>
-              You have {messages.length} unread messages.
-          </h2>
-          :
-          <h2>
-              You don't have unread messages.
-          </h2>
+      messages.length > 0 && !isLogin ? (
+        <h2>You have {messages.length} unread messages.</h2>
+      ) : (
+        <h2>You don't have unread messages.</h2>
+      );
     }
     ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-18. ### What is "key" prop and what is the benefit of using it in arrays of elements?
+15. ### What is "key" prop and what is the benefit of using it in arrays of elements?
 
-    A `key` is a special string attribute you **should** include when creating arrays of elements. *Key* prop helps React identify which items have changed, are added, or are removed.
+    A `key` is a special attribute you **should** include when mapping over arrays to render data. _Key_ prop helps React identify which items have changed, are added, or are removed.
 
-    Most often we use ID from our data as *key*:
+    Keys should be unique among its siblings. Most often we use ID from our data as _key_:
 
     ```jsx harmony
-    const todoItems = todos.map((todo) =>
-      <li key={todo.id}>
-        {todo.text}
-      </li>
-    )
+    const todoItems = todos.map((todo) => <li key={todo.id}>{todo.text}</li>);
     ```
 
-    When you don't have stable IDs for rendered items, you may use the item *index* as a *key* as a last resort:
+    When you don't have stable IDs for rendered items, you may use the item _index_ as a _key_ as a last resort:
 
     ```jsx harmony
-    const todoItems = todos.map((todo, index) =>
-      <li key={index}>
-        {todo.text}
-      </li>
-    )
+    const todoItems = todos.map((todo, index) => (
+      <li key={index}>{todo.text}</li>
+    ));
     ```
+    **Benefits of key:**
+      *   Enables React to **efficiently update and re-render** components.
+      *   Prevents unnecessary re-renders by **reusing** components when possible.
+      *   Helps **maintain internal state** of list items correctly.
 
     **Note:**
 
-    1. Using *indexes* for *keys* is **not recommended** if the order of items may change. This can negatively impact performance and may cause issues with component state.
-    2. If you extract list item as separate component then apply *keys* on list component instead of `li` tag.
+    1. Using _indexes_ for _keys_ is **not recommended** if the order of items may change. This can negatively impact performance and may cause issues with component state.
+    2. If you extract list item as separate component then apply _keys_ on list component instead of `li` tag.
     3. There will be a warning message in the console if the `key` prop is not present on list items.
+    4. The key attribute accepts either string or number and internally convert it as string type.
+    5. Don't generate the key on the fly something like `key={Math.random()}`. Because the keys will never match up between re-renders and DOM created everytime.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-19. ### What is the use of refs?
-
-    The *ref* is used to return a reference to the element. They *should be avoided* in most cases, however, they can be useful when you need a direct access to the DOM element or an instance of a component.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-20. ### How to create refs?
-
-    There are two approaches
-    1. This is a recently added approach. *Refs* are created using `React.createRef()` method and attached to React elements via the `ref` attribute. In order to use *refs* throughout the component, just assign the *ref* to the instance property within constructor.
-
-        ```jsx harmony
-        class MyComponent extends React.Component {
-          constructor(props) {
-            super(props)
-            this.myRef = React.createRef()
-          }
-          render() {
-            return <div ref={this.myRef} />
-          }
-        }
-        ```
-    2. You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element is accessed as follows,
-        ```jsx harmony
-        class SearchBar extends Component {
-           constructor(props) {
-              super(props);
-              this.txtSearch = null;
-              this.state = { term: '' };
-              this.setInputSearchRef = e => {
-                 this.txtSearch = e;
-              }
-           }
-           onInputChange(event) {
-              this.setState({ term: this.txtSearch.value });
-           }
-           render() {
-              return (
-                 <input
-                    value={this.state.term}
-                    onChange={this.onInputChange.bind(this)}
-                    ref={this.setInputSearchRef} />
-              );
-           }
-        }
-        ```
-
-    You can also use *refs* in function components using **closures**.
-    **Note**: You can also use inline ref callbacks even though it is not a recommended approach.
+16. ### What is Virtual DOM?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-21. ### What are forward refs?
+    The _Virtual DOM_ (VDOM) is a lightweight, in-memory representation of _Real DOM_ used by libraries like React to optimize UI rendering. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called _reconciliation_.
 
-    *Ref forwarding* is a feature that lets some components take a *ref* they receive, and pass it further down to a child.
+    **[⬆ Back to Top](#table-of-contents)**
 
-    ```jsx harmony
-    const ButtonElement = React.forwardRef((props, ref) => (
-      <button ref={ref} className="CustomButton">
-        {props.children}
-      </button>
-    ));
+17. ### How Virtual DOM works?
 
-    // Create ref to the DOM button:
-    const ref = React.createRef();
-    <ButtonElement ref={ref}>{'Forward Ref'}</ButtonElement>
-    ```
+    The _Virtual DOM_ works in five simple steps.
 
+    **1. Initial Render**  
+        When a UI component renders for the first time, it returns JSX. React uses this structure to create a Virtual DOM tree, which is a lightweight copy of the actual DOM. This Virtual DOM is then used to build and render the Real DOM in the browser.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-22. ### Which is preferred option with in callback refs and findDOMNode()?
+    **2. State or Props Change**  
+        When the component's state or props change, React creates a new Virtual DOM reflecting the updated UI. However, it doesn't immediately update the Real DOM; instead, it works in memory to prepare for an efficient update.
+               
+      ![vdom](images/vdom1.png)
 
-    It is preferred to use *callback refs* over `findDOMNode()` API. Because `findDOMNode()` prevents certain improvements in React in the future.
+    **3. Diffing Algorithm**  
+        React then compares the new Virtual DOM with the previous one using a process called diffing. It determines what has changed between the two versions and identifies the minimal set of updates needed.
+       
+       ![vdom2](images/vdom2.png)  
 
-    The **legacy** approach of using `findDOMNode`:
+    **4. Reconciliation**  
+        Based on the diffing results, React decides which parts of the Real DOM should be updated. It avoids re-rendering the entire DOM and instead updates only the elements that actually changed.
+        
+       ![vdom3](images/vdom3.png)
 
-    ```javascript
-    class MyComponent extends Component {
-      componentDidMount() {
-        findDOMNode(this).scrollIntoView()
-      }
+    **5. Efficient DOM Updates**  
+        This entire process—working with the Virtual DOM, diffing, and selective updating—makes the UI rendering much faster and more efficient than manipulating the Real DOM directly.
 
-      render() {
-        return <div />
-      }
-    }
-    ```
+    **[⬆ Back to Top](#table-of-contents)**
 
-    The recommended approach is:
+18. ### What is the difference between Shadow DOM and Virtual DOM?
 
-    ```javascript
-    class MyComponent extends Component {
-      constructor(props){
-        super(props);
-        this.node = createRef();
-      }
-      componentDidMount() {
-        this.node.current.scrollIntoView();
-      }
+    The _Shadow DOM_ is a browser technology designed primarily for scoping variables and CSS in _web components_. The _Virtual DOM_ is a concept implemented by libraries in JavaScript on top of browser APIs.
 
-      render() {
-        return <div ref={this.node} />
-      }
-    }
-    ```
+    The key differences in a table format shown below:
 
+    | Feature | Shadow DOM | Virtual DOM |
+    | --- | --- | --- |
+    | Purpose | Encapsulation for Web Components | Efficient UI rendering |
+    | Managed by | Browser | JS frameworks (e.g., React) |
+    | DOM Type | Part of real DOM (scoped) | In-memory representation |
+    | Encapsulation | Yes | No |
+    | Use Case | Web Components, scoped styling | UI diffing and minimal DOM updates |
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-23. ### Why are String Refs legacy?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    If you worked with React before, you might be familiar with an older API where the `ref` attribute is a string, like `ref={'textInput'}`, and the DOM node is accessed as `this.refs.textInput`. We advise against it because *string refs have below issues*, and are considered legacy. String refs were **removed in React v16**.
+19. ### What is React Fiber?
 
-    1. They *force React to keep track of currently executing component*. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
-    2. They are *not composable* — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.
-    3. They *don't work with static analysis* like Flow. Flow can't guess the magic that framework does to make the string ref appear on `this.refs`, as well as its type (which could be different). Callback refs are friendlier to static analysis.
-    4. It doesn't work as most people would expect with the "render callback" pattern (e.g. <DataGrid renderRow={this.renderRow} />)
-       ```jsx harmony
-       class MyComponent extends Component {
-         renderRow = (index) => {
-           // This won't work. Ref will get attached to DataTable rather than MyComponent:
-           return <input ref={'input-' + index} />;
+    **React Fiber** is the **new reconciliation engine** in React, introduced in React 16. It’s a complete rewrite of React’s core algorithm(old stack-based algorithm) for rendering and updating the UI. Fiber enhances React’s ability to handle **asynchronous rendering**, **prioritized updates**(assign priority to different types of updates), and **interruption**(ability to pause, abort, or reuse work) of rendering work, enabling smoother and more responsive user interfaces.
 
-           // This would work though! Callback refs are awesome.
-           return <input ref={input => this['input-' + index] = input} />;
-         }
+    **[⬆ Back to Top](#table-of-contents)**
 
-         render() {
-           return <DataTable data={this.props.data} renderRow={this.renderRow} />
-         }
-       }
-       ```
+20. ### What is the main goal of React Fiber?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-24. ### What is Virtual DOM?
+    The goal of _React Fiber_ is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is **incremental rendering**: the ability to split rendering work into chunks and spread it out over multiple frames.
 
-    The *Virtual DOM* (VDOM) is an in-memory representation of *Real DOM*. The representation of a UI is kept in memory and synced with the "real" DOM. It's a step that happens between the render function being called and the displaying of elements on the screen. This entire process is called *reconciliation*.
+    Its main goals are:
 
+    *   **Incremental Rendering** – Breaks work into chunks for smoother updates.
+    *   **Interruptible Rendering** – Pauses and resumes rendering to keep the UI responsive.
+    *   **Prioritization** – Handles high-priority updates (e.g. animations) before low-priority ones.
+    *   **Concurrency Support** – Enables working on multiple UI versions simultaneously.
+    *   **Better Error Handling** – Supports component-level error boundaries.
+    *   **Suspense Support** – Allows waiting for async data before rendering.
+    *   **Improved DevTools** – Enables better debugging and performance tracking.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-25. ### How Virtual DOM works?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    The *Virtual DOM* works in three simple steps.
+21. ### What are controlled components?
 
-    1. Whenever any underlying data changes, the entire UI is re-rendered in Virtual DOM representation.
+    A **controlled component** is a React component that **fully manages the form element's state**(e.g, elements like `<input>`, `<textarea>`, or `<select>`))  using React's internal state mechanism. i.e, The component does not manage its own internal state — instead, React acts as the single source of truth for form data.
 
-        ![vdom](images/vdom1.png)
+    The controlled components will be implemented using the below steps,
 
-    2. Then the difference between the previous DOM representation and the new one is calculated.
+    1. Initialize the state using `useState` hooks in function components or inside constructor for class components.
+    2. Set the value of the form element to the respective state variable.
+    3. Create an event handler(`onChange`) to handle the user input changes through `useState`'s updater function or `setState` from class component.
+    4. Attach the above event handler to form element's change or click events
 
-        ![vdom2](images/vdom2.png)
+    **Note:** React re-renders the component every time the input value changes.
 
-    3. Once the calculations are done, the real DOM will be updated with only the things that have actually changed.
+   For example, the name input field updates the username using `handleChange` event handler as below,
 
-        ![vdom3](images/vdom3.png)
+   ```javascript
+   import React, { useState } from "react";
 
+   function UserProfile() {
+     const [username, setUsername] = useState("");
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-26. ### What is the difference between Shadow DOM and Virtual DOM?
+     const handleChange = (e) => {
+       setUsername(e.target.value);
+     };
 
-    The *Shadow DOM* is a browser technology designed primarily for scoping variables and CSS in *web components*. The *Virtual DOM* is a concept implemented by libraries in JavaScript on top of browser APIs.
+     return (
+       <form>
+         <label>
+           Name:
+           <input type="text" value={username} onChange={handleChange} />
+         </label>
+       </form>
+     );
+   }
+   ```
+   In these components, DOM does not hold the actual data instead React does.
+   
+   **Benefits:**
 
+   *   Easy to implement **validation**, **conditional formatting**, or **live feedback**.
+   *   Full control over form data.
+   *   Easier to test and debug because the data is centralized in the component’s state.
 
    **[⬆ Back to Top](#table-of-contents)**
-    
-27. ### What is React Fiber?
 
-    Fiber is the new *reconciliation* engine or reimplementation of core algorithm in React v16. The goal of React Fiber is to increase its suitability for areas like animation, layout, gestures, ability to pause, abort, or reuse work and assign priority to different types of updates; and new concurrency primitives.
+22. ### What are uncontrolled components?
+    The **Uncontrolled components** are form elements (like `<input>`, `<textarea>`, or `<select>`) that **manage their own state internally** via the **DOM**, rather than through React state.
+    You can query the DOM using a `ref` to find its current value when you need it. This is a bit more like traditional HTML.
 
+    The uncontrolled components will be implemented using the below steps,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-28. ### What is the main goal of React Fiber?
-
-    The goal of *React Fiber* is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is **incremental rendering**: the ability to split rendering work into chunks and spread it out over multiple frames.
-    
-    *from documentation*
+    1. Create a ref using `useRef` react hook in function component or `React.createRef()` in class based component.
+    2. Attach this `ref` to the form element.
+    3. The form element value can be accessed directly through `ref` in event handlers or `componentDidMount` for class components
 
-    Its main goals are:
-      1. Ability to split interruptible work in chunks.
-      2. Ability to prioritize, rebase and reuse work in progress.
-      3. Ability to yield back and forth between parents and children to support layout in React.
-      4. Ability to return multiple elements from render().
-      5. Better support for error boundaries.
+    In the below UserProfile component, the `username` input is accessed using ref.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-29. ### What are controlled components?
+    ```jsx harmony
+    import React, { useRef } from "react";
 
-    A component that controls the input elements within the forms on subsequent user input is called **Controlled Component**, i.e, every state mutation will have an associated handler function.
+    function UserProfile() {
+      const usernameRef = useRef(null);
 
-    For example, to write all the names in uppercase letters, we use handleChange as below,
+      const handleSubmit = (event) => {
+        event.preventDefault();
+        console.log("The submitted username is: " + usernameRef.current.value);
+      };
 
-    ```javascript
-    handleChange(event) {
-      this.setState({value: event.target.value.toUpperCase()})
+      return (
+        <form onSubmit={handleSubmit}>
+          <label>
+            Username:
+            <input type="text" ref={usernameRef} />
+          </label>
+          <button type="submit">Submit</button>
+        </form>
+      );
     }
     ```
+    **Note:** Here, DOM is in charge of the value. React only accesses the value when needed (via `ref`).
 
+    **Benefits:**
+     *   **Less boilerplate** — no need for `useState` and `onChange`.
+     *   Useful for **quick form setups** or when integrating with **non-React code**.
+     *   Slightly better **performance** in very large forms (fewer re-renders).
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-30. ### What are uncontrolled components?
-
-    The **Uncontrolled Components** are the ones that store their own state internally, and you query the DOM using a ref to find its current value when you need it. This is a bit more like traditional HTML.
+    In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form data is handled by the DOM itself.
 
-    In the below UserProfile component, the `name` input is accessed using ref.
+    <details><summary><b>See Class</b></summary>
+    <p>
 
     ```jsx harmony
     class UserProfile extends React.Component {
       constructor(props) {
-        super(props)
-        this.handleSubmit = this.handleSubmit.bind(this)
-        this.input = React.createRef()
+        super(props);
+        this.handleSubmit = this.handleSubmit.bind(this);
+        this.input = React.createRef();
       }
 
       handleSubmit(event) {
-        alert('A name was submitted: ' + this.input.current.value)
-        event.preventDefault()
+        alert("A name was submitted: " + this.input.current.value);
+        event.preventDefault();
       }
 
       render() {
         return (
           <form onSubmit={this.handleSubmit}>
             <label>
-              {'Name:'}
+              {"Name:"}
               <input type="text" ref={this.input} />
             </label>
             <input type="submit" value="Submit" />
@@ -1070,164 +1262,164 @@ You can download the PDF and Epub version of this repository from the latest run
     }
     ```
 
-    In most cases, it's recommend to use controlled components to implement forms. In a controlled component, form data is handled by a React component. The alternative is uncontrolled components, where form         data is handled by the DOM itself.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-31. ### What is the difference between createElement and cloneElement?
-
-    JSX elements will be transpiled to `React.createElement()` functions to create React elements which are going to be used for the object representation of UI. Whereas `cloneElement` is used to clone an element and pass it new props.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-32. ### What is Lifting State Up in React?
-
-    When several components need to share the same changing data then it is recommended to *lift the shared state up* to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-33. ### What are the different phases of component lifecycle?
-
-    The component lifecycle has three distinct lifecycle phases:
-
-    1. **Mounting:** The component is ready to mount in the browser DOM. This phase covers initialization from `constructor()`, `getDerivedStateFromProps()`, `render()`, and `componentDidMount()` lifecycle methods.
-
-    2. **Updating:** In this phase, the component gets updated in two ways, sending the new props and updating the state either from `setState()` or `forceUpdate()`. This phase covers `getDerivedStateFromProps()`, `shouldComponentUpdate()`, `render()`, `getSnapshotBeforeUpdate()` and `componentDidUpdate()` lifecycle methods.
-
-    3. **Unmounting:** In this last phase, the component is not needed and gets unmounted from the browser DOM. This phase includes `componentWillUnmount()` lifecycle method.
-
-    It's worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows
-
-    1. **Render** The component will render without any side effects. This applies to Pure components and in this phase, React can pause, abort, or restart the render.
+    </p>
+    </details>
 
-    2. **Pre-commit** Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the `getSnapshotBeforeUpdate()`.
+**[⬆ Back to Top](#table-of-contents)**
 
-    3. **Commit** React works with the DOM and executes the final lifecycles respectively `componentDidMount()` for mounting, `componentDidUpdate()` for updating, and `componentWillUnmount()` for unmounting.
+23. ### What is the difference between createElement and cloneElement?
+    Both `React.createElement` and `React.cloneElement` are used to work with React elements, but they serve different purposes.
 
-    React 16.3+ Phases (or an [interactive version](http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/))
+    #### **createElement:** 
+    Creates a new React element from scratch. JSX elements will be transpiled to `React.createElement()` functions to create React elements which are going to be used for the object representation of UI.
+    **Syntax:**
+    ```jsx
+    React.createElement(type, props, ...children)
+    ```
+    **Example:**
+    ```jsx
+    React.createElement('button', { className: 'btn' }, 'Click Me')
+    ```
+    #### **cloneElement:**
+     The `cloneElement` method is used to clone an existing React element and optionally adds or overrides props.
 
-    ![phases 16.4+](images/phases16.4.png)
+    **Syntax:**
+    ```jsx
+    React.cloneElement(element, newProps, ...children)
+    ```
+    **Example:**
+    ```jsx
+    const button = <button className="btn">Click Me</button>;
+    const cloned = React.cloneElement(button, { className: 'btn-primary' });
+    // Result: <button className="btn-primary">Click Me</button>
+    ```
+    **[⬆ Back to Top](#table-of-contents)**
 
-    Before React 16.3
+24. ### What is Lifting State Up in React?
 
-    ![phases 16.2](images/phases.png)
+    When several components need to share the same changing data then it is recommended to _lift the shared state up_ to their closest common ancestor. That means if two child components share the same data from its parent, then move the state to parent instead of maintaining local state in both of the child components.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
+25. ### What are Higher-Order Components?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-34. ### What are the lifecycle methods of React?
+    A _higher-order component_ (_HOC_) is a function that takes a component and returns a new enhanced component with additional props, behavior, or data. It’s a design pattern based on React’s compositional nature, allowing you to reuse logic across multiple components without modifying their internals.
 
-    Before React 16.3
+    We consider HOCs **pure components** because they don’t mutate or copy behavior from the original component—they simply **wrap it**, enhance it, and pass through the necessary props. The wrapped component remains decoupled and reusable.
 
-    - **componentWillMount:** Executed before rendering and is used for App level configuration in your root component.
-    - **componentDidMount:** Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
-    - **componentWillReceiveProps:** Executed when particular prop updates to trigger state transitions.
-    - **shouldComponentUpdate:** Determines if the component will be updated or not. By default it returns `true`. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
-    - **componentWillUpdate:** Executed before re-rendering the component when there are props & state changes confirmed by `shouldComponentUpdate()` which returns true.
-    - **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes.
-    - **componentWillUnmount:** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
+    ```javascript
+    const EnhancedComponent = higherOrderComponent(WrappedComponent);
+    ```
+    Let's take an example of a `withAuth` higher-order component (HOC) in React. This HOC will check if a user is authenticated and either render the wrapped component if authenticated or redirect (or show a message) if not.
 
-    React 16.3+
+    **withAuth HOC Example:**
+    ```jsx
+    import React from 'react';
+    import { Navigate } from 'react-router-dom'; // For redirection (assuming React Router v6)
 
-    - **getDerivedStateFromProps:** Invoked right before calling `render()` and is invoked on *every* render. This exists for rare use cases where you need a derived state. Worth reading [if you need derived state](https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html).
-    - **componentDidMount:** Executed after first rendering and where all AJAX requests, DOM or state updates, and set up event listeners should occur.
-    - **shouldComponentUpdate:** Determines if the component will be updated or not. By default, it returns `true`. If you are sure that the component doesn't need to render after the state or props are updated, you can return a false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives a new prop.
-    - **getSnapshotBeforeUpdate:** Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into `componentDidUpdate()`. This is useful to capture information from the DOM i.e. scroll position.
-    - **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes. This will not fire if `shouldComponentUpdate()` returns `false`.
-    - **componentWillUnmount** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
+    const isAuthenticated = () => {
+      // e.g., check for a valid token in localStorage or context
+      return !!localStorage.getItem('authToken');
+    };
 
+    function withAuth(WrappedComponent) {
+      return function AuthenticatedComponent(props) {
+        if (!isAuthenticated()) {
+          // User is NOT authenticated, redirect to login page
+          return <Navigate to="/login" replace />;
+        }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-35. ### What are Higher-Order Components?
+        // User is authenticated, render the wrapped component
+        return <WrappedComponent {...props} />;
+      };
+    }
 
-    A *higher-order component* (*HOC*) is a function that takes a component and returns a new component. Basically, it's a pattern that is derived from React's compositional nature.
+    export default withAuth;
+    ```
+    **Usage**
+    ```jsx
+    import React from 'react';
+    import withAuth from './withAuth';
 
-    We call them **pure components** because they can accept any dynamically provided child component but they won't modify or copy any behavior from their input components.
+    function Dashboard() {
+      return <h1>Welcome to the Dashboard!</h1>;
+    }
 
-    ```javascript
-    const EnhancedComponent = higherOrderComponent(WrappedComponent)
+    // Wrap Dashboard with withAuth HOC
+    export default withAuth(Dashboard);
     ```
 
     HOC can be used for many use cases:
 
-    1. Code reuse, logic and bootstrap abstraction.
-    2. Render hijacking.
-    3. State abstraction and manipulation.
-    4. Props manipulation.
+    1. Code reuse, logic and bootstrap abstraction (e.g., fetching data, permissions, theming).
+    2. Render hijacking (e.g., conditional rendering or layout changes).
+    3. State abstraction and manipulation(e.g., handling form logic).
+    4. Props manipulation(e.g., injecting additional props or filtering them).
+    
+    Some of the real-world examples of HOCs in react eco-system:
+    1. connect() from react-redux
+    2. withRouter() from React Router v5
+    3. withTranslation() from react-i18next
+    4. withApollo() from Apollo client
+    5. withFormik from Formik library
+    6. withTheme from styled components
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-36. ### How to create props proxy for HOC component?
+26. ### What is children prop?
+    The `children` prop is a special prop in React used to pass elements between the opening and closing tags of a component. It is commonly used in layout and wrapper componnents. 
 
-    You can add/edit props passed to the component using *props proxy* pattern like this:
+    A simple usage of children prop looks as below,
 
     ```jsx harmony
-    function HOC(WrappedComponent) {
-      return class Test extends Component {
-        render() {
-          const newProps = {
-            title: 'New Header',
-            footer: false,
-            showFeatureX: false,
-            showFeatureY: true
-          }
-
-          return <WrappedComponent {...this.props} {...newProps} />
-        }
-      }
+    function MyDiv({ children }){
+        return (
+          <div>
+            {children}
+          </div>;
+        );
     }
-    ```
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-37. ### What is context?
-
-    *Context* provides a way to pass data through the component tree without having to pass props down manually at every level.
-
-    For example, authenticated users, locale preferences, UI themes need to be accessed in the application by many components.
 
-    ```javascript
-    const {Provider, Consumer} = React.createContext(defaultValue)
+    export default function Greeting() {
+      return (
+        <MyDiv>
+          <span>{"Hello"}</span>
+          <span>{"World"}</span>
+        </MyDiv>
+      );
+    }
     ```
+    Here, everything inside `<MyDiv>...</MyDiv>` is passed as children to the custom div component.
 
+    The children can be text, JSX elements, fragments, arrays and functions(for advance use case like render props).
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-38. ### What is children prop?
-
-    *Children* is a prop (`this.props.children`) that allows you to pass components as data to other components, just like any other prop you use. Component tree put between component's opening and closing tag will be passed to that component as `children` prop.
-
-    There are several methods available in the React API to work with this prop. These include `React.Children.map`, `React.Children.forEach`, `React.Children.count`, `React.Children.only`, `React.Children.toArray`.
-
-    A simple usage of children prop looks as below,
+    <details><summary><b>See Class</b></summary>
+    <p>
 
     ```jsx harmony
     const MyDiv = React.createClass({
-      render: function() {
-        return <div>{this.props.children}</div>
-      }
-    })
+      render: function () {
+        return <div>{this.props.children}</div>;
+      },
+    });
 
     ReactDOM.render(
       <MyDiv>
-        <span>{'Hello'}</span>
-        <span>{'World'}</span>
+        <span>{"Hello"}</span>
+        <span>{"World"}</span>
       </MyDiv>,
       node
-    )
+    );
     ```
 
+    </p>
+    </details>
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-39. ### How to write comments in React?
+    **Note:** There are several methods available in the legacy React API to work with this prop. These include `React.Children.map`, `React.Children.forEach`, `React.Children.count`, `React.Children.only`, `React.Children.toArray`.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+27. ### How to write comments in React?
 
     The comments in React/JSX are similar to JavaScript Multiline comments but are wrapped in curly braces.
 
@@ -1250,231 +1442,236 @@ You can download the PDF and Epub version of this repository from the latest run
     </div>
     ```
 
+    You can use `//` and `/* */` in JS logic, hooks, and functions.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-40. ### What is the purpose of using super constructor with props argument?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    A child class constructor cannot make use of `this` reference until the `super()` method has been called. The same applies to ES6 sub-classes as well. The main reason for passing props parameter to `super()` call is to access `this.props` in your child constructors.
+28. ### What is reconciliation?
 
-    **Passing props:**
+    `Reconciliation` is the process through which React updates the Browser DOM and makes React work faster. React use a `diffing algorithm` so that component updates are predictable and faster. React would first calculate the difference between the `real DOM` and the copy of DOM `(Virtual DOM)` when there's an update of components.
+    React stores a copy of Browser DOM which is called `Virtual DOM`. When we make changes or add data, React creates a new Virtual DOM and compares it with the previous one. This comparison is done by `Diffing Algorithm`.
+    Now React compares the Virtual DOM with Real DOM. It finds out the changed nodes and updates only the changed nodes in Real DOM leaving the rest nodes as it is. This process is called _Reconciliation_.
 
-    ```javascript
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super(props)
+    **[⬆ Back to Top](#table-of-contents)**
 
-        console.log(this.props) // prints { name: 'John', age: 42 }
-      }
-    }
-    ```
+29. ### Does the lazy function support named exports?
 
-    **Not passing props:**
+    No, currently `React.lazy` function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
+    Let's take a component file which exports multiple named components,
 
     ```javascript
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super()
+    // MoreComponents.js
+    export const SomeComponent = /* ... */;
+    export const UnusedComponent = /* ... */;
+    ```
+
+    and reexport `MoreComponents.js` components in an intermediate file `IntermediateComponent.js`
 
-        console.log(this.props) // prints undefined
+    ```javascript
+    // IntermediateComponent.js
+    export { SomeComponent as default } from "./MoreComponents.js";
+    ```
 
-        // but props parameter is still available
-        console.log(props) // prints { name: 'John', age: 42 }
-      }
+    Now you can import the module using lazy function as below,
 
-      render() {
-        // no difference outside constructor
-        console.log(this.props) // prints { name: 'John', age: 42 }
-      }
-    }
+    ```javascript
+    import React, { lazy } from "react";
+    const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
     ```
 
-    The above code snippets reveals that `this.props` is different only within the constructor. It would be the same outside the constructor.
+    **[⬆ Back to Top](#table-of-contents)**
 
+30. ### Why React uses `className` over `class` attribute?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-41. ### What is reconciliation?
-
-    When a component's props or state change, React decides whether an actual DOM update is necessary by comparing the newly returned element with the previously rendered one. When they are not equal, React will update the DOM. This process is called *reconciliation*.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-42. ### How to set state with a dynamic key name?
-
-    If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with *computed property names*.
-
-    ```javascript
-    handleInputChange(event) {
-      this.setState({ [event.target.id]: event.target.value })
-    }
-    ```
+    React uses **className** instead of **class** because of a JavaScript naming conflict with the class keyword.
 
+    1. `class` is a reserved keyword in JavaScript
+        In JavaScript, class is used to define ES6 classes:
+      
+        ```js
+        class Person {
+          constructor(name) {
+            this.name = name;
+          }
+        }
+        ```
+        If you try to use class as a variable or property name, it will throw a syntax error. Since JSX is just JavaScript with XML-like syntax, using class directly in JSX would break the parser.
 
-   **[⬆ Back to Top](#table-of-contents)**
+    2. JSX Is JavaScript
     
-43. ### What would be the common mistake of function being called every time the component renders?
-
-    You need to make sure that function is not being called while passing the function as a parameter.
-
-    ```jsx harmony
-    render() {
-      // Wrong: handleClick is called instead of passed as a reference!
-      return <button onClick={this.handleClick()}>{'Click Me'}</button>
-    }
-    ```
-
-    Instead, pass the function itself without parenthesis:
+        When you write JSX like this:
+        ```jsx
+        <div class="btn">Click</div>
+        ```
+        It will be compiled to:
+        ```jsx
+        React.createElement('div', { class: 'btn' }, 'Click');
+        ```
+        But `class` is invalid in this object literal context (since it clashes with the JS keyword), hence React instead uses className.
+        ```jsx
+        <div className="btn">Click</div>
+        ```
+        which compiles to:
+        ```jsx
+        React.createElement('div', { className: 'btn' }, 'Click');
+        ```
+        React then translates `className` to` class` in the final HTML DOM.
 
-    ```jsx harmony
-    render() {
-      // Correct: handleClick is passed as a reference!
-      return <button onClick={this.handleClick}>{'Click Me'}</button>
-    }
-    ```
+    3. Aligns with DOM APIs
+        In vanilla JavaScript, you interact with element classes using:
+        ```js
+        element.className = 'my-class';
+        ```
+        React follows this convention, staying consistent with the DOM API's property name rather than HTML’s attribute.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-44. ### Is lazy function supports named exports?
-    No, currently `React.lazy` function supports default exports only. If you would like to import modules which are named exports, you can create an intermediate module that reexports it as the default. It also ensures that tree shaking keeps working and don’t pull unused components.
-    Let's take a component file which exports multiple named components,
-    ```javascript
-    // MoreComponents.js
-    export const SomeComponent = /* ... */;
-    export const UnusedComponent = /* ... */;
-    ```
-    and reexport `MoreComponents.js` components in an intermediate file `IntermediateComponent.js`
-    ```javascript
-    // IntermediateComponent.js
-    export { SomeComponent as default } from "./MoreComponents.js";
-    ```
-    Now you can import the module using lazy function as below,
-    ```javascript
-    import React, { lazy } from 'react';
-    const SomeComponent = lazy(() => import("./IntermediateComponent.js"));
-    ```
+31. ### What are fragments?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-45. ### Why React uses `className` over `class` attribute?
+    It's a common pattern or practice in React for a component to return multiple elements. _Fragments_ let you group a list of children without adding extra nodes to the DOM.
+    You need to use either `<Fragment>` or a shorter syntax having empty tag (`<></>`).
 
-    `class` is a keyword in JavaScript, and JSX is an extension of JavaScript. That's the principal reason why React uses `className` instead of `class`. Pass a string as the `className` prop.
+    Below is the example of how to use fragment inside _Story_ component.
 
     ```jsx harmony
-    render() {
-      return <span className={'menu navigation-menu'}>{'Menu'}</span>
+    function Story({ title, description, date }) {
+      return (
+        <Fragment>
+          <h2>{title}</h2>
+          <p>{description}</p>
+          <p>{date}</p>
+        </Fragment>
+      );
     }
     ```
 
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-46. ### What are fragments?
-
-    It's a common pattern in React which is used for a component to return multiple elements. *Fragments* let you group a list of children without adding extra nodes to the DOM.
+    It is also possible to render list of fragments inside a loop with the mandatory **key** attribute supplied.
 
     ```jsx harmony
-    render() {
-      return (
-        <React.Fragment>
-          <ChildA />
-          <ChildB />
-          <ChildC />
-        </React.Fragment>
-      )
+    function StoryBook() {
+      return stories.map((story) => (
+        <Fragment key={story.id}>
+          <h2>{story.title}</h2>
+          <p>{story.description}</p>
+          <p>{story.date}</p>
+        </Fragment>
+      ));
     }
     ```
 
-    There is also a *shorter syntax*, but it's not supported in many tools:
+    Usually, you don't need to use `<Fragment>` until there is a need of _key_ attribute. The usage of shorter syntax looks like below.
 
     ```jsx harmony
-    render() {
+    function Story({ title, description, date }) {
       return (
         <>
-          <ChildA />
-          <ChildB />
-          <ChildC />
+          <h2>{title}</h2>
+          <p>{description}</p>
+          <p>{date}</p>
         </>
-      )
+      );
     }
     ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-47. ### Why fragments are better than container divs?
-    Below are the list of reasons,
+32. ### Why fragments are better than container divs?
+
+    Below are the list of reasons to prefer fragments over container DOM elements,
 
     1. Fragments are a bit faster and use less memory by not creating an extra DOM node. This only has a real benefit on very large and deep trees.
-    2. Some CSS mechanisms like *Flexbox* and *CSS Grid* have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
+    2. Some CSS mechanisms like _Flexbox_ and _CSS Grid_ have a special parent-child relationships, and adding divs in the middle makes it hard to keep the desired layout.
     3. The DOM Inspector is less cluttered.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-48. ### What are portals in React?
+33. ### What are portals in React?
 
-    *Portal* is a recommended way to render children into a DOM node that exists outside the DOM hierarchy of the parent component.
+    A Portal is a React feature that enables rendering children into a DOM node that exists outside the parent component's DOM hierarchy, while still preserving the React component hierarchy. Portals help avoid CSS stacking issues—for example, elements with position: fixed may not behave as expected inside a parent with transform. Portals solve this by rendering content (like modals or tooltips) outside such constrained DOM contexts.
 
     ```javascript
-    ReactDOM.createPortal(child, container)
+    ReactDOM.createPortal(child, container);
     ```
+    *   `child`: Any valid React node (e.g., JSX, string, fragment).
+    *   `container`: A real DOM node (e.g., `document.getElementById('modal-root')`).
 
-    The first argument is any render-able React child, such as an element, string, or fragment. The second argument is a DOM element.
+    Even though the content renders elsewhere in the DOM, it still behaves like a normal child in React. It has access to context, state, and event handling.
+
+    **Example:- Modal:**
+    ```jsx
+    function Modal({ children }) {
+      return ReactDOM.createPortal(
+        <div className="modal">{children}</div>,
+        document.body)
+      );
+    }
+    ```
+    The above code will render the modal content into the body element in the HTML, not inside the component's usual location.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-49. ### What are stateless components?
+34. ### What are stateless components?
 
     If the behaviour of a component is independent of its state then it can be a stateless component. You can use either a function or a class for creating stateless components. But unless you need to use a lifecycle hook in your components, you should go for function components. There are a lot of benefits if you decide to use function components here; they are easy to write, understand, and test, a little faster, and you can avoid the `this` keyword altogether.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-50. ### What are stateful components?
+35. ### What are stateful components?
+
+    If the behaviour of a component is dependent on the _state_ of the component then it can be termed as stateful component. These _stateful components_ are either function components with hooks or _class components_.
 
-    If the behaviour of a component is dependent on the *state* of the component then it can be termed as stateful component. These *stateful components* are always *class components* and have a state that gets initialized in the `constructor`.
+    Let's take an example of function stateful component which update the state based on click event,
 
     ```javascript
+    import React, {useState} from 'react';
+
+    const App = (props) => {
+    const [count, setCount] = useState(0);
+    handleIncrement() {
+      setCount(count+1);
+    }
+
+    return (
+      <>
+        <button onClick={handleIncrement}>Increment</button>
+        <span>Counter: {count}</span>
+      </>
+      )
+    }
+    ```
+
+    <details><summary><b>See Class</b></summary>
+    <p>
+    The equivalent class stateful component with a state that gets initialized in the `constructor`.
+
+    ```jsx harmony
     class App extends Component {
       constructor(props) {
-        super(props)
-        this.state = { count: 0 }
+        super(props);
+        this.state = { count: 0 };
+      }
+
+      handleIncrement() {
+        setState({ count: this.state.count + 1 });
       }
 
       render() {
-        // ...
+        <>
+          <button onClick={() => this.handleIncrement}>Increment</button>
+          <span>Count: {count}</span>
+        </>;
       }
     }
     ```
-    **React 16.8 Update:**
-
-     Hooks let you use state and other React features without writing classes.
-
-     *The Equivalent Functional Component*
-
-       ```javascript
-        import React, {useState} from 'react';
-
-        const App = (props) => {
-          const [count, setCount] = useState(0);
-
-          return (
-            // JSX
-          )
-        }
-       ```
 
+    </p>
+    </details>
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-51. ### How to apply validation on props in React?
+36. ### How to apply validation on props in React?
 
-    When the application is running in *development mode*, React will automatically check all props that we set on components to make sure they have *correct type*. If the type is incorrect, React will generate warning messages in the console. It's disabled in *production mode* due to performance impact. The mandatory props are defined with `isRequired`.
+    When the application is running in _development mode_, React will automatically check all props that we set on components to make sure they have _correct type_. If the type is incorrect, React will generate warning messages in the console. It's disabled in _production mode_ due to performance impact. The mandatory props are defined with `isRequired`.
 
     The set of predefined prop types:
 
@@ -1492,14 +1689,14 @@ You can download the PDF and Epub version of this repository from the latest run
     We can define `propTypes` for `User` component as below:
 
     ```jsx harmony
-    import React from 'react'
-    import PropTypes from 'prop-types'
+    import React from "react";
+    import PropTypes from "prop-types";
 
     class User extends React.Component {
       static propTypes = {
         name: PropTypes.string.isRequired,
-        age: PropTypes.number.isRequired
-      }
+        age: PropTypes.number.isRequired,
+      };
 
       render() {
         return (
@@ -1507,49 +1704,50 @@ You can download the PDF and Epub version of this repository from the latest run
             <h1>{`Welcome, ${this.props.name}`}</h1>
             <h2>{`Age, ${this.props.age}`}</h2>
           </>
-        )
+        );
       }
     }
     ```
 
-    **Note:** In React v15.5 *PropTypes* were moved from `React.PropTypes` to `prop-types` library.
+    **Note:** In React v15.5 _PropTypes_ were moved from `React.PropTypes` to `prop-types` library.
 
-    *The Equivalent Functional Component*
+    _The Equivalent Functional Component_
 
     ```jsx harmony
-    import React from 'react'
-    import PropTypes from 'prop-types'
-   
-    function User({name, age}) {
+    import React from "react";
+    import PropTypes from "prop-types";
+
+    function User({ name, age }) {
       return (
         <>
           <h1>{`Welcome, ${name}`}</h1>
           <h2>{`Age, ${age}`}</h2>
         </>
-      )
+      );
     }
 
     User.propTypes = {
-        name: PropTypes.string.isRequired,
-        age: PropTypes.number.isRequired
-      }
+      name: PropTypes.string.isRequired,
+      age: PropTypes.number.isRequired,
+    };
     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-52. ### What are the advantages of React?
+    **[⬆ Back to Top](#table-of-contents)**
+
+37. ### What are the advantages of React?
+
     Below are the list of main advantages of React,
 
-    1. Increases the application's performance with *Virtual DOM*.
+    1. Increases the application's performance with _Virtual DOM_.
     2. JSX makes code easy to read and write.
-    3. It renders both on client and server side (*SSR*).
+    3. It renders both on client and server side (_SSR_).
     4. Easy to integrate with frameworks (Angular, Backbone) since it is only a view library.
     5. Easy to write unit and integration tests with tools such as Jest.
 
+    **[⬆ Back to Top](#table-of-contents)**
+
+38. ### What are the limitations of React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-53. ### What are the limitations of React?
     Apart from the advantages, there are few limitations of React too,
 
     1. React is just a view library, not a full framework.
@@ -1558,70 +1756,17 @@ You can download the PDF and Epub version of this repository from the latest run
     4. The code complexity increases with inline templating and JSX.
     5. Too many smaller components leading to over engineering or boilerplate.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-54. ### What are error boundaries in React v16?
-
-    *Error boundaries* are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
-
-    A class component becomes an error boundary if it defines a new lifecycle method called `componentDidCatch(error, info)` or `static getDerivedStateFromError() `:
-
-    ```jsx harmony
-    class ErrorBoundary extends React.Component {
-      constructor(props) {
-        super(props)
-        this.state = { hasError: false }
-      }
-
-      componentDidCatch(error, info) {
-        // You can also log the error to an error reporting service
-        logErrorToMyService(error, info)
-      }
-
-      static getDerivedStateFromError(error) {
-         // Update state so the next render will show the fallback UI.
-         return { hasError: true };
-       }
-
-      render() {
-        if (this.state.hasError) {
-          // You can render any custom fallback UI
-          return <h1>{'Something went wrong.'}</h1>
-        }
-        return this.props.children
-      }
-    }
-    ```
-
-    After that use it as a regular component:
-
-    ```jsx harmony
-    <ErrorBoundary>
-      <MyWidget />
-    </ErrorBoundary>
-    ```
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-55. ### How are error boundaries handled in React v15?
-
-    React v15 provided very basic support for *error boundaries* using `unstable_handleError` method. It has been renamed to `componentDidCatch` in React v16.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-56. ### What are the recommended ways for static type checking?
+39. ### What are the recommended ways for static type checking?
 
-    Normally we use *PropTypes library* (`React.PropTypes` moved to a `prop-types` package since React v15.5) for *type checking* in the React applications. For large code bases, it is recommended to use *static type checkers* such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
+    Normally we use _PropTypes library_ (`React.PropTypes` moved to a `prop-types` package since React v15.5) for _type checking_ in the React applications. For large code bases, it is recommended to use _static type checkers_ such as Flow or TypeScript, that perform type checking at compile time and provide auto-completion features.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-57. ### What is the use of `react-dom` package?
+40. ### What is the use of `react-dom` package?
 
-    The `react-dom` package provides *DOM-specific methods* that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
+    The `react-dom` package provides _DOM-specific methods_ that can be used at the top level of your app. Most of the components are not required to use this module. Some of the methods of this package are:
 
     1. `render()`
     2. `hydrate()`
@@ -1629,25 +1774,11 @@ You can download the PDF and Epub version of this repository from the latest run
     4. `findDOMNode()`
     5. `createPortal()`
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-58. ### What is the purpose of render method of `react-dom`?
-
-    This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.
-
-    ```
-    ReactDOM.render(element, container, [callback])
-    ```
-
-    If the optional callback is provided, it will be executed after the component is rendered or updated.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-59. ### What is ReactDOMServer?
+41. ### What is ReactDOMServer?
 
-    The `ReactDOMServer` object enables you to render components to static markup (typically used on node server). This object is mainly used for *server-side rendering* (SSR). The following methods can be used in both the server and browser environments:
+    The `ReactDOMServer` object enables you to render components to static markup (typically used on node server). This object is mainly used for _server-side rendering_ (SSR). The following methods can be used in both the server and browser environments:
 
     1. `renderToString()`
     2. `renderToStaticMarkup()`
@@ -1656,22 +1787,23 @@ You can download the PDF and Epub version of this repository from the latest run
 
     ```javascript
     // using Express
-    import { renderToString } from 'react-dom/server'
-    import MyPage from './MyPage'
+    import { renderToString } from "react-dom/server";
+    import MyPage from "./MyPage";
 
-    app.get('/', (req, res) => {
-      res.write('<!DOCTYPE html><html><head><title>My Page</title></head><body>')
-      res.write('<div id="content">')
-      res.write(renderToString(<MyPage/>))
-      res.write('</div></body></html>')
-      res.end()
-    })
+    app.get("/", (req, res) => {
+      res.write(
+        "<!DOCTYPE html><html><head><title>My Page</title></head><body>"
+      );
+      res.write('<div id="content">');
+      res.write(renderToString(<MyPage />));
+      res.write("</div></body></html>");
+      res.end();
+    });
     ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-60. ### How to use innerHTML in React?
+42. ### How to use innerHTML in React?
 
     The `dangerouslySetInnerHTML` attribute is React's replacement for using `innerHTML` in the browser DOM. Just like `innerHTML`, it is risky to use this attribute considering cross-site scripting (XSS) attacks. You just need to pass a `__html` object as key and HTML text as value.
 
@@ -1679,5200 +1811,8040 @@ You can download the PDF and Epub version of this repository from the latest run
 
     ```jsx harmony
     function createMarkup() {
-      return { __html: 'First &middot; Second' }
+      return { __html: "First &middot; Second" };
     }
 
     function MyComponent() {
-      return <div dangerouslySetInnerHTML={createMarkup()} />
+      return <div dangerouslySetInnerHTML={createMarkup()} />;
     }
     ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-61. ### How to use styles in React?
+43. ### How to use styles in React?
 
     The `style` attribute accepts a JavaScript object with camelCased properties rather than a CSS string. This is consistent with the DOM style JavaScript property, is more efficient, and prevents XSS security holes.
 
     ```jsx harmony
     const divStyle = {
-      color: 'blue',
-      backgroundImage: 'url(' + imgUrl + ')'
+      color: "blue",
+      backgroundImage: "url(" + imgUrl + ")",
     };
 
     function HelloWorldComponent() {
-      return <div style={divStyle}>Hello World!</div>
+      return <div style={divStyle}>Hello World!</div>;
     }
     ```
 
     Style keys are camelCased in order to be consistent with accessing the properties on DOM nodes in JavaScript (e.g. `node.style.backgroundImage`).
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-62. ### How events are different in React?
+44. ### How events are different in React?
 
     Handling events in React elements has some syntactic differences:
 
     1. React event handlers are named using camelCase, rather than lowercase.
     2. With JSX you pass a function as the event handler, rather than a string.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-63. ### What will happen if you use `setState()` in constructor?
-
-    When you use `setState()`, then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: *Can only update a mounted or mounting component.* So we need to use `this.state` to initialize variables inside constructor.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-64. ### What is the impact of indexes as keys?
+45. ### What is the impact of indexes as keys?
 
     Keys should be stable, predictable, and unique so that React can keep track of elements.
 
-    In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do.
+    In the below code snippet each element's key will be based on ordering, rather than tied to the data that is being represented. This limits the optimizations that React can do and creates confusing bugs in the application.
 
     ```jsx harmony
-    {todos.map((todo, index) =>
-      <Todo
-        {...todo}
-        key={index}
-      />
-    )}
+    {
+      todos.map((todo, index) => <Todo {...todo} key={index} />);
+    }
     ```
 
-    If you use element data for unique key, assuming todo.id is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
+    If you use element data for unique key, assuming `todo.id` is unique to this list and stable, React would be able to reorder elements without needing to reevaluate them as much.
 
     ```jsx harmony
-    {todos.map((todo) =>
-      <Todo {...todo}
-        key={todo.id} />
-    )}
+    {
+      todos.map((todo) => <Todo {...todo} key={todo.id} />);
+    }
     ```
 
+    **Note:** If you don't specify `key` prop at all, React will use index as a key's value while iterating over an array of data.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-65. ### Is it good to use `setState()` in `componentWillMount()` method?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    Yes, it is safe to use `setState()` inside `componentWillMount()` method. But at the same it is recommended to avoid async initialization in `componentWillMount()` lifecycle method. `componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in `componentDidMount()` instead of `componentWillMount()`.
+46. ### How do you conditionally render components?
+
+    In some cases you want to render different components depending on some state. JSX does not render `false` or `undefined`, so you can use conditional _short-circuiting_ to render a given part of your component only if a certain condition is true.
 
     ```jsx harmony
-    componentDidMount() {
-      axios.get(`api/todos`)
-        .then((result) => {
-          this.setState({
-            messages: [...result.data]
-          })
-        })
-    }
+    const MyComponent = ({ name, address }) => (
+      <div>
+        <h2>{name}</h2>
+        {address && <p>{address}</p>}
+      </div>
+    );
     ```
 
+    If you need an `if-else` condition then use _ternary operator_.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-66. ### What will happen if you use props in initial state?
+    ```jsx harmony
+    const MyComponent = ({ name, address }) => (
+      <div>
+        <h2>{name}</h2>
+        {address ? <p>{address}</p> : <p>{"Address is not available"}</p>}
+      </div>
+    );
+    ```
 
-    If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.
+    **[⬆ Back to Top](#table-of-contents)**
 
-    The below component won't display the updated input value:
+47. ### Why we need to be careful when spreading props on DOM elements?
 
-    ```jsx harmony
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super(props)
+    When we _spread props_ we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with `...rest` operator, so it will add only required props.
 
-        this.state = {
-          records: [],
-          inputValue: this.props.inputValue
-        };
-      }
+    For example,
 
-      render() {
-        return <div>{this.state.inputValue}</div>
-      }
-    }
+    ```jsx harmony
+    const ComponentA = () => (
+      <ComponentB isDisplay={true} className={"componentStyle"} />
+    );
+
+    const ComponentB = ({ isDisplay, ...domProps }) => (
+      <div {...domProps}>{"ComponentB"}</div>
+    );
     ```
 
-    Using props inside render method will update the value:
+    **[⬆ Back to Top](#table-of-contents)**
+
+48. ### How do you memoize a component?
+
+    There are memoize libraries available which can be used on function components.
+
+    For example `moize` library can memoize the component in another component.
 
     ```jsx harmony
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super(props)
+    import moize from "moize";
+    import Component from "./components/Component"; // this module exports a non-memoized component
 
-        this.state = {
-          record: []
-        }
-      }
+    const MemoizedFoo = moize.react(Component);
 
-      render() {
-        return <div>{this.props.inputValue}</div>
-      }
-    }
-    ```
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-67. ### How do you conditionally render components?
-
-    In some cases you want to render different components depending on some state. JSX does not render `false` or `undefined`, so you can use conditional *short-circuiting* to render a given part of your component only if a certain condition is true.
-
-    ```jsx harmony
-    const MyComponent = ({ name, address }) => (
+    const Consumer = () => {
       <div>
-        <h2>{name}</h2>
-        {address &&
-          <p>{address}</p>
-        }
-      </div>
-    )
+        {"I will memoize the following entry:"}
+        <MemoizedFoo />
+      </div>;
+    };
     ```
 
-    If you need an `if-else` condition then use *ternary operator*.
+    **Update:** Since React v16.6.0, we have a `React.memo`. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.
 
-    ```jsx harmony
-    const MyComponent = ({ name, address }) => (
-      <div>
-        <h2>{name}</h2>
-        {address
-          ? <p>{address}</p>
-          : <p>{'Address is not available'}</p>
-        }
-      </div>
-    )
+    ```js
+    const MemoComponent = React.memo(function MemoComponent(props) {
+      /* render using props */
+    });
+    OR;
+    export default React.memo(MyFunctionComponent);
     ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-68. ### Why we need to be careful when spreading props on DOM elements?
-
-    When we *spread props* we run into the risk of adding unknown HTML attributes, which is a bad practice. Instead we can use prop destructuring with `...rest` operator, so it will add only required props.
+49. ### How you implement Server Side Rendering or SSR?
 
-    For example,
+    React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
 
     ```jsx harmony
-    const ComponentA = () =>
-      <ComponentB isDisplay={true} className={'componentStyle'} />
+    import ReactDOMServer from "react-dom/server";
+    import App from "./App";
 
-    const ComponentB = ({ isDisplay, ...domProps }) =>
-      <div {...domProps}>{'ComponentB'}</div>
+    ReactDOMServer.renderToString(<App />);
     ```
 
+    This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-69. ### How you use decorators in React?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    You can *decorate* your *class* components, which is the same as passing the component into a function. **Decorators** are flexible and readable way of modifying component functionality.
+50. ### How to enable production mode in React?
 
-    ```jsx harmony
-    @setTitle('Profile')
-    class Profile extends React.Component {
-        //....
-    }
+    You should use Webpack's `DefinePlugin` method to set `NODE_ENV` to `production`, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
 
-    /*
-      title is a string that will be set as a document title
-      WrappedComponent is what our decorator will receive when
-      put directly above a component class as seen in the example above
-    */
-    const setTitle = (title) => (WrappedComponent) => {
-      return class extends React.Component {
-        componentDidMount() {
-          document.title = title
-        }
+    **[⬆ Back to Top](#table-of-contents)**
 
-        render() {
-          return <WrappedComponent {...this.props} />
-        }
-      }
-    }
-    ```
+51. ### Do Hooks replace render props and higher order components?
 
-    **Note:** Decorators are a feature that didn't make it into ES7, but are currently a *stage 2 proposal*.
+    Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-70. ### How do you memoize a component?
+52. ### What is a switching component?
 
-    There are memoize libraries available which can be used on function components.
+    A _switching component_ is a component that renders one of many components. We need to use object to map prop values to components.
 
-    For example `moize` library can memoize the component in another component.
+    For example, a switching component to display different pages based on `page` prop:
 
     ```jsx harmony
-    import moize from 'moize'
-    import Component from './components/Component' // this module exports a non-memoized component
+    import HomePage from "./HomePage";
+    import AboutPage from "./AboutPage";
+    import ServicesPage from "./ServicesPage";
+    import ContactPage from "./ContactPage";
 
-    const MemoizedFoo = moize.react(Component)
+    const PAGES = {
+      home: HomePage,
+      about: AboutPage,
+      services: ServicesPage,
+      contact: ContactPage,
+    };
 
-    const Consumer = () => {
-      <div>
-        {'I will memoize the following entry:'}
-        <MemoizedFoo/>
-      </div>
-    }
-    ```
+    const Page = (props) => {
+      const Handler = PAGES[props.page] || ContactPage;
 
-    **Update:** Since React v16.6.0, we have a `React.memo`. It provides a higher order component which memoizes component unless the props change. To use it, simply wrap the component using React.memo before you use it.
+      return <Handler {...props} />;
+    };
 
-    ```js
-      const MemoComponent = React.memo(function MemoComponent(props) {
-        /* render using props */
-      });
-      OR
-      export default React.memo(MyFunctionComponent);
+    // The keys of the PAGES object can be used in the prop types to catch dev-time errors.
+    Page.propTypes = {
+      page: PropTypes.oneOf(Object.keys(PAGES)).isRequired,
+    };
     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-71. ### How you implement Server Side Rendering or SSR?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    React is already equipped to handle rendering on Node servers. A special version of the DOM renderer is available, which follows the same pattern as on the client side.
+53. ### What are React Mixins?
 
-    ```jsx harmony
-    import ReactDOMServer from 'react-dom/server'
-    import App from './App'
+    _Mixins_ are a way to totally separate components to have a common functionality. Mixins **should not be used** and can be replaced with _higher-order components_ or _decorators_.
 
-    ReactDOMServer.renderToString(<App />)
-    ```
+    One of the most commonly used mixins is `PureRenderMixin`. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:
 
-    This method will output the regular HTML as a string, which can be then placed inside a page body as part of the server response. On the client side, React detects the pre-rendered content and seamlessly picks up where it left off.
+    ```javascript
+    const PureRenderMixin = require("react-addons-pure-render-mixin");
 
+    const Button = React.createClass({
+      mixins: [PureRenderMixin],
+      // ...
+    });
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-72. ### How to enable production mode in React?
+     <!-- TODO: mixins are deprecated -->
 
-    You should use Webpack's `DefinePlugin` method to set `NODE_ENV` to `production`, by which it strip out things like propType validation and extra warnings. Apart from this, if you minify the code, for example, Uglify's dead-code elimination to strip out development only code and comments, it will drastically reduce the size of your bundle.
+    **[⬆ Back to Top](#table-of-contents)**
 
+54. ### What are the Pointer Events supported in React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-73. ### What is CRA and its benefits?
+    _Pointer Events_ provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the _Pointer Events_ specification.
 
-    The `create-react-app` CLI tool allows you to quickly create & run React applications with no configuration step.
+    The following event types are now available in _React DOM_:
+
+    1. `onPointerDown`
+    2. `onPointerMove`
+    3. `onPointerUp`
+    4. `onPointerCancel`
+    5. `onGotPointerCapture`
+    6. `onLostPointerCapture`
+    7. `onPointerEnter`
+    8. `onPointerLeave`
+    9. `onPointerOver`
+    10. `onPointerOut`
 
-    Let's create Todo App using *CRA*:
+    **[⬆ Back to Top](#table-of-contents)**
 
-    ```console
-    # Installation
-    $ npm install -g create-react-app
+55. ### Why should component names start with capital letter?
 
-    # Create new project
-    $ create-react-app todo-app
-    $ cd todo-app
+    If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
 
-    # Build, test and run
-    $ npm run build
-    $ npm run test
-    $ npm start
+    ```jsx harmony
+    function SomeComponent {
+      // Code goes here
+    }
     ```
-    It includes everything we need to build a React app:
 
-    1. React, JSX, ES6, and Flow syntax support.
-    2. Language extras beyond ES6 like the object spread operator.
-    3. Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
-    4. A fast interactive unit test runner with built-in support for coverage reporting.
-    5. A live development server that warns about common mistakes.
-    6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps.
+    You can define function component whose name starts with lowercase letter, but when it's imported it should have a capital letter. Here lowercase is fine:
 
+    ```jsx harmony
+    function myComponent {
+      render() {
+        return <div />;
+      }
+    }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-74. ### What is the lifecycle methods order in mounting?
+    export default myComponent;
+    ```
 
-    The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.
+    While when imported in another file it should start with capital letter:
 
-    1. `constructor()`
-    2. `static getDerivedStateFromProps()`
-    3. `render()`
-    4. `componentDidMount()`
+    ```jsx harmony
+    import MyComponent from "./myComponent";
+    ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-75. ### What are the lifecycle methods going to be deprecated in React v16?
+56. ### Are custom DOM attributes supported in React v16?
 
-    The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.
+    Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
 
-    1. `componentWillMount()`
-    2. `componentWillReceiveProps()`
-    3. `componentWillUpdate()`
+    For example, let's take a look at the below attribute:
 
-    Starting with React v16.3 these methods are aliased with `UNSAFE_` prefix, and the unprefixed version will be removed in React v17.
+    ```jsx harmony
+    <div mycustomattribute={"something"} />
+    ```
 
+    Would render an empty div to the DOM with React v15:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-76. ### What is the purpose of `getDerivedStateFromProps()` lifecycle method?
+    ```html
+    <div />
+    ```
 
-    The new static `getDerivedStateFromProps()` lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or `null` to indicate that the new props do not require any state updates.
+    In React v16 any unknown attributes will end up in the DOM:
 
-    ```javascript
-    class MyComponent extends React.Component {
-      static getDerivedStateFromProps(props, state) {
-        // ...
-      }
-    }
+    ```html
+    <div mycustomattribute="something" />
     ```
 
-    This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillReceiveProps()`.
+    This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-77. ### What is the purpose of `getSnapshotBeforeUpdate()` lifecycle method?
+57. ### How to loop inside JSX?
 
-    The new `getSnapshotBeforeUpdate()` lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to `componentDidUpdate()`.
+    You can simply use `Array.prototype.map` with ES6 _arrow function_ syntax.
 
-    ```javascript
-    class MyComponent extends React.Component {
-      getSnapshotBeforeUpdate(prevProps, prevState) {
-        // ...
+    For example, the `items` array of objects is mapped into an array of components:
+
+    ```jsx harmony
+    <tbody>
+      {items.map((item) => (
+        <SomeComponent key={item.id} name={item.name} />
+      ))}
+    </tbody>
+    ```
+
+    But you can't iterate using `for` loop:
+
+    ```jsx harmony
+    <tbody>
+      for (let i = 0; i < items.length; i++) {
+        <SomeComponent key={items[i].id} name={items[i].name} />
       }
-    }
+    </tbody>
     ```
 
-    This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillUpdate()`.
+    This is because JSX tags are transpiled into _function calls_, and you can't use statements inside expressions. This may change thanks to `do` expressions which are _stage 1 proposal_.
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-78. ### Do Hooks replace render props and higher order components?
+58. ### How do you access props in attribute quotes?
 
-    Both render props and higher-order components render only a single child but in most of the cases Hooks are a simpler way to serve this by reducing nesting in your tree.
+    React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
 
+    ```jsx harmony
+    <img className="image" src="images/{this.props.image}" />
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-79. ### What is the recommended way for naming components?
+    But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
 
-    It is recommended to name the component by reference instead of using `displayName`.
+    ```jsx harmony
+    <img className="image" src={"images/" + this.props.image} />
+    ```
 
-    Using `displayName` for naming component:
+    Using _template strings_ will also work:
 
-    ```javascript
-    export default React.createClass({
-      displayName: 'TodoApp',
-      // ...
-    })
+    ```jsx harmony
+    <img className="image" src={`images/${this.props.image}`} />
     ```
 
-    The **recommended** approach:
+    **[⬆ Back to Top](#table-of-contents)**
+
+59. ### What is React proptype array with shape?
+
+    If you want to pass an array of objects to a component with a particular shape then use `React.PropTypes.shape()` as an argument to `React.PropTypes.arrayOf()`.
 
     ```javascript
-    export default class TodoApp extends React.Component {
-      // ...
-    }
-    ```
-    also 
-    ```javascript 
-    const TodoApp = () => {
-     //...
-    }
-    export default TodoApp;
+    ReactComponent.propTypes = {
+      arrayWithShape: React.PropTypes.arrayOf(
+        React.PropTypes.shape({
+          color: React.PropTypes.string.isRequired,
+          fontSize: React.PropTypes.number.isRequired,
+        })
+      ).isRequired,
+    };
     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-80. ### What is the recommended ordering of methods in component class?
+    **[⬆ Back to Top](#table-of-contents)**
 
-    *Recommended* ordering of methods from *mounting* to *render stage*:
+60. ### How to conditionally apply class attributes?
 
-    1. `static` methods
-    2. `constructor()`
-    3. `getChildContext()`
-    4. `componentWillMount()`
-    5. `componentDidMount()`
-    6. `componentWillReceiveProps()`
-    7. `shouldComponentUpdate()`
-    8. `componentWillUpdate()`
-    9. `componentDidUpdate()`
-    10. `componentWillUnmount()`
-    11. click handlers or event handlers like `onClickSubmit()` or `onChangeDescription()`
-    12. getter methods for render like `getSelectReason()` or `getFooterContent()`
-    13. optional render methods like `renderNavigation()` or `renderProfilePicture()`
-    14. `render()`
+    You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
 
+    ```jsx harmony
+    <div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-81. ### What is a switching component?
+    Instead you need to move curly braces outside (don't forget to include spaces between class names):
 
-    A *switching component* is a component that renders one of many components. We need to use object to map prop values to components.
+    ```jsx harmony
+    <div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
+    ```
 
-    For example, a switching component to display different pages based on `page` prop:
+    _Template strings_ will also work:
 
     ```jsx harmony
-    import HomePage from './HomePage'
-    import AboutPage from './AboutPage'
-    import ServicesPage from './ServicesPage'
-    import ContactPage from './ContactPage'
+    <div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
+    ```
 
-    const PAGES = {
-      home: HomePage,
-      about: AboutPage,
-      services: ServicesPage,
-      contact: ContactPage
-    }
+    **[⬆ Back to Top](#table-of-contents)**
 
-    const Page = (props) => {
-      const Handler = PAGES[props.page] || ContactPage
+61. ### What is the difference between React and ReactDOM?
 
-      return <Handler {...props} />
-    }
+    The `react` package contains `React.createElement()`, `React.Component`, `React.Children`, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The `react-dom` package contains `ReactDOM.render()`, and in `react-dom/server` we have _server-side rendering_ support with `ReactDOMServer.renderToString()` and `ReactDOMServer.renderToStaticMarkup()`.
 
-    // The keys of the PAGES object can be used in the prop types to catch dev-time errors.
-    Page.propTypes = {
-      page: PropTypes.oneOf(Object.keys(PAGES)).isRequired
-    }
-    ```
+    **[⬆ Back to Top](#table-of-contents)**
 
+62. ### Why ReactDOM is separated from React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-82. ### Why we need to pass a function to setState()?
+    The React team worked on extracting all DOM-related features into a separate library called _ReactDOM_. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, `react-native`, `react-art`, `react-canvas`, and `react-three`, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
 
-    The reason behind for this is that `setState()` is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately after `setState()` is called. That means you should not rely on the current state when calling `setState()` since you can't be sure what that state will be. The solution is to  pass a function to `setState()`, with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature of `setState()`.
+    To build more environments that React can render to, React team planned to split the main React package into two: `react` and `react-dom`. This paves the way to writing components that can be shared between the web version of React and React Native.
 
-    Let's say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
+    **[⬆ Back to Top](#table-of-contents)**
 
-    ```javascript
-    // assuming this.state.count === 0
-    this.setState({ count: this.state.count + 1 })
-    this.setState({ count: this.state.count + 1 })
-    this.setState({ count: this.state.count + 1 })
-    // this.state.count === 1, not 3
-    ```
+63. ### How to use React label element?
 
-    If we pass a function to `setState()`, the count gets incremented correctly.
+    If you try to render a `<label>` element bound to a text input using the standard `for` attribute, then it produces HTML missing that attribute and prints a warning to the console.
 
-    ```javascript
-    this.setState((prevState, props) => ({
-      count: prevState.count + props.increment
-    }))
-    // this.state.count === 3 as expected
+    ```jsx harmony
+    <label for={'user'}>{'User'}</label>
+    <input type={'text'} id={'user'} />
     ```
 
-    **(OR)**
+    Since `for` is a reserved keyword in JavaScript, use `htmlFor` instead.
 
-    ### Why function is preferred over object for `setState()`?
+    ```jsx harmony
+    <label htmlFor={'user'}>{'User'}</label>
+    <input type={'text'} id={'user'} />
+    ```
 
-     React may batch multiple `setState()` calls into a single update for performance. Because `this.props` and `this.state` may be updated asynchronously, you should not rely on their values for calculating the next state.
+    **[⬆ Back to Top](#table-of-contents)**
 
-     This counter example will fail to update as expected:
+64. ### How to combine multiple inline style objects?
 
-     ```javascript
-     // Wrong
-     this.setState({
-       counter: this.state.counter + this.props.increment,
-     })
-     ```
+    You can use _spread operator_ in regular React:
+
+    ```jsx harmony
+    <button style={{ ...styles.panel.button, ...styles.panel.submitButton }}>
+      {"Submit"}
+    </button>
+    ```
 
-     The preferred approach is to call `setState()` with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.
+    If you're using React Native then you can use the array notation:
 
-     ```javascript
-     // Correct
-     this.setState((prevState, props) => ({
-       counter: prevState.counter + props.increment
-     }))
-     ```
+    ```jsx harmony
+    <button style={[styles.panel.button, styles.panel.submitButton]}>
+      {"Submit"}
+    </button>
+    ```
 
+    **[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
+65. ### How to re-render the view when the browser is resized?
 
-    
-83. ### What is strict mode in React?
+    You can use the `useState` hook to manage the width and height state variables, and the `useEffect` hook to add and remove the `resize` event listener. The `[]` dependency array passed to useEffect ensures that the effect only runs once (on mount) and not on every re-render.
 
-    `React.StrictMode` is a useful component for highlighting potential problems in an application. Just like `<Fragment>`, `<StrictMode>` does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for *development mode* only.
+    ```javascript
+    import React, { useState, useEffect } from "react";
+    function WindowDimensions() {
+      const [dimensions, setDimensions] = useState({
+        width: window.innerWidth,
+        height: window.innerHeight,
+      });
 
-    ```jsx harmony
-    import React from 'react'
+      useEffect(() => {
+        function handleResize() {
+          setDimensions({
+            width: window.innerWidth,
+            height: window.innerHeight,
+          });
+        }
+        window.addEventListener("resize", handleResize);
+        return () => window.removeEventListener("resize", handleResize);
+      }, []);
 
-    function ExampleApplication() {
       return (
-        <div>
-          <Header />
-          <React.StrictMode>
-            <div>
-              <ComponentOne />
-              <ComponentTwo />
-            </div>
-          </React.StrictMode>
-          <Header />
-        </div>
-      )
+        <span>
+          {dimensions.width} x {dimensions.height}
+        </span>
+      );
     }
     ```
 
-    In the example above, the *strict mode* checks apply to `<ComponentOne>` and `<ComponentTwo>` components only.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-84. ### What are React Mixins?
+    <details>
+    <summary><h4>Using Class Component</h4></summary>
 
-    *Mixins* are a way to totally separate components to have a common functionality. Mixins **should not be used** and can be replaced with *higher-order components* or *decorators*.
-
-    One of the most commonly used mixins is `PureRenderMixin`. You might be using it in some components to prevent unnecessary re-renders when the props and state are shallowly equal to the previous props and state:
+    You can listen to the `resize` event in `componentDidMount()` and then update the dimensions (`width` and `height`). You should remove the listener in `componentWillUnmount()` method.
 
     ```javascript
-    const PureRenderMixin = require('react-addons-pure-render-mixin')
+    class WindowDimensions extends React.Component {
+      constructor(props) {
+        super(props);
+        this.updateDimensions = this.updateDimensions.bind(this);
+      }
 
-    const Button = React.createClass({
-      mixins: [PureRenderMixin],
-      // ...
-    })
-    ```
-    <!-- TODO: mixins are deprecated -->
+      componentWillMount() {
+        this.updateDimensions();
+      }
 
+      componentDidMount() {
+        window.addEventListener("resize", this.updateDimensions);
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-85. ### Why is `isMounted()` an anti-pattern and what is the proper solution?
+      componentWillUnmount() {
+        window.removeEventListener("resize", this.updateDimensions);
+      }
 
-    The primary use case for `isMounted()` is to avoid calling `setState()` after a component has been unmounted, because it will emit a warning.
+      updateDimensions() {
+        this.setState({
+          width: window.innerWidth,
+          height: window.innerHeight,
+        });
+      }
 
-    ```javascript
-    if (this.isMounted()) {
-      this.setState({...})
+      render() {
+        return (
+          <span>
+            {this.state.width} x {this.state.height}
+          </span>
+        );
+      }
     }
     ```
 
-    Checking `isMounted()` before calling `setState()` does eliminate the warning, but it also defeats the purpose of the warning. Using `isMounted()` is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.
+    </details>
 
-    An optimal solution would be to find places where `setState()` might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled in `componentWillUnmount()`, prior to unmounting.
+**[⬆ Back to Top](#table-of-contents)**
 
+66. ### How to pretty print JSON with React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-86. ### What are the Pointer Events supported in React?
+    We can use `<pre>` tag so that the formatting of the `JSON.stringify()` is retained:
 
-    *Pointer Events* provide a unified way of handling all input events. In the old days we had a mouse and respective event listeners to handle them but nowadays we have many devices which don't correlate to having a mouse, like phones with touch surface or pens. We need to remember that these events will only work in browsers that support the *Pointer Events* specification.
+    ```jsx harmony
+    const data = { name: "John", age: 42 };
 
-    The following event types are now available in *React DOM*:
+    function User {
+        return <pre>{JSON.stringify(data, null, 2)}</pre>;
+    }
 
-    1. `onPointerDown`
-    2. `onPointerMove`
-    3. `onPointerUp`
-    4. `onPointerCancel`
-    5. `onGotPointerCapture`
-    6. `onLostPointerCapture`
-    7. `onPointerEnter`
-    8. `onPointerLeave`
-    9. `onPointerOver`
-    10. `onPointerOut`
+    const container = createRoot(document.getElementById("container"));
 
+    container.render(<User />);
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-87. ### Why should component names start with capital letter?
+      <details><summary><b>See Class</b></summary>
+      <p>
 
-    If you are rendering your component using JSX, the name of that component has to begin with a capital letter otherwise React will throw an error as an unrecognized tag. This convention is because only HTML elements and SVG tags can begin with a lowercase letter.
     ```jsx harmony
-    class SomeComponent extends Component {
-     // Code goes here
-    }
-    ```
-    You can define component class which name starts with lowercase letter, but when it's imported it should have capital letter. Here lowercase is fine:
+    const data = { name: "John", age: 42 };
 
-    ```jsx harmony
-    class myComponent extends Component {
+    class User extends React.Component {
       render() {
-        return <div />
+        return <pre>{JSON.stringify(data, null, 2)}</pre>;
       }
     }
 
-    export default myComponent
+    React.render(<User />, document.getElementById("container"));
     ```
 
-    While when imported in another file it should start with capital letter:
+      </p>
+      </details>
 
-    ```jsx harmony
-    import MyComponent from './myComponent'
-    ```
+**[⬆ Back to Top](#table-of-contents)**
 
+67. ### Why can't you update props in React?
 
-    #### What are the exceptions on React component naming?
+    The React philosophy is that props should be _immutable_(read only) and _top-down_. This means that a parent can send any prop values to a child, but the child can't modify received props.
 
-    The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
-    For example, the below tag can be compiled to a valid component,
+**[⬆ Back to Top](#table-of-contents)**
+
+68. ### How to focus an input element on page load?
+
+    You need to use `useEffect` hook to set focus on input field during page load time for functional component.
 
     ```jsx harmony
-         render() {
-              return (
-                <obj.component/> // `React.createElement(obj.component)`
-              )
-        }
-    ```
+    import React, { useEffect, useRef } from "react";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-88. ### Are custom DOM attributes supported in React v16?
+    const App = () => {
+      const inputElRef = useRef(null);
 
-    Yes. In the past, React used to ignore unknown DOM attributes. If you wrote JSX with an attribute that React doesn't recognize, React would just skip it.
+      useEffect(() => {
+        inputElRef.current.focus();
+      }, []);
 
-    For example, let's take a look at the below attribute:
+      return (
+        <div>
+          <input defaultValue={"Won't focus"} />
+          <input ref={inputElRef} defaultValue={"Will focus"} />
+        </div>
+      );
+    };
 
-    ```jsx harmony
-    <div mycustomattribute={'something'} />
+    ReactDOM.render(<App />, document.getElementById("app"));
     ```
 
-    Would render an empty div to the DOM with React v15:
+      <details><summary><b>See Class</b></summary>
+      <p>
+      You can do it by creating _ref_ for `input` element and using it in `componentDidMount()`:
 
-    ```html
-    <div />
-    ```
+    ```jsx harmony
+    class App extends React.Component {
+      componentDidMount() {
+        this.nameInput.focus();
+      }
 
-    In React v16 any unknown attributes will end up in the DOM:
+      render() {
+        return (
+          <div>
+            <input defaultValue={"Won't focus"} />
+            <input
+              ref={(input) => (this.nameInput = input)}
+              defaultValue={"Will focus"}
+            />
+          </div>
+        );
+      }
+    }
 
-    ```html
-    <div mycustomattribute='something' />
+    ReactDOM.render(<App />, document.getElementById("app"));
     ```
 
-    This is useful for supplying browser-specific non-standard attributes, trying new DOM APIs, and integrating with opinionated third-party libraries.
+      </p>
+      </details>
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-89. ### What is the difference between constructor and getInitialState?
+69. ### How can we find the version of React at runtime in the browser?
 
-    You should initialize state in the constructor when using ES6 classes, and `getInitialState()` method when using `React.createClass()`.
+    You can use `React.version` to get the version.
 
-    **Using ES6 classes:**
+    ```jsx harmony
+    const REACT_VERSION = React.version;
 
-    ```javascript
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super(props)
-        this.state = { /* initial state */ }
-      }
-    }
+    ReactDOM.render(
+      <div>{`React version: ${REACT_VERSION}`}</div>,
+      document.getElementById("app")
+    );
     ```
 
-    **Using `React.createClass()`:**
+**[⬆ Back to Top](#table-of-contents)**
+
+70. ### How to add Google Analytics for React Router?
+
+    Add a listener on the `history` object to record each page view:
 
     ```javascript
-    const MyComponent = React.createClass({
-      getInitialState() {
-        return { /* initial state */ }
-      }
-    })
+    history.listen(function (location) {
+      window.ga("set", "page", location.pathname + location.search);
+      window.ga("send", "pageview", location.pathname + location.search);
+    });
     ```
 
-    **Note:** `React.createClass()` is deprecated and removed in React v16. Use plain JavaScript classes instead.
-
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-90. ### Can you force a component to re-render without calling setState?
+71. ### How do you apply vendor prefixes to inline styles in React?
 
-    By default, when your component's state or props change, your component will re-render. If your `render()` method depends on some other data, you can tell React that the component needs re-rendering by calling `forceUpdate()`.
+    React _does not_ apply _vendor prefixes_ automatically. You need to add vendor prefixes manually.
 
-    ```javascript
-    component.forceUpdate(callback)
+    ```jsx harmony
+    <div
+      style={{
+        transform: "rotate(90deg)",
+        WebkitTransform: "rotate(90deg)", // note the capital 'W' here
+        msTransform: "rotate(90deg)", // 'ms' is the only lowercase vendor prefix
+      }}
+    />
     ```
 
-    It is recommended to avoid all uses of `forceUpdate()` and only read from `this.props` and `this.state` in `render()`.
-
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-91. ### What is the difference between `super()` and `super(props)` in React using ES6 classes?
+72. ### How to import and export components using React and ES6?
 
-    When you want to access `this.props` in `constructor()` then you should pass props to `super()` method.
+    You should use default for exporting the components
 
-    **Using `super(props)`:**
+    ```jsx harmony
+    import User from "user";
 
-    ```javascript
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super(props)
-        console.log(this.props) // { name: 'John', ... }
-      }
+    export default function MyProfile {
+        return <User type="customer">//...</User>;
     }
     ```
 
-    **Using `super()`:**
+    <details><summary><b>See Class</b></summary>
+    <p>
+     ```jsx harmony
+     import React from "react";
+     import User from "user";
 
-    ```javascript
-    class MyComponent extends React.Component {
-      constructor(props) {
-        super()
-        console.log(this.props) // undefined
-      }
+    export default class MyProfile extends React.Component {
+    render() {
+    return <User type="customer">//...</User>;
+    }
     }
-    ```
 
-    Outside `constructor()` both will display same value for `this.props`.
+    ```
+    </p>
+    </details>
 
+    With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-92. ### How to loop inside JSX?
+**[⬆ Back to Top](#table-of-contents)**
 
-    You can simply use `Array.prototype.map` with ES6 *arrow function* syntax.
+73. ### What are the exceptions on React component naming?
 
-    For example, the `items` array of objects is mapped into an array of components:
+    The component names should start with an uppercase letter but there are few exceptions to this convention. The lowercase tag names with a dot (property accessors) are still considered as valid component names.
+    For example, the below tag can be compiled to a valid component,
 
     ```jsx harmony
-    <tbody>
-      {items.map(item => <SomeComponent key={item.id} name={item.name} />)}
-    </tbody>
+         render() {
+              return (
+                <obj.component/> // `React.createElement(obj.component)`
+              )
+        }
     ```
 
-    But you can't iterate using `for` loop:
+    **[⬆ Back to Top](#table-of-contents)**
 
-    ```jsx harmony
-    <tbody>
-      for (let i = 0; i < items.length; i++) {
-        <SomeComponent key={items[i].id} name={items[i].name} />
-      }
-    </tbody>
-    ```
+74. ### Is it possible to use async/await in plain React?
 
-    This is because JSX tags are transpiled into *function calls*, and you can't use statements inside expressions. This may change thanks to `do` expressions which are *stage 1 proposal*.
+    Yes, you can use `async/await` in plain React, as long as your JavaScript environment supports ES2017+. Nowadays most modern browsers and build tools support ES2017+ version. If you're using **Create React App**, **Next.js**, **Remix**, or any modern React setup, `async/await` is supported out of the box through **Babel**.
 
+    ### Example Usage
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-93. ### How do you access props in attribute quotes?
+    ```jsx
+    import { useEffect, useState } from 'react';
 
-    React (or JSX) doesn't support variable interpolation inside an attribute value. The below representation won't work:
+    function UserProfile() {
+      const [user, setUser] = useState(null);
 
-    ```jsx harmony
-    <img className='image' src='images/{this.props.image}' />
-    ```
+      useEffect(() => {
+        const fetchUser = async () => {
+          const response = await fetch('/api/user');
+          const data = await response.json();
+          setUser(data);
+        };
 
-    But you can put any JS expression inside curly braces as the entire attribute value. So the below expression works:
+        fetchUser();
+      }, []);
 
-    ```jsx harmony
-    <img className='image' src={'images/' + this.props.image} />
+      return user ? <div>Hello, {user.name}</div> : <div>Loading...</div>;
+    }
     ```
+    But If you're not using a bundler like **Webpack or Babel**, you will need _Babel_ and [transform-async-to-generator](https://babeljs.io/docs/en/babel-plugin-transform-async-to-generator) plugin. However, React Native ships with Babel and a set of transforms.
 
-    Using *template strings* will also work:
+**[⬆ Back to Top](#table-of-contents)**
 
-    ```jsx harmony
-    <img className='image' src={`images/${this.props.image}`} />
-    ```
+75.  ### What are the common folder structures for React?
 
+     There are two common practices for React project file structure.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-94. ### What is React proptype array with shape?
+     1.  **Grouping by features or routes:**
 
-    If you want to pass an array of objects to a component with a particular shape then use `React.PropTypes.shape()` as an argument to `React.PropTypes.arrayOf()`.
+        One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
 
-    ```javascript
-    ReactComponent.propTypes = {
-      arrayWithShape: React.PropTypes.arrayOf(React.PropTypes.shape({
-        color: React.PropTypes.string.isRequired,
-        fontSize: React.PropTypes.number.isRequired
-      })).isRequired
-    }
-    ```
+        ```
+        common/
+        ├─ Avatar.js
+        ├─ Avatar.css
+        ├─ APIUtils.js
+        └─ APIUtils.test.js
+        feed/
+        ├─ index.js
+        ├─ Feed.js
+        ├─ Feed.css
+        ├─ FeedStory.js
+        ├─ FeedStory.test.js
+        └─ FeedAPI.js
+        profile/
+        ├─ index.js
+        ├─ Profile.js
+        ├─ ProfileHeader.js
+        ├─ ProfileHeader.css
+        └─ ProfileAPI.js
+        ```
 
+     2.  **Grouping by file type:**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-95. ### How to conditionally apply class attributes?
+        Another popular way to structure projects is to group similar files together.
 
-    You shouldn't use curly braces inside quotes because it is going to be evaluated as a string.
+        ```
+        api/
+        ├─ APIUtils.js
+        ├─ APIUtils.test.js
+        ├─ ProfileAPI.js
+        └─ UserAPI.js
+        components/
+        ├─ Avatar.js
+        ├─ Avatar.css
+        ├─ Feed.js
+        ├─ Feed.css
+        ├─ FeedStory.js
+        ├─ FeedStory.test.js
+        ├─ Profile.js
+        ├─ ProfileHeader.js
+        └─ ProfileHeader.css
+        ```
 
-    ```jsx harmony
-    <div className="btn-panel {this.props.visible ? 'show' : 'hidden'}">
-    ```
+**[⬆ Back to Top](#table-of-contents)**
 
-    Instead you need to move curly braces outside (don't forget to include spaces between class names):
+76. ### What are the popular packages for animation?
 
-    ```jsx harmony
-    <div className={'btn-panel ' + (this.props.visible ? 'show' : 'hidden')}>
-    ```
+    _React Transition Group_ and _React Motion_ are popular animation packages in React ecosystem.
 
-    *Template strings* will also work:
+**[⬆ Back to Top](#table-of-contents)**
 
-    ```jsx harmony
-    <div className={`btn-panel ${this.props.visible ? 'show' : 'hidden'}`}>
-    ```
+77. ### What is the benefit of styles modules?
 
+    It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-96. ### What is the difference between React and ReactDOM?
+    For example, these styles could be extracted into a separate component:
 
-    The `react` package contains `React.createElement()`, `React.Component`, `React.Children`, and other helpers related to elements and component classes. You can think of these as the isomorphic or universal helpers that you need to build components. The `react-dom` package contains `ReactDOM.render()`, and in `react-dom/server` we have *server-side rendering* support with `ReactDOMServer.renderToString()` and `ReactDOMServer.renderToStaticMarkup()`.
+    ```javascript
+    export const colors = {
+      white,
+      black,
+      blue,
+    };
 
+    export const space = [0, 8, 16, 32, 64];
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-97. ### Why ReactDOM is separated from React?
+    And then imported individually in other components:
 
-    The React team worked on extracting all DOM-related features into a separate library called *ReactDOM*. React v0.14 is the first release in which the libraries are split. By looking at some of the packages, `react-native`, `react-art`, `react-canvas`, and `react-three`, it has become clear that the beauty and essence of React has nothing to do with browsers or the DOM.
+    ```javascript
+    import { space, colors } from "./styles";
+    ```
 
-    To build more environments that React can render to, React team planned to split the main React package into two: `react` and `react-dom`. This paves the way to writing components that can be shared between the web version of React and React Native.
+**[⬆ Back to Top](#table-of-contents)**
 
+78. ### What are the popular React-specific linters?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-98. ### How to use React label element?
+    ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called `eslint-plugin-react`. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
 
-    If you try to render a `<label>` element bound to a text input using the standard `for` attribute, then it produces HTML missing that attribute and prints a warning to the console.
+    Another popular plugin is `eslint-plugin-jsx-a11y`, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with `alt` text and `tabindex`, for example, will not be picked up by regular plugins.
 
-    ```jsx harmony
-    <label for={'user'}>{'User'}</label>
-    <input type={'text'} id={'user'} />
-    ```
+**[⬆ Back to Top](#table-of-contents)**
 
-    Since `for` is a reserved keyword in JavaScript, use `htmlFor` instead.
+## React Router
 
-    ```jsx harmony
-    <label htmlFor={'user'}>{'User'}</label>
-    <input type={'text'} id={'user'} />
-    ```
+**[⬆ Back to Top](#table-of-contents)**
 
+79. ### What is React Router?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-99. ### How to combine multiple inline style objects?
+    React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.
 
-    You can use *spread operator* in regular React:
+**[⬆ Back to Top](#table-of-contents)**
 
-    ```jsx harmony
-     <button style={{...styles.panel.button, ...styles.panel.submitButton}}>{'Submit'}</button>
-    ```
+80. ### How React Router is different from history library?
 
-    If you're using React Native then you can use the array notation:
+    React Router is a wrapper around the `history` library which handles interaction with the browser's `window.history` with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.
 
-    ```jsx harmony
-    <button style={[styles.panel.button, styles.panel.submitButton]}>{'Submit'}</button>
-    ```
+**[⬆ Back to Top](#table-of-contents)**
 
+81. ### What are the `<Router>` components of React Router v6?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-100. ### How to re-render the view when the browser is resized?
+    React Router v6 provides below 4 `<Router>` components:
 
-     You can listen to the `resize` event in `componentDidMount()` and then update the dimensions (`width` and `height`). You should remove the listener in `componentWillUnmount()` method.
+    1.  `<BrowserRouter>`:Uses the HTML5 history API for standard web apps.
+    2.  `<HashRouter>`:Uses hash-based routing for static servers.
+    3.  `<MemoryRouter>`:Uses in-memory routing for testing and non-browser environments.
+    4.  `<StaticRouter>`:Provides static routing for server-side rendering (SSR).
 
-     ```javascript
-     class WindowDimensions extends React.Component {
-       constructor(props){
-         super(props);
-         this.updateDimensions = this.updateDimensions.bind(this);
-       }
-        
-       componentWillMount() {
-         this.updateDimensions()
-       }
+    The above components will create _browser_, _hash_, _memory_ and _static_ history instances. React Router v6 makes the properties and methods of the `history` instance associated with your router available through the context in the `router` object.
 
-       componentDidMount() {
-         window.addEventListener('resize', this.updateDimensions)
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       componentWillUnmount() {
-         window.removeEventListener('resize', this.updateDimensions)
-       }
+82. ### What is the purpose of `push()` and `replace()` methods of `history`?
 
-       updateDimensions() {
-         this.setState({width: window.innerWidth, height: window.innerHeight})
-       }
+    A history instance has two methods for navigation purpose.
 
-       render() {
-         return <span>{this.state.width} x {this.state.height}</span>
-       }
-     }
-     ```
+    1.  `push()`
+    2.  `replace()`
 
+    If you think of the history as an array of visited locations, `push()` will add a new location to the array and `replace()` will replace the current location in the array with the new one.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-101. ### What is the difference between `setState()` and `replaceState()` methods?
+**[⬆ Back to Top](#table-of-contents)**
 
-     When you use `setState()` the current and previous states are merged. `replaceState()` throws out the current state, and replaces it with only what you provide. Usually `setState()` is used unless you really need to remove all previous keys for some reason. You can also set state to `false`/`null` in `setState()` instead of using `replaceState()`.
+83. ### How do you programmatically navigate using React Router v4?
 
+    There are three different ways to achieve programmatic routing/navigation within components.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-102. ### How to listen to state changes?
+    1.  **Using the `withRouter()` higher-order function:**
 
-     The `componentDidUpdate` lifecycle method will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
+        The `withRouter()` higher-order function will inject the history object as a prop of the component. This object provides `push()` and `replace()` methods to avoid the usage of context.
 
-     ```
-     componentDidUpdate(object prevProps, object prevState)
-     ```
+        ```jsx harmony
+        import { withRouter } from "react-router-dom"; // this also works with 'react-router-native'
+
+        const Button = withRouter(({ history }) => (
+          <button
+            type="button"
+            onClick={() => {
+              history.push("/new-location");
+            }}
+          >
+            {"Click Me!"}
+          </button>
+        ));
+        ```
 
-     **Note:** The previous releases of ReactJS also uses `componentWillUpdate(object nextProps, object nextState)` for state changes. It has been deprecated in latest releases.
+    2.  **Using `<Route>` component and render props pattern:**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-103. ### What is the recommended approach of removing an array element in React state?
+        The `<Route>` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.
 
-     The better approach is to use `Array.prototype.filter()` method.
+        ```jsx harmony
+        import { Route } from "react-router-dom";
+
+        const Button = () => (
+          <Route
+            render={({ history }) => (
+              <button
+                type="button"
+                onClick={() => {
+                  history.push("/new-location");
+                }}
+              >
+                {"Click Me!"}
+              </button>
+            )}
+          />
+        );
+        ```
 
-     For example, let's create a `removeItem()` method for updating the state.
+    3.  **Using context:**
 
-     ```javascript
-     removeItem(index) {
-       this.setState({
-         data: this.state.data.filter((item, i) => i !== index)
-       })
-     }
-     ```
+        This option is not recommended and treated as unstable API.
 
+        ```jsx harmony
+        const Button = (props, context) => (
+          <button
+            type="button"
+            onClick={() => {
+              context.history.push("/new-location");
+            }}
+          >
+            {"Click Me!"}
+          </button>
+        );
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-104. ### Is it possible to use React without rendering HTML?
+        Button.contextTypes = {
+          history: React.PropTypes.shape({
+            push: React.PropTypes.func.isRequired,
+          }),
+        };
+        ```
 
-     It is possible with latest version (>=16.2). Below are the possible options:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```jsx harmony
-     render() {
-       return false
-     }
-     ```
+84. ### How to get query parameters in React Router v4?
 
-     ```jsx harmony
-     render() {
-       return null
-     }
-     ```
+    The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
 
-     ```jsx harmony
-     render() {
-       return []
-     }
-     ```
+    ```javascript
+    const queryString = require("query-string");
+    const parsed = queryString.parse(props.location.search);
+    ```
 
-     ```jsx harmony
-     render() {
-       return <React.Fragment></React.Fragment>
-     }
-     ```
+    You can also use `URLSearchParams` if you want something native:
 
-     ```jsx harmony
-     render() {
-       return <></>
-     }
-     ```
+    ```javascript
+    const params = new URLSearchParams(props.location.search);
+    const foo = params.get("name");
+    ```
 
-     Returning `undefined` won't work.
+    You should use a _polyfill_ for IE11.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-105. ### How to pretty print JSON with React?
+85. ### Why you get "Router may have only one child element" warning?
 
-     We can use `<pre>` tag so that the formatting of the `JSON.stringify()` is retained:
+    You have to wrap your Route's in a `<Switch>` block because `<Switch>` is unique in that it renders a route exclusively.
 
-     ```jsx harmony
-     const data = { name: 'John', age: 42 }
+    At first you need to add `Switch` to your imports:
 
-     class User extends React.Component {
-       render() {
-         return (
-           <pre>
-             {JSON.stringify(data, null, 2)}
-           </pre>
-         )
-       }
-     }
+    ```javascript
+    import { Switch, Router, Route } from "react-router";
+    ```
 
-     React.render(<User />, document.getElementById('container'))
-     ```
+    Then define the routes within `<Switch>` block:
 
+    ```jsx harmony
+    <Router>
+      <Switch>
+        <Route {/* ... */} />
+        <Route {/* ... */} />
+      </Switch>
+    </Router>
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-106. ### Why you can't update props in React?
+**[⬆ Back to Top](#table-of-contents)**
 
-     The React philosophy is that props should be *immutable* and *top-down*. This means that a parent can send any prop values to a child, but the child can't modify received props.
+86. ### How to pass params to `history.push` method in React Router v4?
 
+    While navigating you can pass props to the `history` object:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-107. ### How to focus an input element on page load?
+    ```javascript
+    this.props.history.push({
+      pathname: "/template",
+      search: "?name=sudheer",
+      state: { detail: response.data },
+    });
+    ```
 
-     You can do it by creating *ref* for `input` element and using it in `componentDidMount()`:
+    The `search` property is used to pass query params in `push()` method.
 
-     ```jsx harmony
-     class App extends React.Component{
-       componentDidMount() {
-         this.nameInput.focus()
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       render() {
-         return (
-           <div>
-             <input
-               defaultValue={'Won\'t focus'}
-             />
-             <input
-               ref={(input) => this.nameInput = input}
-               defaultValue={'Will focus'}
-             />
-           </div>
-         )
-       }
-     }
+87. ### How to implement _default_ or _NotFound_ page?
 
-     ReactDOM.render(<App />, document.getElementById('app'))
-     ```
+    A `<Switch>` renders the first child `<Route>` that matches. A `<Route>` with no path always matches. So you just need to simply drop path attribute as below
 
-     Also in Functional component (react 16.08 and above)
-     
-      ```jsx harmony
-      import React, {useEffect, useRef} from 'react';
+    ```jsx harmony
+    <Switch>
+      <Route exact path="/" component={Home} />
+      <Route path="/user" component={User} />
+      <Route component={NotFound} />
+    </Switch>
+    ```
 
-      const App = () => {
-        const inputElRef = useRef(null)
-        
-        useEffect(()=>{
-          inputElRef.current.focus()
-        }, [])
-        
-        return(
-          <div>
-            <input
-              defaultValue={'Won\'t focus'}
-            />
-            <input
-              ref={inputElRef}
-              defaultValue={'Will focus'}
-            />
-          </div>
-        )
-      }
+**[⬆ Back to Top](#table-of-contents)**
 
-      ReactDOM.render(<App />, document.getElementById('app'))
-      ```
-   **[⬆ Back to Top](#table-of-contents)**
-    
-108. ### What are the possible ways of updating objects in state?
+88. ### How to get history on React Router v4?
+
+    Below are the list of steps to get history object on React Router v4,
 
-     1. **Calling `setState()` with an object to merge with state:**
+    1.  Create a module that exports a `history` object and import this module across the project.
 
-         * Using `Object.assign()` to create a copy of the object:
+        For example, create `history.js` file:
 
-             ```javascript
-             const user = Object.assign({}, this.state.user, { age: 42 })
-             this.setState({ user })
-             ```
+        ```javascript
+        import { createBrowserHistory } from "history";
 
-         * Using *spread operator*:
+        export default createBrowserHistory({
+          /* pass a configuration object here if needed */
+        });
+        ```
 
-             ```javascript
-             const user = { ...this.state.user, age: 42 }
-             this.setState({ user })
-             ```
+    2.  You should use the `<Router>` component instead of built-in routers. Import the above `history.js` inside `index.js` file:
 
-     2. **Calling `setState()` with a function:**
+        ```jsx harmony
+        import { Router } from "react-router-dom";
+        import history from "./history";
+        import App from "./App";
+
+        ReactDOM.render(
+          <Router history={history}>
+            <App />
+          </Router>,
+          holder
+        );
+        ```
 
-         ```javascript
-         this.setState(prevState => ({
-           user: {
-             ...prevState.user,
-             age: 42
-           }
-         }))
-         ```
+    3.  You can also use push method of `history` object similar to built-in history object:
 
+        ```javascript
+        // some-other-file.js
+        import history from "./history";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
+        history.push("/go-here");
+        ```
 
-110. ### How can we find the version of React at runtime in the browser?
+**[⬆ Back to Top](#table-of-contents)**
 
-     You can use `React.version` to get the version.
+89. ### How to perform automatic redirect after login?
 
-     ```jsx harmony
-     const REACT_VERSION = React.version
+    The `react-router` package provides `<Redirect>` component in React Router. Rendering a `<Redirect>` will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
 
-     ReactDOM.render(
-       <div>{`React version: ${REACT_VERSION}`}</div>,
-       document.getElementById('app')
-     )
-     ```
+    ```javascript
+    import { Redirect } from "react-router";
 
+    export default function Login {
+        if (this.state.isLoggedIn === true) {
+          return <Redirect to="/your/redirect/page" />;
+        } else {
+          return <div>{"Login Please"}</div>;
+        }
+    }
+    ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-111. ### What are the approaches to include polyfills in your `create-react-app`?
+      <details><summary><b>See Class</b></summary>
+      <p>
 
-     There are approaches to include polyfills in create-react-app,
+    ```jsx
+    import React, { Component } from "react";
+    import { Redirect } from "react-router";
 
-     1. **Manual import from `core-js`:**
+    export default class LoginComponent extends Component {
+      render() {
+        if (this.state.isLoggedIn === true) {
+          return <Redirect to="/your/redirect/page" />;
+        } else {
+          return <div>{"Login Please"}</div>;
+        }
+      }
+    }
+    ```
 
-         Create a file called (something like) `polyfills.js` and import it into root `index.js` file. Run `npm install core-js` or `yarn add core-js` and import your specific required features.
+       </p>
+       </details>
 
-         ```javascript
-         import 'core-js/fn/array/find'
-         import 'core-js/fn/array/includes'
-         import 'core-js/fn/number/is-nan'
-         ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     2. **Using Polyfill service:**
+## React Internationalization
 
-         Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to `index.html`:
+90. ### What is React Intl?
 
-         ```html
-         <script src='https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.includes'></script>
-         ```
+    The _React Intl_ library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of _FormatJS_ which provides bindings to React via its components and API.
 
-         In the above script we had to explicitly request the `Array.prototype.includes` feature as it is not included in the default feature set.
+**[⬆ Back to Top](#table-of-contents)**
 
+91. ### What are the main features of React Intl?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-112. ### How to use https instead of http in create-react-app?
+    Below are the main features of React Intl,
 
-     You just need to use `HTTPS=true` configuration. You can edit your `package.json` scripts section:
+    1.  Display numbers with separators.
+    2.  Display dates and times correctly.
+    3.  Display dates relative to "now".
+    4.  Pluralize labels in strings.
+    5.  Support for 150+ languages.
+    6.  Runs in the browser and Node.
+    7.  Built on standards.
 
-     ```json
-     "scripts": {
-       "start": "set HTTPS=true && react-scripts start"
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     or just run `set HTTPS=true && npm start`
+92. ### What are the two ways of formatting in React Intl?
 
+    The library provides two ways to format strings, numbers, and dates:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-113. ### How to avoid using relative path imports in create-react-app?
+    1.  **Using react components:**
 
-     Create a file called `.env` in the project root and write the import path:
+        ```jsx harmony
+        <FormattedMessage
+          id={"account"}
+          defaultMessage={"The amount is less than minimum balance."}
+        />
+        ```
 
-     ```
-     NODE_PATH=src/app
-     ```
+    2.  **Using an API:**
 
-     After that restart the development server. Now you should be able to import anything inside `src/app` without relative paths.
+        ```javascript
+        const messages = defineMessages({
+          accountMessage: {
+            id: "account",
+            defaultMessage: "The amount is less than minimum balance.",
+          },
+        });
+
+        formatMessage(messages.accountMessage);
+        ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-114. ### How to add Google Analytics for React Router?
+93. ### How to use `<FormattedMessage>` as placeholder using React Intl?
 
-     Add a listener on the `history` object to record each page view:
+    The `<Formatted... />` components from `react-intl` return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API `formatMessage()`. You can inject the `intl` object into your component using `injectIntl()` higher-order component and then format the message using `formatMessage()` available on that object.
 
-     ```javascript
-     history.listen(function (location) {
-       window.ga('set', 'page', location.pathname + location.search)
-       window.ga('send', 'pageview', location.pathname + location.search)
-     })
-     ```
+    ```jsx harmony
+    import React from "react";
+    import { injectIntl, intlShape } from "react-intl";
+
+    const MyComponent = ({ intl }) => {
+      const placeholder = intl.formatMessage({ id: "messageId" });
+      return <input placeholder={placeholder} />;
+    };
 
+    MyComponent.propTypes = {
+      intl: intlShape.isRequired,
+    };
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-115. ### How to update a component every second?
+    export default injectIntl(MyComponent);
+    ```
 
-     You need to use `setInterval()` to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     componentDidMount() {
-       this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
-     }
+94. ### How to access current locale with React Intl?
 
-     componentWillUnmount() {
-       clearInterval(this.interval)
-     }
-     ```
+    You can get the current locale in any component of your application using `injectIntl()`:
 
+    ```jsx harmony
+    import { injectIntl, intlShape } from "react-intl";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-116. ### How do you apply vendor prefixes to inline styles in React?
+    const MyComponent = ({ intl }) => (
+      <div>{`The current locale is ${intl.locale}`}</div>
+    );
 
-     React *does not* apply *vendor prefixes* automatically. You need to add vendor prefixes manually.
+    MyComponent.propTypes = {
+      intl: intlShape.isRequired,
+    };
 
-     ```jsx harmony
-     <div style={{
-       transform: 'rotate(90deg)',
-       WebkitTransform: 'rotate(90deg)', // note the capital 'W' here
-       msTransform: 'rotate(90deg)' // 'ms' is the only lowercase vendor prefix
-     }} />
-     ```
+    export default injectIntl(MyComponent);
+    ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-117. ### How to import and export components using React and ES6?
+95. ### How to format date using React Intl?
 
-     You should use default for exporting the components
+     The `injectIntl()` higher-order component will give you access to the `formatDate()` method via the props in your component. The method is used internally by instances of `FormattedDate` and it returns the string representation of the formatted date.
 
      ```jsx harmony
-     import React from 'react'
-     import User from 'user'
+     import { injectIntl, intlShape } from "react-intl";
 
-     export default class MyProfile extends React.Component {
-       render(){
-         return (
-           <User type="customer">
-             //...
-           </User>
-         )
-       }
-     }
-     ```
+     const stringDate = this.props.intl.formatDate(date, {
+       year: "numeric",
+       month: "numeric",
+       day: "numeric",
+     });
 
-     With the export specifier, the MyProfile is going to be the member and exported to this module and the same can be imported without mentioning the name in other components.
+     const MyComponent = ({ intl }) => (
+       <div>{`The formatted date is ${stringDate}`}</div>
+     );
 
+     MyComponent.propTypes = {
+       intl: intlShape.isRequired,
+     };
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-119. ### Why is a component constructor called only once?
+     export default injectIntl(MyComponent);
+     ```
 
-     React's *reconciliation* algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.
+    **[⬆ Back to Top](#table-of-contents)**
 
+## React Testing
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-120. ### How to define constants in React?
+96. ### What is Shallow Renderer in React testing?
 
-     You can use ES7 `static` field to define constant.
+    _Shallow rendering_ is useful for writing unit test cases in React. It lets you render a component _one level deep_ and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
 
-     ```javascript
-     class MyComponent extends React.Component {
-       static DEFAULT_PAGINATION = 10
-     }
-     ```
-     
+    For example, if you have the following component:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-121. ### How to programmatically trigger click event in React?
+    ```javascript
+    function MyComponent() {
+      return (
+        <div>
+          <span className={"heading"}>{"Title"}</span>
+          <span className={"description"}>{"Description"}</span>
+        </div>
+      );
+    }
+    ```
 
-     You could use the ref prop to acquire a reference to the underlying `HTMLInputElement` object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the `HTMLElement.click` method.
+    Then you can assert as follows:
 
-     This can be done in two steps:
+    ```jsx harmony
+    import ShallowRenderer from "react-test-renderer/shallow";
 
-     1. Create ref in render method:
+    // in your test
+    const renderer = new ShallowRenderer();
+    renderer.render(<MyComponent />);
 
-         ```jsx harmony
-         <input ref={input => this.inputElement = input} />
-         ```
+    const result = renderer.getRenderOutput();
 
-     2. Apply click event in your event handler:
+    expect(result.type).toBe("div");
+    expect(result.props.children).toEqual([
+      <span className={"heading"}>{"Title"}</span>,
+      <span className={"description"}>{"Description"}</span>,
+    ]);
+    ```
 
-         ```javascript
-         this.inputElement.click()
-         ```
+**[⬆ Back to Top](#table-of-contents)**
 
+97. ### What is `TestRenderer` package in React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-122. ### Is it possible to use async/await in plain React?
+    This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or `jsdom`.
 
-     If you want to use `async`/`await` in React, you will need *Babel* and [transform-async-to-generator](https://babeljs.io/docs/en/babel-plugin-transform-async-to-generator) plugin. React Native ships with Babel and a set of transforms.
+    ```jsx harmony
+    import TestRenderer from "react-test-renderer";
 
+    const Link = ({ page, children }) => <a href={page}>{children}</a>;
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-123. ### What are the common folder structures for React?
+    const testRenderer = TestRenderer.create(
+      <Link page={"https://www.facebook.com/"}>{"Facebook"}</Link>
+    );
 
-     There are two common practices for React project file structure.
+    console.log(testRenderer.toJSON());
+    // {
+    //   type: 'a',
+    //   props: { href: 'https://www.facebook.com/' },
+    //   children: [ 'Facebook' ]
+    // }
+    ```
 
-     1. **Grouping by features or routes:**
+**[⬆ Back to Top](#table-of-contents)**
 
-         One common way to structure projects is locate CSS, JS, and tests together, grouped by feature or route.
+98. ### What is the purpose of ReactTestUtils package?
 
-         ```
-         common/
-         ├─ Avatar.js
-         ├─ Avatar.css
-         ├─ APIUtils.js
-         └─ APIUtils.test.js
-         feed/
-         ├─ index.js
-         ├─ Feed.js
-         ├─ Feed.css
-         ├─ FeedStory.js
-         ├─ FeedStory.test.js
-         └─ FeedAPI.js
-         profile/
-         ├─ index.js
-         ├─ Profile.js
-         ├─ ProfileHeader.js
-         ├─ ProfileHeader.css
-         └─ ProfileAPI.js
-         ```
+    _ReactTestUtils_ are provided in the `with-addons` package and allow you to perform actions against a simulated DOM for the purpose of unit testing.
 
-     2. **Grouping by file type:**
+**[⬆ Back to Top](#table-of-contents)**
 
-         Another popular way to structure projects is to group similar files together.
+99. ### What is Jest?
 
-         ```
-         api/
-         ├─ APIUtils.js
-         ├─ APIUtils.test.js
-         ├─ ProfileAPI.js
-         └─ UserAPI.js
-         components/
-         ├─ Avatar.js
-         ├─ Avatar.css
-         ├─ Feed.js
-         ├─ Feed.css
-         ├─ FeedStory.js
-         ├─ FeedStory.test.js
-         ├─ Profile.js
-         ├─ ProfileHeader.js
-         └─ ProfileHeader.css
-         ```
+    _Jest_ is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a `jsdom` environment. It's often used for testing components.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-124. ### What are the popular packages for animation?
+100. ### What are the advantages of Jest over Jasmine?
 
-     *React Transition Group* and *React Motion* are popular animation packages in React ecosystem.
+     There are couple of advantages compared to Jasmine:
 
+     - Automatically finds tests to execute in your source code.
+     - Automatically mocks dependencies when running your tests.
+     - Allows you to test asynchronous code synchronously.
+     - Runs your tests with a fake DOM implementation (via `jsdom`) so that your tests can be run on the command line.
+     - Runs tests in parallel processes so that they finish sooner.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-125. ### What is the benefit of styles modules?
+**[⬆ Back to Top](#table-of-contents)**
 
-     It is recommended to avoid hard coding style values in components. Any values that are likely to be used across different UI components should be extracted into their own modules.
+101. ### Give a simple example of Jest test case
 
-     For example, these styles could be extracted into a separate component:
+     Let's write a test for a function that adds two numbers in `sum.js` file:
 
      ```javascript
-     export const colors = {
-       white,
-       black,
-       blue
-     }
+     const sum = (a, b) => a + b;
 
-     export const space = [
-       0,
-       8,
-       16,
-       32,
-       64
-     ]
+     export default sum;
      ```
 
-     And then imported individually in other components:
+     Create a file named `sum.test.js` which contains actual test:
 
      ```javascript
-     import { space, colors } from './styles'
-     ```
+     import sum from "./sum";
 
+     test("adds 1 + 2 to equal 3", () => {
+       expect(sum(1, 2)).toBe(3);
+     });
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-126. ### What are the popular React-specific linters?
-
-     ESLint is a popular JavaScript linter. There are plugins available that analyse specific code styles. One of the most common for React is an npm package called `eslint-plugin-react`. By default, it will check a number of best practices, with rules checking things from keys in iterators to a complete set of prop types.
-
-     Another popular plugin is `eslint-plugin-jsx-a11y`, which will help fix common issues with accessibility. As JSX offers slightly different syntax to regular HTML, issues with `alt` text and `tabindex`, for example, will not be picked up by regular plugins.
+     And then add the following section to your `package.json`:
 
+     ```json
+     {
+       "scripts": {
+         "test": "jest"
+       }
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-127. ### How to make AJAX call and in which component lifecycle methods should I make an AJAX call?
+     Finally, run `yarn test` or `npm test` and Jest will print a result:
 
-     You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in `fetch`. You should fetch data in the `componentDidMount()` lifecycle method. This is so you can use `setState()` to update your component when the data is retrieved.
+     ```console
+     $ yarn test
+     PASS ./sum.test.js
+     ✓ adds 1 + 2 to equal 3 (2ms)
+     ```
 
-     For example, the employees list fetched from API and set local state:
+## React Redux
 
-     ```jsx harmony
-     class MyComponent extends React.Component {
-       constructor(props) {
-         super(props)
-         this.state = {
-           employees: [],
-           error: null
-         }
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       componentDidMount() {
-         fetch('https://api.example.com/items')
-           .then(res => res.json())
-           .then(
-             (result) => {
-               this.setState({
-                 employees: result.employees
-               })
-             },
-             (error) => {
-               this.setState({ error })
-             }
-           )
-       }
+102. ### What is flux?
 
-       render() {
-         const { error, employees } = this.state
-         if (error) {
-           return <div>Error: {error.message}</div>;
-         } else {
-           return (
-             <ul>
-               {employees.map(employee => (
-                 <li key={employee.name}>
-                   {employee.name}-{employee.experience}
-                 </li>
-               ))}
-             </ul>
-           )
-         }
-       }
-     }
-     ```
+       **Flux** is an **application architecture** (not a framework or library) designed by Facebook to manage **data flow** in React applications. It was created as an alternative to the traditional **MVC (Model-View-Controller)** pattern, and it emphasizes a **unidirectional data flow** to make state changes more predictable and easier to debug.
 
+       Flux complements React by organizing the way data moves through your application, especially in large-scale or complex projects.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-128. ### What are render props?
+       #### Core Concepts of Flux
 
-     **Render Props** is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
+       Flux operates using **four key components**, each with a specific responsibility:
+       *   **Actions**
+             *   Plain JavaScript objects or functions that describe _what happened_ (e.g., user interactions or API responses).
+             *   Example: `{ type: 'ADD_TODO', payload: 'Buy milk' }`
+       *   **Dispatcher**
+             *   A central hub that receives actions and **dispatches** them to the appropriate stores.
+             *   There is **only one dispatcher** in a Flux application.
+       *   **Stores**
+             *   Hold the **application state** and business logic.
+             *   Respond to actions from the dispatcher and update themselves accordingly.
+             *   They **emit change events** that views can listen to.
+       *   **Views (React Components)**
+             *   Subscribe to stores and **re-render** when the data changes.
+             *   They can also trigger new actions (e.g., on user input).
 
-     ```jsx harmony
-     <DataProvider render={data => (
-       <h1>{`Hello ${data.target}`}</h1>
-     )}/>
-     ```
 
-     Libraries such as React Router and DownShift are using this pattern.
+       The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
 
-## React Router
+       ![flux](images/flux.png)
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-129. ### What is React Router?
+103.  ### What is Redux?
+       Redux is a predictable state container for JavaScript applications, most commonly used with React. It helps you manage and centralize your application’s state in a single source of truth, enabling easier debugging, testing, and maintenance—especially in large or complex applications. Redux core is tiny library(about 2.5kB gzipped) and has no dependencies.
 
-     React Router is a powerful routing library built on top of React that helps you add new screens and flows to your application incredibly quickly, all while keeping the URL in sync with what's being displayed on the page.
+**[⬆ Back to Top](#table-of-contents)**
 
+104. ### What are the core principles of Redux?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-130. ### How React Router is different from history library?
+     Redux follows three fundamental principles:
 
-     React Router is a wrapper around the `history` library which handles interaction with the browser's `window.history` with its browser and hash histories. It also provides memory history which is useful for environments that don't have global history, such as mobile app development (React Native) and unit testing with Node.
+     1. **Single source of truth:** The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
+   
+      ```jsx
+      const store = createStore(reducer);
+      ```
+     2. **State is read-only:** The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
+      ```js
+      const action = { type: 'INCREMENT' };
+      store.dispatch(action);
+      ```
+     3. **Changes are made with pure functions(Reducers):** To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.
+      
+      ```jsx
+      function counter(state = 0, action) {
+        switch (action.type) {
+          case 'INCREMENT':
+            return state + 1;
+          case 'DECREMENT':
+            return state - 1;
+          default:
+            return state;
+        }
+      }
+      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-131. ### What are the `<Router>` components of React Router v4?
+105. ### What are the downsides of Redux compared to Flux?
 
-     React Router v4 provides below 3 `<Router>` components:
+     While Redux offers a powerful and predictable state management solution, it comes with a few trade-offs when compared to Flux. These include:
 
-     1. `<BrowserRouter>`
-     2. `<HashRouter>`
-     3. `<MemoryRouter>`
+     1.  **Immutability is essential**  
+        Redux enforces a strict immutability model for state updates, which differs from Flux’s more relaxed approach. This means you must avoid mutating state directly. Many Redux-related libraries assume immutability, so your team must be disciplined in writing pure update logic. You can use tools like `redux-immutable-state-invariant`, `Immer`, or `Immutable.js` to help enforce this practice, especially during development.
+     2.  **Careful selection of complementary packages**  
+        Redux is more minimal by design and provides extension points such as middleware and store enhancers. This has led to a large ecosystem, but it also means you must thoughtfully choose and configure additional packages for features like undo/redo, persistence, or form handling—something Flux explicitly leaves out but may be simpler to manage in smaller setups.
+     3.  **Limited static type integration**  
+        While Flux has mature support for static type checking with tools like Flow, Redux’s type integration is less seamless. Although TypeScript is commonly used with Redux now, early Flow support was limited, and more boilerplate was required for static type safety. This may affect teams that rely heavily on type systems for large codebases.
 
-     The above components will create *browser*, *hash*, and *memory* history instances. React Router v4 makes the properties and methods of the `history` instance associated with your router available through the context in the `router` object.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-132. ### What is the purpose of `push()` and `replace()` methods of `history`?
+106. ### What is the difference between `mapStateToProps()` and `mapDispatchToProps()`?
 
-     A history instance has two methods for navigation purpose.
+     `mapStateToProps()` is a utility which helps your component get updated state (which is updated by some other components):
 
-     1. `push()`
-     2. `replace()`
+     ```javascript
+     const mapStateToProps = (state) => {
+       return {
+         todos: getVisibleTodos(state.todos, state.visibilityFilter),
+       };
+     };
+     ```
 
-     If you think of the history as an array of visited locations, `push()` will add a new location to the array and `replace()` will replace the current location in the array with the new one.
+     `mapDispatchToProps()` is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):
 
+     ```javascript
+     const mapDispatchToProps = (dispatch) => {
+       return {
+         onTodoClick: (id) => {
+           dispatch(toggleTodo(id));
+         },
+       };
+     };
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-133. ### How do you programmatically navigate using React Router v4?
+     It is recommended to always use the “object shorthand” form for the `mapDispatchToProps`.
 
-     There are three different ways to achieve programmatic routing/navigation within components.
+     Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.
 
-     1. **Using the `withRouter()` higher-order function:**
+     ```javascript
+     const mapDispatchToProps = {
+       onTodoClick,
+     };
+     ```
 
-         The `withRouter()` higher-order function will inject the history object as a prop of the component. This object provides `push()` and `replace()` methods to avoid the usage of context.
+**[⬆ Back to Top](#table-of-contents)**
 
-         ```jsx harmony
-         import { withRouter } from 'react-router-dom' // this also works with 'react-router-native'
+107. ### Can I dispatch an action in reducer?
 
-         const Button = withRouter(({ history }) => (
-           <button
-             type='button'
-             onClick={() => { history.push('/new-location') }}
-           >
-             {'Click Me!'}
-           </button>
-         ))
-         ```
+     Dispatching an action within a reducer is an **anti-pattern**. Your reducer should be _without side effects_, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
 
-     2. **Using `<Route>` component and render props pattern:**
+**[⬆ Back to Top](#table-of-contents)**
 
-         The `<Route>` component passes the same props as `withRouter()`, so you will be able to access the history methods through the history prop.
+108. ### How to access Redux store outside a component?
 
-         ```jsx harmony
-         import { Route } from 'react-router-dom'
+     You just need to export the store from the module where it created with `createStore()`. Also, it shouldn't pollute the global window object.
 
-         const Button = () => (
-           <Route render={({ history }) => (
-             <button
-               type='button'
-               onClick={() => { history.push('/new-location') }}
-             >
-               {'Click Me!'}
-             </button>
-           )} />
-         )
-         ```
+     ```javascript
+     store = createStore(myReducer);
 
-     3. **Using context:**
-
-         This option is not recommended and treated as unstable API.
-
-         ```jsx harmony
-         const Button = (props, context) => (
-           <button
-             type='button'
-             onClick={() => {
-               context.history.push('/new-location')
-             }}
-           >
-             {'Click Me!'}
-           </button>
-         )
-
-         Button.contextTypes = {
-           history: React.PropTypes.shape({
-             push: React.PropTypes.func.isRequired
-           })
-         }
-         ```
+     export default store;
+     ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-134. ### How to get query parameters in React Router v4?
+109. ### What are the drawbacks of MVW pattern?
 
-     The ability to parse query strings was taken out of React Router v4 because there have been user requests over the years to support different implementation. So the decision has been given to users to choose the implementation they like. The recommended approach is to use query strings library.
+     1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
+     2. Due to circular dependencies, a complicated model was created around models and views.
+     3. Lot of data changes happens for collaborative applications(like Google Docs).
+     4. No way to do undo (travel back in time) easily without adding so much extra code.
 
-     ```javascript
-     const queryString = require('query-string');
-     const parsed = queryString.parse(props.location.search);
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     You can also use `URLSearchParams` if you want something native:
+110. ### Are there any similarities between Redux and RxJS?
 
-     ```javascript
-     const params = new URLSearchParams(props.location.search)
-     const foo = params.get('name')
-     ```
+     These libraries are very different for very different purposes, but there are some vague similarities.
 
-     You should use a *polyfill* for IE11.
+     Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-135. ### Why you get "Router may have only one child element" warning?
+111. ### How to reset state in Redux?
 
-     You have to wrap your Route's in a `<Switch>` block because `<Switch>` is unique in that it renders a route exclusively.
+     You need to write a _root reducer_ in your application which delegate handling the action to the reducer generated by `combineReducers()`.
 
-     At first you need to add `Switch` to your imports:
+     For example, let us take `rootReducer()` to return the initial state after `USER_LOGOUT` action. As we know, reducers are supposed to return the initial state when they are called with `undefined` as the first argument, no matter the action.
 
      ```javascript
-     import { Switch, Router, Route } from 'react-router'
-     ```
+     const appReducer = combineReducers({
+       /* your app's top-level reducers */
+     });
 
-     Then define the routes within `<Switch>` block:
+     const rootReducer = (state, action) => {
+       if (action.type === "USER_LOGOUT") {
+         state = undefined;
+       }
 
-     ```jsx harmony
-     <Router>
-       <Switch>
-         <Route {/* ... */} />
-         <Route {/* ... */} />
-       </Switch>
-     </Router>
+       return appReducer(state, action);
+     };
      ```
 
+     In case of using `redux-persist`, you may also need to clean your storage. `redux-persist` keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-136. ### How to pass params to `history.push` method in React Router v4?
+     ```javascript
+     const appReducer = combineReducers({
+       /* your app's top-level reducers */
+     });
 
-     While navigating you can pass props to the `history` object:
+     const rootReducer = (state, action) => {
+       if (action.type === "USER_LOGOUT") {
+         Object.keys(state).forEach((key) => {
+           storage.removeItem(`persist:${key}`);
+         });
 
-     ```javascript
-     this.props.history.push({
-       pathname: '/template',
-       search: '?name=sudheer',
-       state: { detail: response.data }
-     })
+         state = undefined;
+       }
+
+       return appReducer(state, action);
+     };
      ```
 
-     The `search` property is used to pass query params in `push()` method.
+**[⬆ Back to Top](#table-of-contents)**
 
+112. ### What is the difference between React context and React Redux?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-137. ### How to implement *default* or *NotFound* page?
+     You can use **Context** in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for.
 
-     A `<Switch>` renders the first child `<Route>` that matches. A `<Route>` with no path always matches. So you just need to simply drop path attribute as below
+     Whereas **Redux** is much more powerful and provides a large number of features that the Context API doesn't provide. Also, React Redux uses context internally but it doesn't expose this fact in the public API.
 
-     ```jsx harmony
-     <Switch>
-       <Route exact path="/" component={Home}/>
-       <Route path="/user" component={User}/>
-       <Route component={NotFound} />
-     </Switch>
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
+113. ### Why are Redux state functions called reducers?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-138. ### How to get history on React Router v4?
-     Below are the list of steps to get history object on React Router v4,
+     Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could _reduce_ a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
 
-     1. Create a module that exports a `history` object and import this module across the project.
+**[⬆ Back to Top](#table-of-contents)**
 
-         For example, create `history.js` file:
+114. ### How to make AJAX request in Redux?
 
-         ```javascript
-         import { createBrowserHistory } from 'history'
+     You can use `redux-thunk` middleware which allows you to define async actions.
 
-         export default createBrowserHistory({
-           /* pass a configuration object here if needed */
-         })
-         ```
+     Let's take an example of fetching specific account as an AJAX call using _fetch API_:
 
-     2. You should use the `<Router>` component instead of built-in routers. Import the above `history.js` inside `index.js` file:
+     ```javascript
+     export function fetchAccount(id) {
+       return (dispatch) => {
+         dispatch(setLoadingAccountState()); // Show a loading spinner
+         fetch(`/account/${id}`, (response) => {
+           dispatch(doneFetchingAccount()); // Hide loading spinner
+           if (response.status === 200) {
+             dispatch(setAccount(response.json)); // Use a normal function to set the received state
+           } else {
+             dispatch(someError);
+           }
+         });
+       };
+     }
 
-         ```jsx harmony
-         import { Router } from 'react-router-dom'
-         import history from './history'
-         import App from './App'
+     function setAccount(data) {
+       return { type: "SET_Account", data: data };
+     }
+     ```
 
-         ReactDOM.render((
-           <Router history={history}>
-             <App />
-           </Router>
-         ), holder)
-         ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     3. You can also use push method of `history` object similar to built-in history object:
+115. ### Should I keep all component's state in Redux store?
 
-         ```javascript
-         // some-other-file.js
-         import history from './history'
+     Keep your data in the Redux store, and the UI related state internally in the component.
 
-         history.push('/go-here')
-         ```
+**[⬆ Back to Top](#table-of-contents)**
 
+116. ### What is the proper way to access Redux store?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-139. ### How to perform automatic redirect after login?
+     The best way to access your store in a component is to use the `connect()` function, that creates a new component that wraps around your existing one. This pattern is called _Higher-Order Components_, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
 
-     The `react-router` package provides `<Redirect>` component in React Router. Rendering a `<Redirect>` will navigate to a new location. Like server-side redirects, the new location will override the current location in the history stack.
+     Let's take an example of `<FilterLink>` component using connect:
 
      ```javascript
-     import React, { Component } from 'react'
-     import { Redirect } from 'react-router'
+     import { connect } from "react-redux";
+     import { setVisibilityFilter } from "../actions";
+     import Link from "../components/Link";
 
-     export default class LoginComponent extends Component {
-       render() {
-         if (this.state.isLoggedIn === true) {
-           return <Redirect to="/your/redirect/page" />
-         } else {
-           return <div>{'Login Please'}</div>
-         }
-       }
-     }
-     ```
+     const mapStateToProps = (state, ownProps) => ({
+       active: ownProps.filter === state.visibilityFilter,
+     });
 
-## React Internationalization
+     const mapDispatchToProps = (dispatch, ownProps) => ({
+       onClick: () => dispatch(setVisibilityFilter(ownProps.filter)),
+     });
 
+     const FilterLink = connect(mapStateToProps, mapDispatchToProps)(Link);
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-140. ### What is React Intl?
+     export default FilterLink;
+     ```
 
-     The *React Intl* library makes internationalization in React straightforward, with off-the-shelf components and an API that can handle everything from formatting strings, dates, and numbers, to pluralization. React Intl is part of *FormatJS* which provides bindings to React via its components and API.
+     Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using `connect()` over accessing the store directly (using context API).
 
+     ```javascript
+     function MyComponent {
+       someMethod() {
+         doSomethingWith(this.context.store);
+       }
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-141. ### What are the main features of React Intl?
-     Below are the main features of React Intl,
+**[⬆ Back to Top](#table-of-contents)**
 
-        1. Display numbers with separators.
-        2. Display dates and times correctly.
-        3. Display dates relative to "now".
-        4. Pluralize labels in strings.
-        5. Support for 150+ languages.
-        6. Runs in the browser and Node.
-        7. Built on standards.
+117. ### What is the difference between component and container in React Redux?
 
+     **Component** is a class or function component that describes the presentational part of your application.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-142. ### What are the two ways of formatting in React Intl?
+     **Container** is an informal term for a component that is connected to a Redux store. Containers _subscribe_ to Redux state updates and _dispatch_ actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
 
-     The library provides two ways to format strings, numbers, and dates:
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. **Using react components:**
+118. ### What is the purpose of the constants in Redux?
 
-         ```jsx harmony
-         <FormattedMessage
-           id={'account'}
-           defaultMessage={'The amount is less than minimum balance.'}
-         />
-         ```
+     Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a `ReferenceError` immediately.
 
-     2. **Using an API:**
+     Normally we will save them in a single file (`constants.js` or `actionTypes.js`).
 
-         ```javascript
-         const messages = defineMessages({
-           accountMessage: {
-             id: 'account',
-             defaultMessage: 'The amount is less than minimum balance.',
-           }
-         })
+     ```javascript
+     export const ADD_TODO = "ADD_TODO";
+     export const DELETE_TODO = "DELETE_TODO";
+     export const EDIT_TODO = "EDIT_TODO";
+     export const COMPLETE_TODO = "COMPLETE_TODO";
+     export const COMPLETE_ALL = "COMPLETE_ALL";
+     export const CLEAR_COMPLETED = "CLEAR_COMPLETED";
+     ```
 
-         formatMessage(messages.accountMessage)
-         ```
+     In Redux, you use them in two places:
 
+     1. **During action creation:**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-143. ### How to use `<FormattedMessage>` as placeholder using React Intl?
+        Let's take `actions.js`:
 
-     The `<Formatted... />` components from `react-intl` return elements, not plain text, so they can't be used for placeholders, alt text, etc. In that case, you should use lower level API `formatMessage()`. You can inject the `intl` object into your component using `injectIntl()` higher-order component and then format the message using `formatMessage()` available on that object.
+        ```javascript
+        import { ADD_TODO } from "./actionTypes";
 
-     ```jsx harmony
-     import React from 'react'
-     import { injectIntl, intlShape } from 'react-intl'
+        export function addTodo(text) {
+          return { type: ADD_TODO, text };
+        }
+        ```
 
-     const MyComponent = ({ intl }) => {
-       const placeholder = intl.formatMessage({id: 'messageId'})
-       return <input placeholder={placeholder} />
-     }
+     2. **In reducers:**
 
-     MyComponent.propTypes = {
-       intl: intlShape.isRequired
-     }
+        Let's create `reducer.js`:
 
-     export default injectIntl(MyComponent)
-     ```
+        ```javascript
+        import { ADD_TODO } from "./actionTypes";
+
+        export default (state = [], action) => {
+          switch (action.type) {
+            case ADD_TODO:
+              return [
+                ...state,
+                {
+                  text: action.text,
+                  completed: false,
+                },
+              ];
+            default:
+              return state;
+          }
+        };
+        ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-144. ### How to access current locale with React Intl?
+119. ### What are the different ways to write `mapDispatchToProps()`?
 
-     You can get the current locale in any component of your application using `injectIntl()`:
+     There are a few ways of binding _action creators_ to `dispatch()` in `mapDispatchToProps()`.
 
-     ```jsx harmony
-     import { injectIntl, intlShape } from 'react-intl'
+     Below are the possible options:
 
-     const MyComponent = ({ intl }) => (
-       <div>{`The current locale is ${intl.locale}`}</div>
-     )
+     ```javascript
+     const mapDispatchToProps = (dispatch) => ({
+       action: () => dispatch(action()),
+     });
+     ```
 
-     MyComponent.propTypes = {
-       intl: intlShape.isRequired
-     }
+     ```javascript
+     const mapDispatchToProps = (dispatch) => ({
+       action: bindActionCreators(action, dispatch),
+     });
+     ```
 
-     export default injectIntl(MyComponent)
+     ```javascript
+     const mapDispatchToProps = { action };
      ```
 
+     The third option is just a shorthand for the first one.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-145. ### How to format date using React Intl?
+**[⬆ Back to Top](#table-of-contents)**
 
-     The `injectIntl()` higher-order component will give you access to the `formatDate()` method via the props in your component. The method is used internally by instances of `FormattedDate` and it returns the string representation of the formatted date.
+120. ### What is the use of the `ownProps` parameter in `mapStateToProps()` and `mapDispatchToProps()`?
+
+     If the `ownProps` parameter is specified, React Redux will pass the props that were passed to the component into your _connect_ functions. So, if you use a connected component:
 
      ```jsx harmony
-     import { injectIntl, intlShape } from 'react-intl'
+     import ConnectedComponent from "./containers/ConnectedComponent";
 
-     const stringDate = this.props.intl.formatDate(date, {
-       year: 'numeric',
-       month: 'numeric',
-       day: 'numeric'
-     })
+     <ConnectedComponent user={"john"} />;
+     ```
 
-     const MyComponent = ({intl}) => (
-       <div>{`The formatted date is ${stringDate}`}</div>
-     )
+     The `ownProps` inside your `mapStateToProps()` and `mapDispatchToProps()` functions will be an object:
 
-     MyComponent.propTypes = {
-       intl: intlShape.isRequired
+     ```javascript
+     {
+       user: "john";
      }
-
-     export default injectIntl(MyComponent)
      ```
 
-## React Testing
+     You can use this object to decide what to return from those functions.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-146. ### What is Shallow Renderer in React testing?
+121. ### How to structure Redux top level directories?
 
-     *Shallow rendering* is useful for writing unit test cases in React. It lets you render a component *one level deep* and assert facts about what its render method returns, without worrying about the behavior of child components, which are not instantiated or rendered.
+     Most of the applications has several top-level directories as below:
 
-     For example, if you have the following component:
+     1. **Components**: Used for _dumb_ components unaware of Redux.
+     2. **Containers**: Used for _smart_ components connected to Redux.
+     3. **Actions**: Used for all action creators, where file names correspond to part of the app.
+     4. **Reducers**: Used for all reducers, where files name correspond to state key.
+     5. **Store**: Used for store initialization.
 
-     ```javascript
-     function MyComponent() {
-       return (
-         <div>
-           <span className={'heading'}>{'Title'}</span>
-           <span className={'description'}>{'Description'}</span>
-         </div>
-       )
-     }
-     ```
+     This structure works well for small and medium size apps.
 
-     Then you can assert as follows:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```jsx harmony
-     import ShallowRenderer from 'react-test-renderer/shallow'
+122. ### What is redux-saga?
 
-     // in your test
-     const renderer = new ShallowRenderer()
-     renderer.render(<MyComponent />)
+     `redux-saga` is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
 
-     const result = renderer.getRenderOutput()
+     It is available in NPM:
 
-     expect(result.type).toBe('div')
-     expect(result.props.children).toEqual([
-       <span className={'heading'}>{'Title'}</span>,
-       <span className={'description'}>{'Description'}</span>
-     ])
+     ```console
+     $ npm install --save redux-saga
      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-147. ### What is `TestRenderer` package in React?
+123. ### What is the mental model of redux-saga?
 
-     This package provides a renderer that can be used to render components to pure JavaScript objects, without depending on the DOM or a native mobile environment. This package makes it easy to grab a snapshot of the platform view hierarchy (similar to a DOM tree) rendered by a ReactDOM or React Native without using a browser or `jsdom`.
+     _Saga_ is like a separate thread in your application, that's solely responsible for side effects. `redux-saga` is a redux _middleware_, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.
 
-     ```jsx harmony
-     import TestRenderer from 'react-test-renderer'
+**[⬆ Back to Top](#table-of-contents)**
 
-     const Link = ({page, children}) => <a href={page}>{children}</a>
+124. ### What are the differences between `call()` and `put()` in redux-saga?
 
-     const testRenderer = TestRenderer.create(
-       <Link page={'https://www.facebook.com/'}>{'Facebook'}</Link>
-     )
+     Both `call()` and `put()` are effect creator functions. `call()` function is used to create effect description, which instructs middleware to call the promise. `put()` function creates an effect, which instructs middleware to dispatch an action to the store.
 
-     console.log(testRenderer.toJSON())
-     // {
-     //   type: 'a',
-     //   props: { href: 'https://www.facebook.com/' },
-     //   children: [ 'Facebook' ]
-     // }
+     Let's take example of how these effects work for fetching particular user data.
+
+     ```javascript
+     function* fetchUserSaga(action) {
+       // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
+       // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
+       const userData = yield call(api.fetchUser, action.userId);
+
+       // Instructing middleware to dispatch corresponding action.
+       yield put({
+         type: "FETCH_USER_SUCCESS",
+         userData,
+       });
+     }
      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-148. ### What is the purpose of ReactTestUtils package?
+125. ### What is Redux Thunk?
 
-     *ReactTestUtils* are provided in the `with-addons` package and allow you to perform actions against a simulated DOM for the purpose of unit testing.
+     _Redux Thunk_ middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods `dispatch()` and `getState()` as parameters.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-149. ### What is Jest?
+126. ### What are the differences between `redux-saga` and `redux-thunk`?
 
-     *Jest* is a JavaScript unit testing framework created by Facebook based on Jasmine and provides automated mock creation and a `jsdom` environment. It's often used for testing components.
+     Both _Redux Thunk_ and _Redux Saga_ take care of dealing with side effects. In most of the scenarios, Thunk uses _Promises_ to deal with them, whereas Saga uses _Generators_. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-150. ### What are the advantages of Jest over Jasmine?
+127. ### What is Redux DevTools?
 
-     There are couple of advantages compared to Jasmine:
+     _Redux DevTools_ is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
 
-     - Automatically finds tests to execute in your source code.
-     - Automatically mocks dependencies when running your tests.
-     - Allows you to test asynchronous code synchronously.
-     - Runs your tests with a fake DOM implementation (via `jsdom`) so that your tests can be run on the command line.
-     - Runs tests in parallel processes so that they finish sooner.
+**[⬆ Back to Top](#table-of-contents)**
 
+128. ### What are the features of Redux DevTools?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-151. ### Give a simple example of Jest test case
+     Some of the main features of Redux DevTools are below,
 
-     Let's write a test for a function that adds two numbers in `sum.js` file:
+     1. Lets you inspect every state and action payload.
+     2. Lets you go back in time by _cancelling_ actions.
+     3. If you change the reducer code, each _staged_ action will be re-evaluated.
+     4. If the reducers throw, you will see during which action this happened, and what the error was.
+     5. With `persistState()` store enhancer, you can persist debug sessions across page reloads.
 
-     ```javascript
-     const sum = (a, b) => a + b
+**[⬆ Back to Top](#table-of-contents)**
 
-     export default sum
-     ```
+129. ### What are Redux selectors and why use them?
 
-     Create a file named `sum.test.js` which contains actual test:
+     _Selectors_ are functions that take Redux state as an argument and return some data to pass to the component.
 
-     ```javascript
-     import sum from './sum'
+     For example, to get user details from the state:
 
-     test('adds 1 + 2 to equal 3', () => {
-       expect(sum(1, 2)).toBe(3)
-     })
+     ```javascript
+     const getUserData = (state) => state.user.data;
      ```
 
-     And then add the following section to your `package.json`:
+     These selectors have two main benefits,
 
-     ```json
-     {
-       "scripts": {
-         "test": "jest"
-       }
-     }
-     ```
+     1. The selector can compute derived data, allowing Redux to store the minimal possible state
+     2. The selector is not recomputed unless one of its arguments changes
 
-     Finally, run `yarn test` or `npm test` and Jest will print a result:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```console
-     $ yarn test
-     PASS ./sum.test.js
-     ✓ adds 1 + 2 to equal 3 (2ms)
-     ```
+130. ### What is Redux Form?
 
-## React Redux
+     _Redux Form_ works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-152. ### What is flux?
+131. ### What are the main features of Redux Form?
 
-     *Flux* is an *application design paradigm* used as a replacement for the more traditional MVC pattern. It is not a framework or a library but a new kind of architecture that complements React and the concept of Unidirectional Data Flow. Facebook uses this pattern internally when working with React.
+     Some of the main features of Redux Form are:
 
-     The workflow between dispatcher, stores and views components with distinct inputs and outputs as follows:
+     1. Field values persistence via Redux store.
+     2. Validation (sync/async) and submission.
+     3. Formatting, parsing and normalization of field values.
 
-     ![flux](images/flux.png)
+**[⬆ Back to Top](#table-of-contents)**
 
+132. ### How to add multiple middlewares to Redux?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-153. ### What is Redux?
+     You can use `applyMiddleware()`.
 
-     *Redux* is a predictable state container for JavaScript apps based on the *Flux design pattern*. Redux can be used together with React, or with any other view library. It is tiny (about 2kB) and has no dependencies.
+     For example, you can add `redux-thunk` and `logger` passing them as arguments to `applyMiddleware()`:
 
+     ```javascript
+     import { createStore, applyMiddleware } from "redux";
+     const createStoreWithMiddleware = applyMiddleware(
+       ReduxThunk,
+       logger
+     )(createStore);
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-154. ### What are the core principles of Redux?
+**[⬆ Back to Top](#table-of-contents)**
 
-     Redux follows three fundamental principles:
+133. ### How to set initial state in Redux?
 
-     1. **Single source of truth:** The state of your whole application is stored in an object tree within a single store. The single state tree makes it easier to keep track of changes over time and debug or inspect the application.
-     2. **State is read-only:** The only way to change the state is to emit an action, an object describing what happened. This ensures that neither the views nor the network callbacks will ever write directly to the state.
-     3. **Changes are made with pure functions:** To specify how the state tree is transformed by actions, you write reducers. Reducers are just pure functions that take the previous state and an action as parameters, and return the next state.
+     You need to pass initial state as second argument to createStore:
 
+     ```javascript
+     const rootReducer = combineReducers({
+       todos: todos,
+       visibilityFilter: visibilityFilter,
+     });
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-155. ### What are the downsides of Redux compared to Flux?
+     const initialState = {
+       todos: [{ id: 123, name: "example", completed: false }],
+     };
 
-     Instead of saying downsides we can say that there are few compromises of using Redux over Flux. Those are as follows:
+     const store = createStore(rootReducer, initialState);
+     ```
 
-     1. **You will need to learn to avoid mutations:** Flux is un-opinionated about mutating data, but Redux doesn't like mutations and many packages complementary to Redux assume you never mutate the state. You can enforce this with dev-only packages like `redux-immutable-state-invariant`, Immutable.js, or instructing your team to write non-mutating code.
-     2. **You're going to have to carefully pick your packages:** While Flux explicitly doesn't try to solve problems such as undo/redo, persistence, or forms, Redux has extension points such as middleware and store enhancers, and it has spawned a rich ecosystem.
-     3. **There is no nice Flow integration yet:** Flux currently lets you do very impressive static type checks which Redux doesn't support yet.
+**[⬆ Back to Top](#table-of-contents)**
 
+134. ### How Relay is different from Redux?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-156. ### What is the difference between `mapStateToProps()` and `mapDispatchToProps()`?
+     Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via _GraphQL_ queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
 
-     `mapStateToProps()` is a utility which helps your component get updated state (which is updated by some other components):
+135. ### What is an action in Redux?
 
-     ```javascript
-     const mapStateToProps = (state) => {
-       return {
-         todos: getVisibleTodos(state.todos, state.visibilityFilter)
-       }
-     }
-     ```
+     _Actions_ are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
 
-     `mapDispatchToProps()` is a utility which will help your component to fire an action event (dispatching action which may cause change of application state):
+     For example, let's take an action which represents adding a new todo item:
 
-     ```javascript
-     const mapDispatchToProps = (dispatch) => {
-       return {
-         onTodoClick: (id) => {
-           dispatch(toggleTodo(id))
-         }
-       }
+     ```
+     {
+       type: ADD_TODO,
+       text: 'Add todo item'
      }
      ```
-     
-     It is recommended to always use the “object shorthand” form for the `mapDispatchToProps`.
-        
-     Redux wraps it in another function that looks like (…args) => dispatch(onTodoClick(…args)), and pass that wrapper function as a prop to your component.
-      
-      ```javascript
-       const mapDispatchToProps = ({
-         onTodoClick
-       })
-      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-157. ### Can I dispatch an action in reducer?
+## React Native
 
-     Dispatching an action within a reducer is an **anti-pattern**. Your reducer should be *without side effects*, simply digesting the action payload and returning a new state object. Adding listeners and dispatching actions within the reducer can lead to chained actions and other side effects.
+**[⬆ Back to Top](#table-of-contents)**
 
+136. ### What is the difference between React Native and React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-158. ### How to access Redux store outside a component?
+     **React** is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
 
-     You just need to export the store from the module where it created with `createStore()`. Also, it shouldn't pollute the global window object.
+     **React Native** is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
 
-     ```javascript
-     store = createStore(myReducer)
+**[⬆ Back to Top](#table-of-contents)**
 
-     export default store
-     ```
+137. ### How to test React Native apps?
 
+     React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-159. ### What are the drawbacks of MVW pattern?
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. DOM manipulation is very expensive which causes applications to behave slow and inefficient.
-     3. Due to circular dependencies, a complicated model was created around models and views.
-     3. Lot of data changes happens for collaborative applications(like Google Docs).
-     4. No way to do undo (travel back in time) easily without adding so much extra code.
+138. ### How to do logging in React Native?
 
+     You can use `console.log`, `console.warn`, etc. As of React Native v0.29 you can simply run the following to see logs in the console:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-160. ### Are there any similarities between Redux and RxJS?
+     ```
+     $ react-native log-ios
+     $ react-native log-android
+     ```
 
-     These libraries are very different for very different purposes, but there are some vague similarities.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Redux is a tool for managing state throughout the application. It is usually used as an architecture for UIs. Think of it as an alternative to (half of) Angular. RxJS is a reactive programming library. It is usually used as a tool to accomplish asynchronous tasks in JavaScript. Think of it as an alternative to Promises. Redux uses the Reactive paradigm because the Store is reactive. The Store observes actions from a distance, and changes itself. RxJS also uses the Reactive paradigm, but instead of being an architecture, it gives you basic building blocks, Observables, to accomplish this pattern.
+139. ### How to debug your React Native?
 
+     Follow the below steps to debug React Native app:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-161. ### How to dispatch an action on load?
+     1. Run your application in the iOS simulator.
+     2. Press `Command + D` and a webpage should open up at `http://localhost:8081/debugger-ui`.
+     3. Enable _Pause On Caught Exceptions_ for a better debugging experience.
+     4. Press `Command + Option + I` to open the Chrome Developer tools, or open it via `View` -> `Developer` -> `Developer Tools`.
+     5. You should now be able to debug as you normally would.
 
-     You can dispatch an action in `componentDidMount()` method and in `render()` method you can verify the data.
+## React supported libraries & Integration
 
-     ```javascript
-     class App extends Component {
-       componentDidMount() {
-         this.props.fetchData()
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       render() {
-         return this.props.isLoaded
-           ? <div>{'Loaded'}</div>
-           : <div>{'Not Loaded'}</div>
-       }
-     }
+140. ### What is reselect and how it works?
 
-     const mapStateToProps = (state) => ({
-       isLoaded: state.isLoaded
-     })
+     _Reselect_ is a **selector library** (for Redux) which uses _memoization_ concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.
 
-     const mapDispatchToProps = { fetchData }
+     Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
 
-     export default connect(mapStateToProps, mapDispatchToProps)(App)
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
+141. ### What is Flow?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-162. ### How to use `connect()` from React Redux?
+     _Flow_ is a _static type checker_ designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving `null`, unlike most type systems.
 
-     You need to follow two steps to use your store in your container:
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. **Use `mapStateToProps()`:** It maps the state variables from your store to the props that you specify.
-     2. **Connect the above props to your container:** The object returned by the `mapStateToProps` function is connected to the container. You can import `connect()` from `react-redux`.
+142. ### What is the difference between Flow and PropTypes?
 
-         ```jsx harmony
-         import React from 'react'
-         import { connect } from 'react-redux'
+     Flow is a _static analysis tool_ (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
 
-         class App extends React.Component {
-           render() {
-             return <div>{this.props.containerData}</div>
-           }
-         }
+     PropTypes is a _basic type checker_ (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
 
-         function mapStateToProps(state) {
-           return { containerData: state.data }
-         }
+**[⬆ Back to Top](#table-of-contents)**
 
-         export default connect(mapStateToProps)(App)
-         ```
+143. ### How to use Font Awesome icons in React?
 
+     The below steps followed to include Font Awesome in React:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-163. ### How to reset state in Redux?
+     1. Install `font-awesome`:
 
-     You need to write a *root reducer* in your application which delegate handling the action to the reducer generated by `combineReducers()`.
+        ```console
+        $ npm install --save font-awesome
+        ```
 
-     For example, let us take `rootReducer()` to return the initial state after `USER_LOGOUT` action. As we know, reducers are supposed to return the initial state when they are called with `undefined` as the first argument, no matter the action.
+     2. Import `font-awesome` in your `index.js` file:
 
-     ```javascript
-     const appReducer = combineReducers({
-       /* your app's top-level reducers */
-     })
+        ```javascript
+        import "font-awesome/css/font-awesome.min.css";
+        ```
 
-     const rootReducer = (state, action) => {
-       if (action.type === 'USER_LOGOUT') {
-         state = undefined
-       }
+     3. Add Font Awesome classes in `className`:
 
-       return appReducer(state, action)
-     }
-     ```
+        ```javascript
+        function MyComponent {
+          return <div><i className={'fa fa-spinner'} /></div>
+        }
+        ```
 
-     In case of using `redux-persist`, you may also need to clean your storage. `redux-persist` keeps a copy of your state in a storage engine. First, you need to import the appropriate storage engine and then, to parse the state before setting it to undefined and clean each storage state key.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     const appReducer = combineReducers({
-       /* your app's top-level reducers */
-     })
+144. ### What is React Dev Tools?
 
-     const rootReducer = (state, action) => {
-       if (action.type === 'USER_LOGOUT') {
-         Object.keys(state).forEach(key => {
-           storage.removeItem(`persist:${key}`)
-         })
+     _React Developer Tools_ let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
 
-         state = undefined
-       }
+     The official extensions available for different browsers or environments.
 
-       return appReducer(state, action)
-     }
-     ```
+     1. **Chrome extension**
+     2. **Firefox extension**
+     3. **Standalone app** (Safari, React Native, etc)
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-164. ### Whats the purpose of `at` symbol in the Redux connect decorator?
+145. ### Why is DevTools not loading in Chrome for local files?
 
-     The **@** symbol is in fact a JavaScript expression used to signify decorators. *Decorators* make it possible to annotate and modify classes and properties at design time.
+     If you opened a local HTML file in your browser (`file://...`) then you must first open _Chrome Extensions_ and check `Allow access to file URLs`.
 
-     Let's take an example setting up Redux without and with a decorator.
+**[⬆ Back to Top](#table-of-contents)**
 
-     * **Without decorator:**
+146. ### How to use Polymer in React?
 
-         ```javascript
-         import React from 'react'
-         import * as actionCreators from './actionCreators'
-         import { bindActionCreators } from 'redux'
-         import { connect } from 'react-redux'
+     You need to follow below steps to use Polymer in React,
 
-         function mapStateToProps(state) {
-           return { todos: state.todos }
-         }
+     1. Create a Polymer element:
 
-         function mapDispatchToProps(dispatch) {
-           return { actions: bindActionCreators(actionCreators, dispatch) }
-         }
+        ```jsx harmony
+        <link
+          rel="import"
+          href="../../bower_components/polymer/polymer.html"
+        />;
+        Polymer({
+          is: "calendar-element",
+          ready: function () {
+            this.textContent = "I am a calendar";
+          },
+        });
+        ```
 
-         class MyApp extends React.Component {
-           // ...define your main app here
-         }
+     2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the `index.html` of your React application:
 
-         export default connect(mapStateToProps, mapDispatchToProps)(MyApp)
-         ```
+        ```html
+        <link
+          rel="import"
+          href="./src/polymer-components/calendar-element.html"
+        />
+        ```
 
-     * **With decorator:**
+     3. Use that element in the JSX file:
 
-         ```javascript
-         import React from 'react'
-         import * as actionCreators from './actionCreators'
-         import { bindActionCreators } from 'redux'
-         import { connect } from 'react-redux'
+        ```javascript
+        export default function MyComponent {
+          return <calendar-element />;
+        }
+        ```
 
-         function mapStateToProps(state) {
-           return { todos: state.todos }
-         }
+**[⬆ Back to Top](#table-of-contents)**
 
-         function mapDispatchToProps(dispatch) {
-           return { actions: bindActionCreators(actionCreators, dispatch) }
-         }
+147. ### What are the advantages of React over Vue.js?
 
-         @connect(mapStateToProps, mapDispatchToProps)
-         export default class MyApp extends React.Component {
-           // ...define your main app here
-         }
-         ```
+     React has the following advantages over Vue.js:
 
-     The above examples are almost similar except the usage of decorator. The decorator syntax isn't built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.
+     1. Gives more flexibility in large apps developing.
+     2. Easier to test.
+     3. Suitable for mobile apps creating.
+     4. More information and solutions available.
 
+**Note:** The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-165. ### What is the difference between React context and React Redux?
+**[⬆ Back to Top](#table-of-contents)**
 
-     You can use **Context** in your application directly and is going to be great for passing down data to deeply nested components which what it was designed for.
+148. ### What is the difference between React and Angular?
 
-     Whereas **Redux** is much more powerful and provides a large number of features that the Context API doesn't provide. Also, React Redux uses context internally but it doesn't expose this fact in the public API.
+     Let's see the difference between React and Angular in a table format.
 
+     | React                                                                                       | Angular                                                                                                                            |
+     | ------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------- |
+     | React is a library and has only the View layer                                              | Angular is a framework and has complete MVC functionality                                                                          |
+     | React handles rendering on the server side                                                  | AngularJS renders only on the client side but Angular 2 and above renders on the server side                                       |
+     | React uses JSX that looks like HTML in JS which can be confusing                            | Angular follows the template approach for HTML, which makes code shorter and easy to understand                                    |
+     | React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower                                                            |
+     | In React, data flows only in one way and hence debugging is easy                            | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-166. ### Why are Redux state functions called reducers?
+**Note:** The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
 
-     Reducers always return the accumulation of the state (based on all previous and current actions). Therefore, they act as a reducer of state. Each time a Redux reducer is called, the state and action are passed as parameters. This state is then reduced (or accumulated) based on the action, and then the next state is returned. You could *reduce* a collection of actions and an initial state (of the store) on which to perform these actions to get the resulting final state.
+**[⬆ Back to Top](#table-of-contents)**
 
+149. ### Why React tab is not showing up in DevTools?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-167. ### How to make AJAX request in Redux?
+     When the page loads, _React DevTools_ sets a global named `__REACT_DEVTOOLS_GLOBAL_HOOK__`, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.
 
-     You can use `redux-thunk` middleware which allows you to define async actions.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Let's take an example of fetching specific account as an AJAX call using *fetch API*:
+150. ### What are Styled Components?
 
-     ```javascript
-     export function fetchAccount(id) {
-       return dispatch => {
-         dispatch(setLoadingAccountState()) // Show a loading spinner
-         fetch(`/account/${id}`, (response) => {
-           dispatch(doneFetchingAccount()) // Hide loading spinner
-           if (response.status === 200) {
-             dispatch(setAccount(response.json)) // Use a normal function to set the received state
-           } else {
-             dispatch(someError)
-           }
-         })
-       }
-     }
+     `styled-components` is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.
 
-     function setAccount(data) {
-      return { type: 'SET_Account', data: data }
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
+151. ### Give an example of Styled Components?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-168. ### Should I keep all component's state in Redux store?
+     Lets create `<Title>` and `<Wrapper>` components with specific styles for each.
 
-      Keep your data in the Redux store, and the UI related state internally in the component.
+     ```javascript
+     import React from "react";
+     import styled from "styled-components";
 
+     // Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
+     const Title = styled.h1`
+       font-size: 1.5em;
+       text-align: center;
+       color: palevioletred;
+     `;
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-169. ### What is the proper way to access Redux store?
+     // Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
+     const Wrapper = styled.section`
+       padding: 4em;
+       background: papayawhip;
+     `;
+     ```
 
-     The best way to access your store in a component is to use the `connect()` function, that creates a new component that wraps around your existing one. This pattern is called *Higher-Order Components*, and is generally the preferred way of extending a component's functionality in React. This allows you to map state and action creators to your component, and have them passed in automatically as your store updates.
+     These two variables, `Title` and `Wrapper`, are now components that you can render just like any other react component.
 
-     Let's take an example of `<FilterLink>` component using connect:
+     ```jsx harmony
+     <Wrapper>
+       <Title>{"Lets start first styled component!"}</Title>
+     </Wrapper>
+     ```
 
-     ```javascript
-     import { connect } from 'react-redux'
-     import { setVisibilityFilter } from '../actions'
-     import Link from '../components/Link'
+**[⬆ Back to Top](#table-of-contents)**
 
-     const mapStateToProps = (state, ownProps) => ({
-       active: ownProps.filter === state.visibilityFilter
-     })
+152. ### What is Relay?
 
-     const mapDispatchToProps = (dispatch, ownProps) => ({
-       onClick: () => dispatch(setVisibilityFilter(ownProps.filter))
-     })
+     Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
 
-     const FilterLink = connect(
-       mapStateToProps,
-       mapDispatchToProps
-     )(Link)
+**[⬆ Back to Top](#table-of-contents)**
 
-     export default FilterLink
-     ```
-
-     Due to it having quite a few performance optimizations and generally being less likely to cause bugs, the Redux developers almost always recommend using `connect()` over accessing the store directly (using context API).
+## Miscellaneous
 
-     ```javascript
-     class MyComponent {
-       someMethod() {
-         doSomethingWith(this.context.store)
-       }
-     }
-     ```
+153. ### What are the main features of Reselect library?
 
+     Let's see the main features of Reselect library,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-170. ### What is the difference between component and container in React Redux?
+     1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
+     2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
+     3. Selectors are composable. They can be used as input to other selectors.
 
-     **Component** is a class or function component that describes the presentational part of your application.
+154. #### Give an example of Reselect usage?
 
-     **Container** is an informal term for a component that is connected to a Redux store. Containers *subscribe* to Redux state updates and *dispatch* actions, and they usually don't render DOM elements; they delegate rendering to presentational child components.
+     Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:
 
+     ```javascript
+     import { createSelector } from "reselect";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-171. ### What is the purpose of the constants in Redux?
+     const shopItemsSelector = (state) => state.shop.items;
+     const taxPercentSelector = (state) => state.shop.taxPercent;
 
-     Constants allows you to easily find all usages of that specific functionality across the project when you use an IDE. It also prevents you from introducing silly bugs caused by typos – in which case, you will get a `ReferenceError` immediately.
+     const subtotalSelector = createSelector(shopItemsSelector, (items) =>
+       items.reduce((acc, item) => acc + item.value, 0)
+     );
 
-     Normally we will save them in a single file (`constants.js` or `actionTypes.js`).
+     const taxSelector = createSelector(
+       subtotalSelector,
+       taxPercentSelector,
+       (subtotal, taxPercent) => subtotal * (taxPercent / 100)
+     );
 
-     ```javascript
-     export const ADD_TODO = 'ADD_TODO'
-     export const DELETE_TODO = 'DELETE_TODO'
-     export const EDIT_TODO = 'EDIT_TODO'
-     export const COMPLETE_TODO = 'COMPLETE_TODO'
-     export const COMPLETE_ALL = 'COMPLETE_ALL'
-     export const CLEAR_COMPLETED = 'CLEAR_COMPLETED'
-     ```
+     export const totalSelector = createSelector(
+       subtotalSelector,
+       taxSelector,
+       (subtotal, tax) => ({ total: subtotal + tax })
+     );
 
-     In Redux, you use them in two places:
+     let exampleState = {
+       shop: {
+         taxPercent: 8,
+         items: [
+           { name: "apple", value: 1.2 },
+           { name: "orange", value: 0.95 },
+         ],
+       },
+     };
 
-     1. **During action creation:**
+     console.log(subtotalSelector(exampleState)); // 2.15
+     console.log(taxSelector(exampleState)); // 0.172
+     console.log(totalSelector(exampleState)); // { total: 2.322 }
+     ```
 
-         Let's take `actions.js`:
+**[⬆ Back to Top](#table-of-contents)**
 
-         ```javascript
-         import { ADD_TODO } from './actionTypes';
+155. ### Can Redux only be used with React?
 
-         export function addTodo(text) {
-           return { type: ADD_TODO, text }
-         }
-         ```
+     Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
 
-     2. **In reducers:**
+**[⬆ Back to Top](#table-of-contents)**
 
-         Let's create `reducer.js`:
-
-         ```javascript
-         import { ADD_TODO } from './actionTypes'
-
-         export default (state = [], action) => {
-           switch (action.type) {
-             case ADD_TODO:
-               return [
-                 ...state,
-                 {
-                   text: action.text,
-                   completed: false
-                 }
-               ];
-             default:
-               return state
-           }
-         }
-         ```
+156. ### Do you need to have a particular build tool to use Redux?
 
+     Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-172. ### What are the different ways to write `mapDispatchToProps()`?
+**[⬆ Back to Top](#table-of-contents)**
 
-     There are a few ways of binding *action creators* to `dispatch()` in `mapDispatchToProps()`.
+157. ### How Redux Form `initialValues` get updated from state?
 
-     Below are the possible options:
+     You need to add `enableReinitialize : true` setting.
 
      ```javascript
-     const mapDispatchToProps = (dispatch) => ({
-      action: () => dispatch(action())
-     })
+     const InitializeFromStateForm = reduxForm({
+       form: "initializeFromState",
+       enableReinitialize: true,
+     })(UserEdit);
      ```
 
-     ```javascript
-     const mapDispatchToProps = (dispatch) => ({
-      action: bindActionCreators(action, dispatch)
-     })
-     ```
+     If your `initialValues` prop gets updated, your form will update too.
 
-     ```javascript
-     const mapDispatchToProps = { action }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     The third option is just a shorthand for the first one.
+158. ### How React PropTypes allow different types for one prop?
 
+     You can use `oneOfType()` method of `PropTypes`.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-173. ### What is the use of the `ownProps` parameter in `mapStateToProps()` and `mapDispatchToProps()`?
+     For example, the height property can be defined with either `string` or `number` type as below:
 
-     If the `ownProps` parameter is specified, React Redux will pass the props that were passed to the component into your *connect* functions. So, if you use a connected component:
+     ```javascript
+     Component.propTypes = {
+       size: PropTypes.oneOfType([PropTypes.string, PropTypes.number]),
+     };
+     ```
 
-     ```jsx harmony
-     import ConnectedComponent from './containers/ConnectedComponent';
+**[⬆ Back to Top](#table-of-contents)**
 
-     <ConnectedComponent user={'john'} />
-     ```
+159. ### Can I import an SVG file as react component?
 
-     The `ownProps` inside your `mapStateToProps()` and `mapDispatchToProps()` functions will be an object:
+     You can import SVG directly as component instead of loading it as a file. This feature is available with `react-scripts@2.0.0` and higher.
 
-     ```javascript
-     { user: 'john' }
+     ```jsx harmony
+     import { ReactComponent as Logo } from "./logo.svg";
+
+     const App = () => (
+       <div>
+         {/* Logo is an actual react component */}
+         <Logo />
+       </div>
+     );
      ```
 
-     You can use this object to decide what to return from those functions.
+     **Note**: Don't forget about the curly braces in the import.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-174. ### How to structure Redux top level directories?
+160. ### What is render hijacking in react?
 
-     Most of the applications has several top-level directories as below:
+     The concept of render hijacking is the ability to control what a component will output from another component. It means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
 
-     1. **Components**: Used for *dumb* components unaware of Redux.
-     2. **Containers**: Used for *smart* components connected to Redux.
-     3. **Actions**: Used for all action creators, where file names correspond to part of the app.
-     4. **Reducers**: Used for all reducers, where files name correspond to state key.
-     5. **Store**: Used for store initialization.
+**[⬆ Back to Top](#table-of-contents)**
 
-     This structure works well for small and medium size apps.
+161. ### How to pass numbers to React component?
 
+     We can pass `numbers` as `props` to React component using curly braces `{}` where as `strings` in double quotes `""` or single quotes `''`
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-175. ### What is redux-saga?
+     ```jsx
+     import React from "react";
 
-     `redux-saga` is a library that aims to make side effects (asynchronous things like data fetching and impure things like accessing the browser cache) in React/Redux applications easier and better.
+     const ChildComponent = ({ name, age }) => {
+       return (
+         <>
+           My Name is {name} and Age is {age}
+         </>
+       );
+     };
 
-     It is available in NPM:
+     const ParentComponent = () => {
+       return (
+         <>
+           <ChildComponent name="Chetan" age={30} />
+         </>
+       );
+     };
 
-     ```console
-     $ npm install --save redux-saga
+     export default ParentComponent;
      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-176. ### What is the mental model of redux-saga?
-
-     *Saga* is like a separate thread in your application, that's solely responsible for side effects. `redux-saga` is a redux *middleware*, which means this thread can be started, paused and cancelled from the main application with normal Redux actions, it has access to the full Redux application state and it can dispatch Redux actions as well.
+162. ### Do I need to keep all my state into Redux? Should I ever use react internal state?
 
+     It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-177. ### What are the differences between `call()` and `put()` in redux-saga?
+     Below are the rules of thumb to determine what kind of data should be put into Redux
 
-     Both `call()` and `put()` are effect creator functions. `call()` function is used to create effect description, which instructs middleware to call the promise. `put()` function creates an effect, which instructs middleware to dispatch an action to the store.
+     1. Do other parts of the application care about this data?
+     2. Do you need to be able to create further derived data based on this original data?
+     3. Is the same data being used to drive multiple components?
+     4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
+     5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?
 
-     Let's take example of how these effects work for fetching particular user data.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     function* fetchUserSaga(action) {
-       // `call` function accepts rest arguments, which will be passed to `api.fetchUser` function.
-       // Instructing middleware to call promise, it resolved value will be assigned to `userData` variable
-       const userData = yield call(api.fetchUser, action.userId)
+163. ### What is the purpose of registerServiceWorker in React?
 
-       // Instructing middleware to dispatch corresponding action.
-       yield put({
-         type: 'FETCH_USER_SUCCESS',
-         userData
-       })
-     }
-     ```
+     React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.
 
+     ```jsx
+     import React from "react";
+     import ReactDOM from "react-dom";
+     import App from "./App";
+     import registerServiceWorker from "./registerServiceWorker";
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-178. ### What is Redux Thunk?
+     ReactDOM.render(<App />, document.getElementById("root"));
+     registerServiceWorker();
+     ```
 
-     *Redux Thunk* middleware allows you to write action creators that return a function instead of an action. The thunk can be used to delay the dispatch of an action, or to dispatch only if a certain condition is met. The inner function receives the store methods `dispatch()` and `getState()` as parameters.
+**[⬆ Back to Top](#table-of-contents)**
 
+164. ### What is React memo function?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-179. ### What are the differences between `redux-saga` and `redux-thunk`?
+     Class components can be restricted from re-rendering when their input props are the same using **PureComponent or shouldComponentUpdate**. Now you can do the same with function components by wrapping them in **React.memo**.
 
-     Both *Redux Thunk* and *Redux Saga* take care of dealing with side effects. In most of the scenarios, Thunk uses *Promises* to deal with them, whereas Saga uses *Generators*. Thunk is simple to use and Promises are familiar to many developers, Sagas/Generators are more powerful but you will need to learn them. But both middleware can coexist, so you can start with Thunks and introduce Sagas when/if you need them.
+     ```jsx
+     const MyComponent = React.memo(function MyComponent(props) {
+       /* only rerenders if props change */
+     });
+     ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-180. ### What is Redux DevTools?
+165. ### What is React lazy function?
 
-     *Redux DevTools* is a live-editing time travel environment for Redux with hot reloading, action replay, and customizable UI. If you don't want to bother with installing Redux DevTools and integrating it into your project, consider using Redux DevTools Extension for Chrome and Firefox.
+     The `React.lazy` function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the `OtherComponent` when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
 
+     ```jsx
+     const OtherComponent = React.lazy(() => import("./OtherComponent"));
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-181. ### What are the features of Redux DevTools?
-     Some of the main features of Redux DevTools are below,
+     function MyComponent() {
+       return (
+         <div>
+           <OtherComponent />
+         </div>
+       );
+     }
+     ```
 
-        1. Lets you inspect every state and action payload.
-        2. Lets you go back in time by *cancelling* actions.
-        3. If you change the reducer code, each *staged* action will be re-evaluated.
-        4. If the reducers throw, you will see during which action this happened, and what the error was.
-        5. With `persistState()` store enhancer, you can persist debug sessions across page reloads.
+     **Note:**
+     `React.lazy` and `Suspense` is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-182. ### What are Redux selectors and why to use them?
+166. ### How to prevent unnecessary updates using setState?
 
-     *Selectors* are functions that take Redux state as an argument and return some data to pass to the component.
+     You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return **null** to stop re-rendering otherwise return the latest state value.
 
-     For example, to get user details from the state:
+     For example, the user profile information is conditionally rendered as follows,
 
-     ```javascript
-     const getUserData = state => state.user.data
+     ```jsx
+     getUserProfile = (user) => {
+       const latestAddress = user.address;
+       this.setState((state) => {
+         if (state.address === latestAddress) {
+           return null;
+         } else {
+           return { title: latestAddress };
+         }
+       });
+     };
      ```
 
-     These selectors have two main benefits,
-
-     1. The selector can compute derived data, allowing Redux to store the minimal possible state
-     2. The selector is not recomputed unless one of its arguments changes
-
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-183. ### What is Redux Form?
+167. ### How do you render Array, Strings and Numbers in React 16 Version?
 
-     *Redux Form* works with React and Redux to enable a form in React to use Redux to store all of its state. Redux Form can be used with raw HTML5 inputs, but it also works very well with common UI frameworks like Material UI, React Widgets and React Bootstrap.
+     **Arrays**: Unlike older releases, you don't need to make sure **render** method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.
 
+     For example, let us take the below list of developers,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-184. ### What are the main features of Redux Form?
-     Some of the main features of Redux Form are:
+     ```jsx
+     const ReactJSDevs = () => {
+       return [
+         <li key="1">John</li>,
+         <li key="2">Jackie</li>,
+         <li key="3">Jordan</li>,
+       ];
+     };
+     ```
 
-       1. Field values persistence via Redux store.
-       2. Validation (sync/async) and submission.
-       3. Formatting, parsing and normalization of field values.
+     You can also merge this array of items in another array component.
 
+     ```jsx
+     const JSDevs = () => {
+       return (
+         <ul>
+           <li>Brad</li>
+           <li>Brodge</li>
+           <ReactJSDevs />
+           <li>Brandon</li>
+         </ul>
+       );
+     };
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-185. ### How to add multiple middlewares to Redux?
+     **Strings and Numbers:** You can also return string and number type from the render method.
 
-     You can use `applyMiddleware()`.
+     ```jsx
+     render() {
+      return 'Welcome to ReactJS questions';
+     }
+     // Number
+     render() {
+      return 2018;
+     }
+     ```
 
-     For example, you can add `redux-thunk` and `logger` passing them as arguments to `applyMiddleware()`:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     import { createStore, applyMiddleware } from 'redux'
-     const createStoreWithMiddleware = applyMiddleware(ReduxThunk, logger)(createStore)
-     ```
+168. ### What are hooks?
 
+     Hooks is a special JavaScript function that allows you use state and other React features without writing a class. This pattern has been introduced as a new feature in React 16.8 and helped to isolate the stateful logic from the components.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-186. ### How to set initial state in Redux?
+     Let's see an example of useState hook:
 
-     You need to pass initial state as second argument to createStore:
+     ```jsx
+     import { useState } from "react";
 
-     ```javascript
-     const rootReducer = combineReducers({
-       todos: todos,
-       visibilityFilter: visibilityFilter
-     })
+     function Example() {
+       // Declare a new state variable, which we'll call "count"
+       const [count, setCount] = useState(0);
 
-     const initialState = {
-       todos: [{ id: 123, name: 'example', completed: false }]
+       return (
+         <>
+           <p>You clicked {count} times</p>
+           <button onClick={() => setCount(count + 1)}>Click me</button>
+         </>
+       );
      }
-
-     const store = createStore(
-       rootReducer,
-       initialState
-     )
      ```
 
+     **Note:** Hooks can be used inside an existing function component without rewriting the component.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-187. ### How Relay is different from Redux?
+**[⬆ Back to Top](#table-of-contents)**
 
-     Relay is similar to Redux in that they both use a single store. The main difference is that relay only manages state originated from the server, and all access to the state is used via *GraphQL* queries (for reading data) and mutations (for changing data). Relay caches the data for you and optimizes data fetching for you, by fetching only changed data and nothing more.
+169. ### What rules need to be followed for hooks?
 
-188. ### What is an action in Redux?
+     You need to follow two rules in order to use hooks,
 
-     *Actions* are plain JavaScript objects or payloads of information that send data from your application to your store. They are the only source of information for the store. Actions must have a type property that indicates the type of action being performed.
+     1. **Call Hooks only at the top level of your react functions:** You should always use hooks at the top level of react function before any early returns. i.e, You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple re-renders due to `useState` and `useEffect` calls.
 
-     For example, let's take an action which represents adding a new todo item:
+     Let's see the difference using an example,
+     **Correct usage:**:
+     ```jsx
+     function UserProfile() {
+      // Correct: Hooks called at the top level
+      const [name, setName] = useState('John');
+      const [country, setCountry] = useState('US');
 
-     ```
-     {
-       type: ADD_TODO,
-       text: 'Add todo item'
+      return (
+        <div>
+          <h1>Name: {name}</h1>
+          <p>Country: {country}</p>
+        </div>
+      );
      }
      ```
+     **Incorrect usage:**:
+     ```jsx
+     function UserProfile() {
+      const [name, setName] = useState('John');
 
+      if (name === 'John') {
+        // Incorrect: useState is called inside a conditional
+        const [country, setCountry] = useState('US'); 
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
+      return (
+        <div>
+          <h1>Name: {name}</h1>
+          <p>Country: {country}</p> {/* This will throw an error if the name condition isn't met */}
+        </div>
+      );
+     }
+     ```
+     The `useState` hook for the country field is being called conditionally within an `if` block. This can lead to inconsistent state behavior and may cause hooks to be called in a different order on each re-render.
 
-## React Native
+     2. **Call Hooks from React Functions only:** You shouldn’t call Hooks from regular JavaScript functions or class components. Instead, you should call them from either function components or custom hooks.
 
+     Let's find the difference of correct and incorrect usage with below examples,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-188. ### What is the difference between React Native and React?
-
-     **React** is a JavaScript library, supporting both front end web and being run on the server, for building user interfaces and web applications.
+     **Correct usage:**:
+     ```jsx
+     //Example1:
+     function Counter() {
+      // Correct: useState is used inside a functional component
+      const [count, setCount] = useState(0);
 
-     **React Native** is a mobile framework that compiles to native app components, allowing you to build native mobile applications (iOS, Android, and Windows) in JavaScript that allows you to use React to build your components, and implements React under the hood.
+      return <div>Counter: {count}</div>;
+     }
+     //Example2:
+     function useFetchData(url) {
+      const [data, setData] = useState(null);
 
+      useEffect(() => {
+        fetch(url)
+          .then((response) => response.json())
+          .then((data) => setData(data));
+      }, [url]);
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-189. ### How to test React Native apps?
+      return data;
+     }
 
-     React Native can be tested only in mobile simulators like iOS and Android. You can run the app in your mobile using expo app (https://expo.io) Where it syncs using QR code, your mobile and computer should be in same wireless network.
+     function UserProfile() {
+      // Correct: Using a custom hook here
+      const user = useFetchData('https://some-api.com/user');
 
+      return (
+        <div>
+          <h1>{user ? user.name : 'Loading profile...'}</h1>
+        </div>
+      );
+     }
+     ```
+     **Incorrect usage:**:
+     ```jsx
+      //Example1
+      function normalFunction() {
+        // Incorrect: Can't call hooks in normal functions
+        const [count, setCount] = useState(0); 
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-190. ### How to do logging in React Native?
+      //Example2
+      function fetchData(url) {
+        // Incorrect: Hooks can't be used in non-React functions
+        const [data, setData] = useState(null);
 
-     You can use `console.log`, `console.warn`, etc. As of React Native v0.29 you can simply run the following to see logs in the console:
+        useEffect(() => {
+          fetch(url)
+            .then((response) => response.json())
+            .then((data) => setData(data));
+        }, [url]);
 
+        return data;
+      }
      ```
-     $ react-native log-ios
-     $ react-native log-android
-     ```
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-191. ### How to debug your React Native?
+     In the above incorrect usage example, both `useState` and `useEffect` are used in non-React functions(`normalFunction` and `fetchData`), which is not allowed.
 
-     Follow the below steps to debug React Native app:
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. Run your application in the iOS simulator.
-     2. Press `Command + D` and a webpage should open up at `http://localhost:8081/debugger-ui`.
-     3. Enable *Pause On Caught Exceptions* for a better debugging experience.
-     4. Press `Command + Option + I` to open the Chrome Developer tools, or open it via `View` -> `Developer` -> `Developer Tools`.
-     5. You should now be able to debug as you normally would.
+170.   ### How to ensure hooks followed the rules in your project?
 
-## React supported libraries & Integration
+        React team released an ESLint plugin called **eslint-plugin-react-hooks** that enforces Hook's two rules. It is part of Hooks API. You can add this plugin to your project using the below command,
 
+        ```javascript
+        npm install eslint-plugin-react-hooks --save-dev
+        ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-192. ### What is reselect and how it works?
+        And apply the below config in your ESLint config file,
 
-     *Reselect* is a **selector library** (for Redux) which uses *memoization* concept. It was originally written to compute derived data from Redux-like applications state, but it can't be tied to any architecture or library.
+        ```javascript
+        // Your ESLint configuration
+        {
+          "plugins": [
+            // ...
+            "react-hooks"
+          ],
+          "rules": {
+            // ...
+            "react-hooks/rules-of-hooks": "error"
+          }
+        }
+        ```
 
-     Reselect keeps a copy of the last inputs/outputs of the last call, and recomputes the result only if one of the inputs changes. If the the same inputs are provided twice in a row, Reselect returns the cached output. It's memoization and cache are fully customizable.
+        This plugin also provide another important rule through `react-hooks/exhaustive-deps`. It ensures that the dependencies of useEffect, useCallback, and useMemo hooks are correctly listed to avoid potential bugs.
 
+        ```jsx
+        useEffect(() => {
+          // Forgetting `message` will result in incorrect behavior
+          console.log(message);
+        }, []); // Here `message` should be a dependency
+        ```
+        The recommended `eslint-config-react-app` preset already includes the hooks rules of this plugin.
+        For example, the linter enforce proper naming convention for hooks. If you rename your custom hooks which as prefix "use" to something else then linter won't allow you to call built-in hooks such as useState, useEffect etc inside of your custom hook anymore.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-193. ### What is Flow?
+        **Note:** This plugin is intended to use in Create React App by default.
 
-     *Flow* is a *static type checker* designed to find type errors in JavaScript. Flow types can express much more fine-grained distinctions than traditional type systems. For example, Flow helps you catch errors involving `null`, unlike most type systems.
+**[⬆ Back to Top](#table-of-contents)**
 
+171. ### What are the differences between Flux and Redux?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-194. ### What is the difference between Flow and PropTypes?
+     Below are the major differences between Flux and Redux
 
-     Flow is a *static analysis tool* (static checker) which uses a superset of the language, allowing you to add type annotations to all of your code and catch an entire class of bugs at compile time.
+     | Flux                                           | Redux                                      |
+     | ---------------------------------------------- | ------------------------------------------ |
+     | State is mutable                               | State is immutable                         |
+     | The Store contains both state and change logic | The Store and change logic are separate    |
+     | There are multiple stores exist                | There is only one store exist              |
+     | All the stores are disconnected and flat       | Single store with hierarchical reducers    |
+     | It has a singleton dispatcher                  | There is no concept of dispatcher          |
+     | React components subscribe to the store        | Container components uses connect function |
 
-     PropTypes is a *basic type checker* (runtime checker) which has been patched onto React. It can't check anything other than the types of the props being passed to a given component. If you want more flexible typechecking for your entire project Flow/TypeScript are appropriate choices.
+**[⬆ Back to Top](#table-of-contents)**
 
+172. ### What are the benefits of React Router V4?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-195. ### How to use Font Awesome icons in React?
+     Below are the main benefits of React Router V4 module,
 
-     The below steps followed to include Font Awesome in React:
+     1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(`<BrowserRouter>`) which wraps specific child router components(`<Route>`).
+     2. You don't need to manually set history. The router module will take care history by wrapping routes with `<BrowserRouter>` component.
+     3. The application size is reduced by adding only the specific router module(Web, core, or native)
 
-     1. Install `font-awesome`:
+**[⬆ Back to Top](#table-of-contents)**
 
-         ```console
-         $ npm install --save font-awesome
-         ```
+173. ### Can you describe about componentDidCatch lifecycle method signature?
 
-     2. Import `font-awesome` in your `index.js` file:
+     The **componentDidCatch** lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
 
-         ```javascript
-         import 'font-awesome/css/font-awesome.min.css'
-         ```
+     1. error: - The error object which was thrown
+     2. info: - An object with a componentStack key contains the information about which component threw the error.
 
-     3. Add Font Awesome classes in `className`:
+     The method structure would be as follows
 
-         ```javascript
-         render() {
-           return <div><i className={'fa fa-spinner'} /></div>
-         }
-         ```
+     ```javascript
+     componentDidCatch(error, info);
+     ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-196. ### What is React Dev Tools?
+174. ### In which scenarios do error boundaries not catch errors?
 
-     *React Developer Tools* let you inspect the component hierarchy, including component props and state. It exists both as a browser extension (for Chrome and Firefox), and as a standalone app (works with other environments including Safari, IE, and React Native).
+     Below are the cases in which error boundaries don't work,
 
-     The official extensions available for different browsers or environments.
-     1. **Chrome extension**
-     2. **Firefox extension**
-     3. **Standalone app** (Safari, React Native, etc)
+     1. Inside Event handlers
+     2. Asynchronous code using **setTimeout or requestAnimationFrame** callbacks
+     3. During Server side rendering
+     4. When errors thrown in the error boundary code itself
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-197. ### Why is DevTools not loading in Chrome for local files?
+175. ### What is the behavior of uncaught errors in react 16?
+     In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
 
-     If you opened a local HTML file in your browser (`file://...`) then you must first open *Chrome Extensions* and check `Allow access to file URLs`.
+**[⬆ Back to Top](#table-of-contents)**
 
+176. ### What is the proper placement for error boundaries?
+     The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,
+     1. You can wrap top-level route components to display a generic error message for the entire application.
+     2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-198. ### How to use Polymer in React?
-     You need to follow below steps to use Polymer in React,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. Create a Polymer element:
+177. ### What is the benefit of component stack trace from error boundary?
 
-         ```jsx harmony
-         <link rel='import' href='../../bower_components/polymer/polymer.html' />
-         Polymer({
-           is: 'calender-element',
-           ready: function() {
-             this.textContent = 'I am a calender'
-           }
-         })
-         ```
+     Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.
 
-     2. Create the Polymer component HTML tag by importing it in a HTML document, e.g. import it in the `index.html` of your React application:
+     For example, BuggyCounter component displays the component stack trace as below,
 
-         ```html
-         <link rel='import' href='./src/polymer-components/calender-element.html'>
-         ```
+     ![stacktrace](images/error_boundary.png)
 
-     3. Use that element in the JSX file:
+**[⬆ Back to Top](#table-of-contents)**
 
-         ```javascript
-         import React from 'react'
+178. ### What are default props?
 
-         class MyComponent extends React.Component {
-           render() {
-             return (
-               <calender-element />
-             )
-           }
-         }
+     The _defaultProps_ can be defined as a property on the component to set the default values for the props. These default props are used when props not supplied(i.e., undefined props), but not for `null` or `0` as props. That means, If you provide null value then it remains null value. It's the same behavior with 0 as well.
 
-         export default MyComponent
-         ```
+     For example, let us create color default prop for the button component,
 
+     ```javascript
+     function MyButton {
+       // ...
+     }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-199. ### What are the advantages of React over Vue.js?
+     MyButton.defaultProps = {
+       color: "red",
+     };
+     ```
 
-     React has the following advantages over Vue.js:
+     If `props.color` is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses the default value
 
-     1. Gives more flexibility in large apps developing.
-     2. Easier to test.
-     3. Suitable for mobile apps creating.
-     4. More information and solutions available.
+     ```javascript
+     function MyButton() {
+       return <MyButton />; // props.color will contain red value
+     }
+     ```
 
-   **Note:** The above list of advantages are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-200. ### What is the difference between React and Angular?
-     Let's see the difference between React and Angular in a table format.
+179. ### What is the purpose of displayName class property?
 
-     | React | Angular |
-     | ----- | ------- |
-     | React is a library and has only the View layer | Angular is a framework and has complete MVC functionality |
-     | React handles rendering on the server side | AngularJS renders only on the client side but Angular 2 and above renders on the server side |
-     | React uses JSX that looks like HTML in JS which can be confusing | Angular follows the template approach for HTML, which makes code shorter and easy to understand |
-     | React Native, which is a React type to build mobile applications are faster and more stable | Ionic, Angular's mobile native app is relatively less stable and slower |
-     | In React, data flows only in one way and hence debugging is easy | In Angular, data flows both way i.e it has two-way data binding between children and parent and hence debugging is often difficult |
+     The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.
 
-   **Note:** The above list of differences are purely opinionated and it vary based on the professional experience. But they are helpful as base parameters.
+     For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-201. ### Why React tab is not showing up in DevTools?
+     ```javascript
+     function withSubscription(WrappedComponent) {
+       class WithSubscription extends React.Component {
+         /* ... */
+       }
+       WithSubscription.displayName = `WithSubscription(${getDisplayName(
+         WrappedComponent
+       )})`;
+       return WithSubscription;
+     }
+     function getDisplayName(WrappedComponent) {
+       return (
+         WrappedComponent.displayName || WrappedComponent.name || "Component"
+       );
+     }
+     ```
 
-     When the page loads, *React DevTools* sets a global named `__REACT_DEVTOOLS_GLOBAL_HOOK__`, then React communicates with that hook during initialization. If the website is not using React or if React fails to communicate with DevTools then it won't show up the tab.
+**[⬆ Back to Top](#table-of-contents)**
 
+180. ### What is the browser support for react applications?
+     React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use **es5-shim and es5-sham** polyfill then it even support old browsers that doesn't support ES5 methods.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-202. ### What are Styled Components?
+**[⬆ Back to Top](#table-of-contents)**
 
-     `styled-components` is a JavaScript library for styling React applications. It removes the mapping between styles and components, and lets you write actual CSS augmented with JavaScript.
+181. ### What is code-splitting?
 
+     Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-203. ### Give an example of Styled Components?
+     For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.
 
-     Lets create `<Title>` and `<Wrapper>` components with specific styles for each.
+     **moduleA.js**
 
      ```javascript
-     import React from 'react'
-     import styled from 'styled-components'
+     const moduleA = "Hello";
 
-     // Create a <Title> component that renders an <h1> which is centered, red and sized at 1.5em
-     const Title = styled.h1`
-       font-size: 1.5em;
-       text-align: center;
-       color: palevioletred;
-     `
-
-     // Create a <Wrapper> component that renders a <section> with some padding and a papayawhip background
-     const Wrapper = styled.section`
-       padding: 4em;
-       background: papayawhip;
-     `
+     export { moduleA };
      ```
 
-     These two variables, `Title` and `Wrapper`, are now components that you can render just like any other react component.
+     **App.js**
 
-     ```jsx harmony
-     <Wrapper>
-       <Title>{'Lets start first styled component!'}</Title>
-     </Wrapper>
-     ```
+     ```javascript
+     export default function App {
+       function handleClick() {
+         import("./moduleA")
+           .then(({ moduleA }) => {
+             // Use moduleA
+           })
+           .catch((err) => {
+             // Handle failure
+           });
+       };
 
+      return (
+        <div>
+          <button onClick={this.handleClick}>Load</button>
+        </div>
+      );
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-204. ### What is Relay?
+  <details><summary><b>See Class</b></summary>
+    <p>
 
-     Relay is a JavaScript framework for providing a data layer and client-server communication to web applications using the React view layer.
+  ```javascript
+    import React, { Component } from "react";
 
+     class App extends Component {
+       handleClick = () => {
+         import("./moduleA")
+           .then(({ moduleA }) => {
+             // Use moduleA
+           })
+           .catch((err) => {
+             // Handle failure
+           });
+       };
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-205. ### How to use TypeScript in `create-react-app` application?
-     Starting from react-scripts@2.1.0 or higher, there is a built-in support for typescript. i.e, `create-react-app` now supports typescript natively. You can just pass `--typescript` option as below
-     ```bash
-     npx create-react-app my-app --typescript
+       render() {
+         return (
+           <div>
+             <button onClick={this.handleClick}>Load</button>
+           </div>
+         );
+       }
+     }
 
-     # or
+     export default App;
+  ```
 
-     yarn create react-app my-app --typescript
-     ```
-     But for lower versions of react scripts, just supply `--scripts-version` option as `react-scripts-ts` while you create a new project. `react-scripts-ts` is a set of adjustments to take the standard `create-react-app` project pipeline and bring TypeScript into the mix.
+  </p>
+</details>
 
-     Now the project layout should look like the following:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```
-     my-app/
-     ├─ .gitignore
-     ├─ images.d.ts
-     ├─ node_modules/
-     ├─ public/
-     ├─ src/
-     │  └─ ...
-     ├─ package.json
-     ├─ tsconfig.json
-     ├─ tsconfig.prod.json
-     ├─ tsconfig.test.json
-     └─ tslint.json
-     ```
+182. ### What are Keyed Fragments?
 
-## Miscellaneous
+     The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,
 
+     ```javascript
+     function Glossary(props) {
+       return (
+         <dl>
+           {props.items.map((item) => (
+             // Without the `key`, React will fire a key warning
+             <React.Fragment key={item.id}>
+               <dt>{item.term}</dt>
+               <dd>{item.description}</dd>
+             </React.Fragment>
+           ))}
+         </dl>
+       );
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-206. ### What are the main features of Reselect library?
+     **Note:** key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
 
-     Let's see the main features of Reselect library,
+**[⬆ Back to Top](#table-of-contents)**
 
-       1. Selectors can compute derived data, allowing Redux to store the minimal possible state.
-       2. Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
-       3. Selectors are composable. They can be used as input to other selectors.
+183. ### Does React support all HTML attributes?
 
-207. #### Give an example of Reselect usage?
+     As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
 
-     Let's take calculations and different amounts of a shipment order with the simplified usage of Reselect:
+     Let us take few props with respect to standard HTML attributes,
 
      ```javascript
-     import { createSelector } from 'reselect'
+     <div tabIndex="-1" />      // Just like node.tabIndex DOM API
+     <div className="Button" /> // Just like node.className DOM API
+     <input readOnly={true} />  // Just like node.readOnly DOM API
+     ```
 
-     const shopItemsSelector = state => state.shop.items
-     const taxPercentSelector = state => state.shop.taxPercent
+     These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
 
-     const subtotalSelector = createSelector(
-       shopItemsSelector,
-       items => items.reduce((acc, item) => acc + item.value, 0)
-     )
+**[⬆ Back to Top](#table-of-contents)**
 
-     const taxSelector = createSelector(
-       subtotalSelector,
-       taxPercentSelector,
-       (subtotal, taxPercent) => subtotal * (taxPercent / 100)
-     )
+184. ### When component props defaults to true?
 
-     export const totalSelector = createSelector(
-       subtotalSelector,
-       taxSelector,
-       (subtotal, tax) => ({ total: subtotal + tax })
-     )
+     If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.
 
-     let exampleState = {
-       shop: {
-         taxPercent: 8,
-         items: [
-           { name: 'apple', value: 1.20 },
-           { name: 'orange', value: 0.95 },
-         ]
-       }
-     }
+     For example, below expressions are equivalent,
 
-     console.log(subtotalSelector(exampleState)) // 2.15
-     console.log(taxSelector(exampleState))      // 0.172
-     console.log(totalSelector(exampleState))    // { total: 2.322 }
+     ```javascript
+     <MyInput autocomplete />
+
+     <MyInput autocomplete={true} />
      ```
 
+     **Note:** It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, `{name}` which is short for `{name: name}`)
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-209. ### Does the statics object work with ES6 classes in React?
+**[⬆ Back to Top](#table-of-contents)**
 
-     No, `statics` only works with `React.createClass()`:
+185. ### What is NextJS and major features of it?
 
-     ```javascript
-     someComponent= React.createClass({
-       statics: {
-         someMethod: function() {
-           // ..
-         }
-       }
-     })
-     ```
+     Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,
 
-     But you can write statics inside ES6+ classes as below,
+     1. Server-rendered by default
+     2. Automatic code splitting for faster page loads
+     3. Simple client-side routing (page based)
+     4. Webpack-based dev environment which supports (HMR)
+     5. Able to implement with Express or any other Node.js HTTP server
+     6. Customizable with your own Babel and Webpack configurations
 
-     ```javascript
-     class Component extends React.Component {
-       static propTypes = {
-         // ...
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       static someMethod() {
-         // ...
-       }
-     }
-     ```
+186. ### How do you pass an event handler to a component?
 
-     or writing them outside class as below,
+     You can pass event handlers and other functions as props to child components. The functions can be passed to child component as below,
 
-     ```javascript
-     class Component extends React.Component {
-        ....
+     ```jsx
+     function Button({ onClick }) {
+       return <button onClick={onClick}>Download</button>;
      }
 
-     Component.propTypes = {...}
-     Component.someMethod = function(){....}
-     ```
+     export default function downloadExcel() {
+       function handleClick() {
+         alert("Downloaded");
+       }
 
+       return <Button onClick={handleClick}></Button>;
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-210. ### Can Redux only be used with React?
+**[⬆ Back to Top](#table-of-contents)**
 
-     Redux can be used as a data store for any UI layer. The most common usage is with React and React Native, but there are bindings available for Angular, Angular 2, Vue, Mithril, and more. Redux simply provides a subscription mechanism which can be used by any other code.
+187. ### How to prevent a function from being called multiple times?
 
+     If you use an event handler such as **onClick or onScroll** and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-211. ### Do you need to have a particular build tool to use Redux?
+     1. **Throttling:** Changes based on a time based frequency. For example, it can be used using \_.throttle lodash function
+     2. **Debouncing:** Publish changes after a period of inactivity. For example, it can be used using \_.debounce lodash function
+     3. **RequestAnimationFrame throttling:** Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
 
-     Redux is originally written in ES6 and transpiled for production into ES5 with Webpack and Babel. You should be able to use it regardless of your JavaScript build process. Redux also offers a UMD build that can be used directly without any build process at all.
+**[⬆ Back to Top](#table-of-contents)**
 
+188. ### How JSX prevents Injection Attacks?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-212. ### How Redux Form `initialValues` get updated from state?
+     React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
 
-     You need to add `enableReinitialize : true` setting.
+     For example, you can embed user input as below,
 
      ```javascript
-     const InitializeFromStateForm = reduxForm({
-       form: 'initializeFromState',
-       enableReinitialize : true
-     })(UserEdit)
+     const name = response.potentiallyMaliciousInput;
+     const element = <h1>{name}</h1>;
      ```
 
-     If your `initialValues` prop gets updated, your form will update too.
+     This way you can prevent XSS(Cross-site-scripting) attacks in the application.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-213. ### How React PropTypes allow different types for one prop?
+189. ### How do you update rendered elements?
 
-     You can use `oneOfType()` method of `PropTypes`.
+     You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.
 
-     For example, the height property can be defined with either `string` or `number` type as below:
+     For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
 
      ```javascript
-     Component.propTypes = {
-       size: PropTypes.oneOfType([
-         PropTypes.string,
-         PropTypes.number
-       ])
+     function tick() {
+       const element = (
+         <div>
+           <h1>Hello, world!</h1>
+           <h2>It is {new Date().toLocaleTimeString()}.</h2>
+         </div>
+       );
+       ReactDOM.render(element, document.getElementById("root"));
      }
+
+     setInterval(tick, 1000);
      ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-214. ### Can I import an SVG file as react component?
+190. ### How do you say that props are readonly?
 
-     You can import SVG directly as component instead of loading it as a file. This feature is available with `react-scripts@2.0.0` and higher.
+     When you declare a component as a function or a class, it must never modify its own props.
 
-     ```jsx harmony
-     import { ReactComponent as Logo } from './logo.svg'
+     Let us take a below capital function,
 
-     const App = () => (
-       <div>
-         {/* Logo is an actual react component */}
-         <Logo />
-       </div>
-     )
+     ```javascript
+     function capital(amount, interest) {
+       return amount + interest;
+     }
      ```
 
-     **Note**: Don't forget about the curly braces in the import.
-
+     The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-215. ### Why are inline ref callbacks or functions not recommended?
+**[⬆ Back to Top](#table-of-contents)**
 
-     If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
+191. ### What are the conditions to safely use the index as a key?
 
-     ```jsx
-     class UserForm extends Component {
-       handleSubmit = () => {
-         console.log("Input Value is: ", this.input.value)
-       }
+     There are three conditions to make sure, it is safe use the index as a key.
 
+     1. The list and items are static– they are not computed and do not change
+     2. The items in the list have no ids
+     3. The list is never reordered or filtered.
 
-       render () {
-        return (
-          <form onSubmit={this.handleSubmit}>
-            <input
-              type='text'
-              ref={(input) => this.input = input} /> // Access DOM input in handle submit
-            <button type='submit'>Submit</button>
-          </form>
-        )
-      }
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
+192. ### Should keys be globally unique?
 
-     ```jsx
-     class UserForm extends Component {
-      handleSubmit = () => {
-        console.log("Input Value is: ", this.input.value)
-      }
+     The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
 
-      setSearchInput = (input) => {
-        this.input = input
-      }
+     For example, the below `Book` component uses two arrays with different arrays,
 
-      render () {
-        return (
-          <form onSubmit={this.handleSubmit}>
-            <input
-              type='text'
-              ref={this.setSearchInput} /> // Access DOM input in handle submit
-            <button type='submit'>Submit</button>
-          </form>
-        )
-      }
+     ```javascript
+     function Book(props) {
+       const index = (
+         <ul>
+           {props.pages.map((page) => (
+             <li key={page.id}>{page.title}</li>
+           ))}
+         </ul>
+       );
+       const content = props.pages.map((page) => (
+         <div key={page.id}>
+           <h3>{page.title}</h3>
+           <p>{page.content}</p>
+           <p>{page.pageNumber}</p>
+         </div>
+       ));
+       return (
+         <div>
+           {index}
+           <hr />
+           {content}
+         </div>
+       );
      }
      ```
 
-    **Note:** In React v16.3,
-   **[⬆ Back to Top](#table-of-contents)**
-    
-216. ### What is render hijacking in react?
+**[⬆ Back to Top](#table-of-contents)**
 
-     The concept of render hijacking is the ability to control what a component will output from another component. It  means that you decorate your component by wrapping it into a Higher-Order component. By wrapping, you can inject additional props or make other changes, which can cause changing logic of rendering. It does not actually enable hijacking, but by using HOC you make your component behave differently.
+193. ### What is the popular choice for form handling?
 
+     `Formik` is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-217. ### What are HOC factory implementations?
-     There are two main ways of implementing HOCs in React.
+     In detail, You can categorize them as follows,
 
-     1. Props Proxy (PP) and
-     2. Inheritance Inversion (II).
+     1. Getting values in and out of form state
+     2. Validation and error messages
+     3. Handling form submission
 
-     But they follow different approaches for manipulating the *WrappedComponent*.
+     It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
 
-     **Props Proxy**
+**[⬆ Back to Top](#table-of-contents)**
 
-     In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name **Props Proxy**.
+194. ### What are the advantages of formik over redux form library?
 
-     ```jsx
+     Below are the main reasons to recommend formik over redux form library,
 
-     function ppHOC(WrappedComponent) {
-      return class PP extends React.Component {
-        render() {
-          return <WrappedComponent {...this.props}/>
-        }
-      }
-     }
-     ```
-     **Inheritance Inversion**
+     1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
+     2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
+     3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB
 
-     In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems **inverse**.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```jsx
-     function iiHOC(WrappedComponent) {
-      return class Enhancer extends WrappedComponent {
-        render() {
-          return super.render()
-        }
-      }
-     }
-     ```
+195. ### Why are you not required to use inheritance?
+     In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
+     Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-218. ### How to pass numbers to React component?
+**[⬆ Back to Top](#table-of-contents)**
 
-     You should be passing the numbers via curly braces({}) where as strings in quotes
+196. ### Can I use web components in react application?
 
-     ```jsx
-        React.render(<User age={30} department={"IT"} />, document.getElementById('container'));
-     ```
+     Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-219. ### Do I need to keep all my state into Redux? Should I ever use react internal state?
-     It is up to the developer's decision, i.e., it is developer's job to determine what kinds of state make up your application, and where each piece of state should live. Some users prefer to keep every single piece of data in Redux, to maintain a fully serializable and controlled version of their application at all times. Others prefer to keep non-critical or UI state, such as “is this dropdown currently open”, inside a component's internal state.
+     For example, let us use `Vaadin` date picker web component as below,
 
-     Below are the thumb rules to determine what kind of data should be put into Redux
-     1. Do other parts of the application care about this data?
-     2. Do you need to be able to create further derived data based on this original data?
-     3. Is the same data being used to drive multiple components?
-     4. Is there value to you in being able to restore this state to a given point in time (ie, time travel debugging)?
-     5. Do you want to cache the data (i.e, use what's in state if it's already there instead of re-requesting it)?
+     ```javascript
+     import "./App.css";
+     import "@vaadin/vaadin-date-picker";
+     export default function App() {
+       return (
+         <div className="App">
+           <vaadin-date-picker label="When were you born?"></vaadin-date-picker>
+         </div>
+       );
+     }
+     ```
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-220. ### What is the purpose of registerServiceWorker in React?
+197. ### What is dynamic import?
 
-     React creates a service worker for you without any configuration by default. The service worker is a web API that helps you cache your assets and other files so that when the user is offline or on a slow network, he/she can still see results on the screen, as such, it helps you build a better user experience, that's what you should know about service worker for now. It's all about adding offline capabilities to your site.
+     You can achieve code-splitting in your app using dynamic import.
 
-     ```jsx
-        import React from 'react';
-        import ReactDOM from 'react-dom';
-        import App from './App';
-        import registerServiceWorker from './registerServiceWorker';
+     Let's take an example of addition,
+
+     1. **Normal Import**
 
-        ReactDOM.render(<App />, document.getElementById('root'));
-        registerServiceWorker();
+     ```javascript
+     import { add } from "./math";
+     console.log(add(10, 20));
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-221. ### What is React memo function?
+     2. **Dynamic Import**
 
-     Class components can be restricted from re-rendering when their input props are the same using **PureComponent or shouldComponentUpdate**. Now you can do the same with function components by wrapping them in **React.memo**.
-     ```jsx
-     const MyComponent = React.memo(function MyComponent(props) {
-      /* only rerenders if props change */
+     ```javascript
+     import("./math").then((math) => {
+       console.log(math.add(10, 20));
      });
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-222. ### What is React lazy function?
-     The `React.lazy` function lets you render a dynamic import as a regular component. It will automatically load the bundle containing the `OtherComponent` when the component gets rendered. This must return a Promise which resolves to a module with a default export containing a React component.
-     ```jsx
-     const OtherComponent = React.lazy(() => import('./OtherComponent'));
-
-     function MyComponent() {
-      return (
-        <div>
-          <OtherComponent />
-        </div>
-      );
-     }
-     ```
-     **Note:**
-     `React.lazy` and `Suspense` is not yet available for server-side rendering. If you want to do code-splitting in a server rendered app, we still recommend React Loadable.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-223. ### How to prevent unnecessary updates using setState?
-     You can compare the current value of the state with an existing state value and decide whether to rerender the page or not. If the values are the same then you need to return **null** to stop re-rendering otherwise return the latest state value.
+198. ### What are loadable components?
 
-     For example, the user profile information is conditionally rendered as follows,
-     ```jsx
-     getUserProfile = user => {
-       const latestAddress = user.address;
-       this.setState(state => {
-         if (state.address === latestAddress) {
-           return null;
-         } else {
-           return { title: latestAddress };
-         }
-       });
-     };
-     ```
+     With the release of React 18, React.lazy and Suspense are now available for server-side rendering. However, prior to React 18, it was recommended to use Loadable Components for code-splitting in a server-side rendered app because React.lazy and Suspense were not available for server-side rendering. Loadable Components lets you render a dynamic import as a regular component. For example, you can use Loadable Components to load the OtherComponent in a separate bundle like this:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-224. ### How do you render Array, Strings and Numbers in React 16 Version?
-     **Arrays**: Unlike older releases, you don't need to make sure **render** method return a single element in React16. You are able to return multiple sibling elements without a wrapping element by returning an array.
+     ```javascript
+     import loadable from "@loadable/component";
 
-     For example, let us take the below list of developers,
+     const OtherComponent = loadable(() => import("./OtherComponent"));
 
-     ```jsx
-     const ReactJSDevs = () => {
-       return [
-         <li key="1">John</li>,
-         <li key="2">Jackie</li>,
-         <li key="3">Jordan</li>
-       ];
-     }
-     ```
-     You can also merge this array of items in another array component.
-     ```jsx
-     const JSDevs = () => {
+     function MyComponent() {
        return (
-         <ul>
-           <li>Brad</li>
-           <li>Brodge</li>
-           <ReactJSDevs/>
-           <li>Brandon</li>
-         </ul>
+         <div>
+           <OtherComponent />
+         </div>
        );
      }
      ```
-     **Strings and Numbers:** You can also return string and number type from the render method.
 
-     ```jsx
-     render() {
-      return 'Welcome to ReactJS questions';
-     }
-     // Number
-     render() {
-      return 2018;
-     }
-     ```
+     Now OtherComponent will be loaded in a separated bundle
+     Loadable Components provides additional benefits beyond just code-splitting, such as automatic code reloading, error handling, and preloading. By using Loadable Components, you can ensure that your application loads quickly and efficiently, providing a better user experience for your users.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-225. ### How to use class field declarations syntax in React classes?
-     React Class Components can be made much more concise using the class field declarations. You can initialize the local state without using the constructor and declare class methods by using arrow functions without the extra need to bind them.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Let's take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,
-     ```jsx
-     class Counter extends Component {
-       state = { value: 0 };
+199. ### What is suspense component?
 
-       handleIncrement = () => {
-         this.setState(prevState => ({
-           value: prevState.value + 1
-         }));
-       };
+     React Suspense is a built-in feature that lets you defer rendering part of your component tree until some condition(asynchronous operation) is met—usually, data or code has finished loading. While waiting, Suspense lets you display a fallback UI like a spinner or placeholder.
 
-       handleDecrement = () => {
-         this.setState(prevState => ({
-           value: prevState.value - 1
-         }));
-       };
 
-       render() {
-         return (
-           <div>
-             {this.state.value}
+     1. Lazy loading components uses suspense feature,
 
-             <button onClick={this.handleIncrement}>+</button>
-             <button onClick={this.handleDecrement}>-</button>
-           </div>
-         )
-       }
-     }
-     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
+        If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using **Suspense** component.
     
-226. ### What are hooks?
-     Hooks is a special function (introduced as a new feature in React 16.8) that lets you use state and other React features without writing a class.
+        ```javascript
+        const OtherComponent = React.lazy(() => import("./OtherComponent"));
 
-     Let's see an example of useState hook:
-     ```jsx
-     import { useState } from 'react';
+        function MyComponent() {
+          return (
+            <div>
+              <Suspense fallback={<div>Loading...</div>}>
+                <OtherComponent />
+              </Suspense>
+            </div>
+          );
+        }
+        ```
+        The above component shows fallback UI instead real component until `OtherComponent` is fully loaded.
 
-     function Example() {
-       // Declare a new state variable, which we'll call "count"
-       const [count, setCount] = useState(0);
+     2. As an another example, suspend until async data(data fetching) is ready
+      ```jsx
+        function UserProfile() {
+          const user = use(fetchUser()); // throws a promise internally
+          return <div>{user.name}</div>;
+        }
 
-       return (
-         <div>
-           <p>You clicked {count} times</p>
-           <button onClick={() => setCount(count + 1)}>
-             Click me
-           </button>
-         </div>
-       );
-     }
-     ```
-     **Note:** Hooks can be used inside an existing function component.
+        function App() {
+          return (
+            <Suspense fallback={<div>Loading user...</div>}>
+              <UserProfile />
+            </Suspense>
+          );
+        }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-227. ### What rules need to be followed for hooks?
+    ```
+     
 
-     You need to follow two rules in order to use hooks,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. Call Hooks only at the top level of your react functions. i.e, You shouldn’t call Hooks inside loops, conditions, or nested functions. This will ensure that Hooks are called in the same order each time a component renders and it preserves the state of Hooks between multiple useState and useEffect calls.
-     2. Call Hooks from React Functions only. i.e, You shouldn’t call Hooks from regular JavaScript functions.
+200. ### What is route based code splitting?
 
+     One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
+
+     Let us take an example of route based website using libraries like React Router with React.lazy,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-228. ### How to ensure hooks followed the rules in your project?
-     React team released an ESLint plugin called **eslint-plugin-react-hooks** that enforces these two rules. You can add this plugin to your project using the below command,
-     ```javascript
-     npm install eslint-plugin-react-hooks@next
-     ```
-     And apply the below config in your ESLint config file,
      ```javascript
-     // Your ESLint configuration
-     {
-       "plugins": [
-         // ...
-         "react-hooks"
-       ],
-       "rules": {
-         // ...
-         "react-hooks/rules-of-hooks": "error"
-       }
-     }
-     ```
-     **Note:** This plugin is intended to use in Create React App by default.
+     import { BrowserRouter as Router, Route, Switch } from "react-router-dom";
+     import React, { Suspense, lazy } from "react";
 
+     const Home = lazy(() => import("./routes/Home"));
+     const About = lazy(() => import("./routes/About"));
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-229. ### What are the differences between Flux and Redux?
-     Below are the major differences between Flux and Redux
+     const App = () => (
+       <Router>
+         <Suspense fallback={<div>Loading...</div>}>
+           <Switch>
+             <Route exact path="/" component={Home} />
+             <Route path="/about" component={About} />
+           </Switch>
+         </Suspense>
+       </Router>
+     );
+     ```
 
-     | Flux | Redux |
-     | ----- | ------- |
-     | State is mutable | State is immutable |
-     | The Store contains both state and change logic | The Store and change logic are separate |
-     | There are multiple stores exist | There is only one store exist |
-     | All the stores are disconnected and flat | Single store with hierarchical reducers|
-     | It has a singleton dispatcher | There is no concept of dispatcher |
-     | React components subscribe to the store | Container components uses connect function|
+     In the above code, the code splitting will happen at each route level.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-230. ### What are the benefits of React Router V4?
-     Below are the main benefits of React Router V4 module,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. In React Router v4(version 4), the API is completely about components. A router can be visualized as a single component(`<BrowserRouter>`) which wraps specific child router components(`<Route>`).
-     2. You don't need to manually set history. The router module will take care history by wrapping routes with  `<BrowserRouter>` component.
-     3. The application size is reduced by adding only the specific router module(Web, core, or native)
+201. ### What is the purpose of default value in context?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-231. ### Can you describe about componentDidCatch lifecycle method signature?
-     The **componentDidCatch** lifecycle method is invoked after an error has been thrown by a descendant component. The method receives two parameters,
-     1. error: - The error object which was thrown
-     2. info: - An object with a componentStack key contains the information about which component threw the error.
+     The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
+
+     Below code snippet provides default theme value as Luna.
 
-     The method structure would be as follows
      ```javascript
-     componentDidCatch(error, info)
+     const MyContext = React.createContext(defaultValue);
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-232. ### In which scenarios error boundaries do not catch errors?
-     Below are the cases in which error boundaries doesn't work,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. Inside Event handlers
-     2. Asynchronous code using **setTimeout or requestAnimationFrame** callbacks
-     3. During Server side rendering
-     4. When errors thrown in the error boundary code itself
+202. ### What is diffing algorithm?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-233. ### Why do you not need error boundaries for event handlers?
-     Error boundaries do not catch errors inside event handlers.
-     
-     React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.
+     React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n³) where n is the number of elements in the tree.
 
-     If you need to catch an error inside an event handler, use the regular JavaScript try / catch statement:
+     In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
 
-     ```javascript
-     class MyComponent extends React.Component {
-       constructor(props) {
-         super(props);
-         this.state = { error: null };
-         this.handleClick = this.handleClick.bind(this);
-       }
+     1. Two elements of different types will produce different trees.
+     2. The developer can hint at which child elements may be stable across different renders with a key prop.
 
-       handleClick() {
-         try {
-           // Do something that could throw
-         } catch (error) {
-           this.setState({ error });
-         }
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       render() {
-         if (this.state.error) {
-           return <h1>Caught an error.</h1>
-         }
-         return <button onClick={this.handleClick}>Click Me</button>
-       }
-     }
-     ```
-     Note that the above example is demonstrating regular JavaScript behavior and doesn’t use error boundaries.
+203. ### What are the rules covered by diffing algorithm?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-234. ### What is the difference between try catch block and error boundaries?
-     Try catch block works with imperative code whereas error boundaries are meant for declarative code to render on the screen.
+     When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
 
-     For example, the try catch block used for below imperative code
-     ```javascript
-     try {
-       showButton();
-     } catch (error) {
-       // ...
-     }
-     ```
-     Whereas error boundaries wrap declarative code as below,
-     ```javascript
-     <ErrorBoundary>
-       <MyComponent />
-     </ErrorBoundary>
-     ```
-     So if an error occurs in a **componentDidUpdate** method caused by a **setState** somewhere deep in the tree, it will still correctly propagate to the closest error boundary.
+     1. **Elements Of Different Types:**
+        Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example, elements <a> to <img>, or from <Article> to <Comment> of different types lead a full rebuild.
+     2. **DOM Elements Of The Same Type:**
+        When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
 
+        ```javascript
+        <div className="show" title="ReactJS" />
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-235. ### What is the behavior of uncaught errors in react 16?
-     In React 16, errors that were not caught by any error boundary will result in unmounting of the whole React component tree. The reason behind this decision is that it is worse to leave corrupted UI in place than to completely remove it. For example, it is worse for a payments app to display a wrong amount than to render nothing.
+        <div className="hide" title="ReactJS" />
+        ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-236. ### What is the proper placement for error boundaries?
-     The granularity of error boundaries usage is up to the developer based on project needs. You can follow either of these approaches,
-     1. You can wrap top-level route components to display a generic error message for the entire application.
-     2. You can also wrap individual components in an error boundary to protect them from crashing the rest of the application.
+     3. **Component Elements Of The Same Type:**
+        When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
+     4. **Recursing On Children:**
+        when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-237. ### What is the benefit of component stack trace from error boundary?
-     Apart from error messages and javascript stack, React16 will display the component stack trace with file names and line numbers using error boundary concept.
+        ```javascript
+        <ul>
+          <li>first</li>
+          <li>second</li>
+        </ul>
 
-     For example, BuggyCounter component displays the component stack trace as below,
+        <ul>
+          <li>first</li>
+          <li>second</li>
+          <li>third</li>
+        </ul>
 
-     ![stacktrace](images/error_boundary.png)
+        ```
 
+     5. **Handling keys:**
+        React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-238. ### What is the required method to be defined for a class component?
-     The `render()` method is the only required method in a class component. i.e, All methods other than render method are optional for a class component.
+     ```javascript
+     <ul>
+       <li key="2015">Duke</li>
+       <li key="2016">Villanova</li>
+     </ul>
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-239. ### What are the possible return types of render method?
-     Below are the list of following types used and return from render method,
+     <ul>
+       <li key="2014">Connecticut</li>
+       <li key="2015">Duke</li>
+       <li key="2016">Villanova</li>
+     </ul>
+     ```
 
-     1. **React elements:** Elements that instruct React to render a DOM node. It includes html elements such as `<div/>` and user defined elements.
-     2. **Arrays and fragments:** Return multiple elements to render as Arrays and Fragments to wrap multiple elements
-     3. **Portals:** Render children into a different DOM subtree.
-     4. **String and numbers:** Render both Strings and Numbers as text nodes in the DOM
-     5. **Booleans or null:** Doesn't render anything but these types are used to conditionally render content.
+**[⬆ Back to Top](#table-of-contents)**
 
+204. ### When do you need to use refs?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-240. ### What is the main purpose of constructor?
-     The constructor is mainly used for two purposes,
+     There are few use cases to go for refs,
 
-     1. To initialize local state by assigning object to this.state
-     2. For binding event handler methods to the instance
-     For example, the below code covers both the above cases,
-     ```javascript
-     constructor(props) {
-       super(props);
-       // Don't call this.setState() here!
-       this.state = { counter: 0 };
-       this.handleClick = this.handleClick.bind(this);
-     }
-     ```
+     1. Managing focus, text selection, or media playback.
+     2. Triggering imperative animations.
+     3. Integrating with third-party DOM libraries.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-241. ### Is it mandatory to define constructor for React component?
-     No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-242. ### What are default props?
-     The defaultProps are defined as a property on the component class to set the default props for the class. This is used for undefined props, but not for null props.
+205. ### Must prop be named as render for render props?
 
-     For example, let us create color default prop for the button component,
+     Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e, Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,
 
      ```javascript
-     class MyButton extends React.Component {
-       // ...
-     }
+     <Mouse
+       children={(mouse) => (
+         <p>
+           The mouse position is {mouse.x}, {mouse.y}
+         </p>
+       )}
+     />
+     ```
 
-     MyButton.defaultProps = {
-       color: 'red'
-     };
+     Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,
 
+     ```javascript
+     <Mouse>
+       {(mouse) => (
+         <p>
+           The mouse position is {mouse.x}, {mouse.y}
+         </p>
+       )}
+     </Mouse>
      ```
 
-     If `props.color` is not provided then it will set the default value to 'red'. i.e, Whenever you try to access the color prop it uses default value
+     While using this above technique(without any name), explicitly state that children should be a function in your propTypes.
+
      ```javascript
-     render() {
-        return <MyButton /> ; // props.color will be set to red
-      }
+     Mouse.propTypes = {
+       children: PropTypes.func.isRequired,
+     };
      ```
-     **Note:** If you provide null value then it remains null value.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-243. ### Why should not call setState in componentWillUnmount?
-     You should not call `setState()` in `componentWillUnmount()` because once a component instance is unmounted, it will never be mounted again.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-244. ### What is the purpose of getDerivedStateFromError?
-     This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value to update state.
+206. ### What are the problems of using render props with pure components?
+     If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.
 
-     The signature of the lifecycle method is as follows,
-     ```javascript
-     static getDerivedStateFromError(error)
-     ```
-     Let us take error boundary use case with the above lifecycle method for demonstration purpose,
-     ```javascript
-     class ErrorBoundary extends React.Component {
-       constructor(props) {
-         super(props);
-         this.state = { hasError: false };
-       }
+**[⬆ Back to Top](#table-of-contents)**
 
-       static getDerivedStateFromError(error) {
-         // Update state so the next render will show the fallback UI.
-         return { hasError: true };
-       }
+207. ### What is windowing technique?
+     Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.
 
-       render() {
-         if (this.state.hasError) {
-           // You can render any custom fallback UI
-           return <h1>Something went wrong.</h1>;
-         }
+**[⬆ Back to Top](#table-of-contents)**
 
-         return this.props.children;
-       }
-     }
+208. ### How do you print falsy values in JSX?
+
+     The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,
+
+     ```javascript
+     <div>My JavaScript variable is {String(myVariable)}.</div>
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-245. ### What is the methods order when component re-rendered?
-     An update can be caused by changes to props or state. The below methods are called in the following order when a component is being re-rendered.
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. static getDerivedStateFromProps()
-     2. shouldComponentUpdate()
-     3. render()
-     4. getSnapshotBeforeUpdate()
-     5. componentDidUpdate()
+209. ### What is the typical use case of portals?
+     React Portals are primarily used to render UI components such as **modals, tooltips, dropdowns, hovercards, and notifications** outside of their parent component's DOM tree. This helps avoid common CSS issues caused by parent elements, such as:
 
+     *   `**overflow: hidden**` on parent elements clipping or hiding child elements like modals or tooltips,
+     *   **stacking context and** `**z-index**` **conflicts** created by parent containers that prevent child elements from appearing above other content.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-246. ### What are the methods invoked during error handling?
-     Below methods are called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.
+     That means, you need to visually “break out” of its container. By rendering these UI elements into a separate DOM node (often directly under `<body>`), portals ensure they appear above all other content and are not restricted by the parent’s CSS or layout constraints, resulting in correct positioning and visibility regardless of the parent’s styling.
 
-     1. static getDerivedStateFromError()
-     2. componentDidCatch()
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-247. ### What is the purpose of displayName class property?
-     The displayName string is used in debugging messages. Usually, you don’t need to set it explicitly because it’s inferred from the name of the function or class that defines the component. You might want to set it explicitly if you want to display a different name for debugging purposes or when you create a higher-order component.
+210.   ### How do you set default value for uncontrolled component?
 
-     For example, To ease debugging, choose a display name that communicates that it’s the result of a withSubscription HOC.
+     In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a **defaultValue** attribute instead of **value**.
 
      ```javascript
-     function withSubscription(WrappedComponent) {
-       class WithSubscription extends React.Component {/* ... */}
-       WithSubscription.displayName = `WithSubscription(${getDisplayName(WrappedComponent)})`;
-       return WithSubscription;
-     }
-     function getDisplayName(WrappedComponent) {
-       return WrappedComponent.displayName || WrappedComponent.name || 'Component';
+     render() {
+       return (
+         <form onSubmit={this.handleSubmit}>
+           <label>
+             User Name:
+             <input
+               defaultValue="John"
+               type="text"
+               ref={this.input} />
+           </label>
+           <input type="submit" value="Submit" />
+         </form>
+       );
      }
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-248. ### What is the browser support for react applications?
-     React supports all popular browsers, including Internet Explorer 9 and above, although some polyfills are required for older browsers such as IE 9 and IE 10. If you use  **es5-shim and es5-sham** polyfill then it even support old browsers that doesn't support ES5 methods.
+     The same applies for `select` and `textArea` inputs. But you need to use **defaultChecked** for `checkbox` and `radio` inputs.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-249. ### What is the purpose of unmountComponentAtNode method?
-     This method is available from react-dom package and it removes a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns true if a component was unmounted and false if there was no component to unmount.
+**[⬆ Back to Top](#table-of-contents)**
 
-     The method signature would be as follows,
-     ```javascript
-     ReactDOM.unmountComponentAtNode(container)
-     ```
+211. ### What is your favorite React stack?
+     Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
+     You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-250. ### What is code-splitting?
-     Code-Splitting is a feature supported by bundlers like Webpack and Browserify which can create multiple bundles that can be dynamically loaded at runtime. The react project supports code splitting via dynamic import() feature.
+**[⬆ Back to Top](#table-of-contents)**
 
-     For example, in the below code snippets, it will make moduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.
-     **moduleA.js**
-     ```javascript
-     const moduleA = 'Hello';
+212. ### What is the difference between Real DOM and Virtual DOM?
 
-     export { moduleA };
-     ```
-     **App.js**
-     ```javascript
-     import React, { Component } from 'react';
+     Below are the main differences between Real DOM and Virtual DOM,
 
-     class App extends Component {
-       handleClick = () => {
-         import('./moduleA')
-           .then(({ moduleA }) => {
-             // Use moduleA
-           })
-           .catch(err => {
-             // Handle failure
-           });
-       };
+     | Real DOM                             | Virtual DOM                          |
+     | ------------------------------------ | ------------------------------------ |
+     | Updates are slow                     | Updates are fast                     |
+     | DOM manipulation is very expensive.  | DOM manipulation is very easy        |
+     | You can update HTML directly.        | You Can’t directly update HTML       |
+     | It causes too much of memory wastage | There is no memory wastage           |
+     | Creates a new DOM if element updates | It updates the JSX if element update |
 
-       render() {
-         return (
-           <div>
-             <button onClick={this.handleClick}>Load</button>
-           </div>
-         );
-       }
-     }
+**[⬆ Back to Top](#table-of-contents)**
 
-     export default App;
+213. ### How to add Bootstrap to a react application?
 
-     ```
+     Bootstrap can be added to your React app in a three possible ways,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-251. ### What is the benefit of strict mode?
-     The <StrictMode> will be  helpful in the below cases
+     1. Using the Bootstrap CDN:
+        This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
+     2. Bootstrap as Dependency:
+        If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application
+        ```javascript
+        npm install bootstrap
+        ```
+     3. React Bootstrap Package:
+        In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
+        1. react-bootstrap
+        2. reactstrap
 
-     1. Identifying components with **unsafe lifecycle methods**.
-     2. Warning about **legacy string ref** API usage.
-     3. Detecting unexpected **side effects**.
-     4. Detecting **legacy context** API.
-     5. Warning about deprecated findDOMNode usage
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-252. ### What are Keyed Fragments?
-     The Fragments declared with the explicit <React.Fragment> syntax may have keys. The general use case is mapping a collection to an array of fragments as below,
+214. ### Can you list down top websites or applications using react as front end framework?
+
+     Below are the `top 10 websites` using React as their front-end framework,
+
+     1. Facebook
+     2. Uber
+     3. Instagram
+     4. WhatsApp
+     5. Khan Academy
+     6. Airbnb
+     7. Dropbox
+     8. Flipboard
+     9. Netflix
+     10. PayPal
+
+**[⬆ Back to Top](#table-of-contents)**
+
+215. ### Is it recommended to use CSS In JS technique in React?
+     React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate \*.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+216. ### Do I need to rewrite all my class components with hooks?
+     No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+217. ### What is useEffect hook? How to fetch data with React Hooks?
+
+     The `useEffect` hook is a React Hook that lets you perform **side effects** in function components. Side effects are operations that interact with the outside world or system and aren't directly related to rendering UI — such as fetching data, setting up subscriptions, timers, manually manipulating the DOM, etc.
+
+     In function components, useEffect replaces the class component lifecycle methods(`componentDidMount`, `componentDidUpdate` and `componentWillUnmount`) with a single, unified API.     
+
+     **Syntax**
+     ```js
+     useEffect(() => {
+        // Side effect logic here
+
+        return () => {
+        // Cleanup logic (optional)
+        };
+        }, [dependencies]);
+     ```
+     This effect hook can be used to fetch data from an API and to set the data in the local state of the component with the useState hook’s update function.
+
+     Here is an example of fetching a list of ReactJS articles from an API using fetch.
 
      ```javascript
-     function Glossary(props) {
+     import React from "react";
+
+     function App() {
+       const [data, setData] = React.useState({ hits: [] });
+
+       React.useEffect(() => {
+         fetch("http://hn.algolia.com/api/v1/search?query=react")
+           .then((response) => response.json())
+           .then((data) => setData(data));
+       }, []);
+
        return (
-         <dl>
-           {props.items.map(item => (
-             // Without the `key`, React will fire a key warning
-             <React.Fragment key={item.id}>
-               <dt>{item.term}</dt>
-               <dd>{item.description}</dd>
-             </React.Fragment>
+         <ul>
+           {data.hits.map((item) => (
+             <li key={item.objectID}>
+               <a href={item.url}>{item.title}</a>
+             </li>
            ))}
-         </dl>
+         </ul>
        );
      }
+
+     export default App;
      ```
 
-     **Note:** key is the only attribute that can be passed to Fragment. In the future, there might be a support for additional attributes, such as event handlers.
+     A popular way to simplify this is by using the library axios.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-253. ### Does React support all HTML attributes?
-     As of React 16, both standard or custom DOM attributes are fully supported. Since React components often take both custom and DOM-related props, React uses the camelCase convention just like the DOM APIs.
+     We provided an empty array as second argument to the useEffect hook to avoid activating it on component updates. This way, it only fetches on component mount.
 
-     Let us take few props with respect to standard HTML attributes,
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     <div tabIndex="-1" />      // Just like node.tabIndex DOM API
-     <div className="Button" /> // Just like node.className DOM API
-     <input readOnly={true} />  // Just like node.readOnly DOM API
-     ```
-     These props work similarly to the corresponding HTML attributes, with the exception of the special cases. It also support all SVG attributes.
+218. ### Is Hooks cover all use cases for classes?
+     Hooks doesn't cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon **getSnapshotBeforeUpdate** and **componentDidCatch** lifecycles yet.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-254. ### What are the limitations with HOCs?
+**[⬆ Back to Top](#table-of-contents)**
 
-     Higher-order components come with a few caveats apart from its benefits. Below are the few listed in an order,
-     1. **Don’t use HOCs inside the render method:**
-        It is not recommended to apply a HOC to a component within the render method of a component.
-        ```javascript
-        render() {
-          // A new version of EnhancedComponent is created on every render
-          // EnhancedComponent1 !== EnhancedComponent2
-          const EnhancedComponent = enhance(MyComponent);
-          // That causes the entire subtree to unmount/remount each time!
-          return <EnhancedComponent />;
-        }
-        ```
-        The above code impacts on performance by remounting a component that causes the state of that component and all of its children to be lost. Instead, apply HOCs outside the component definition so that the resulting component is created only once.
+219. ### What is the stable release for hooks support?
 
-     2. **Static methods must be copied over:**
-        When you apply a HOC to a component the new component does not have any of the static methods of the original component
-        ```javascript
-        // Define a static method
-        WrappedComponent.staticMethod = function() {/*...*/}
-        // Now apply a HOC
-        const EnhancedComponent = enhance(WrappedComponent);
+     React includes a stable implementation of React Hooks in 16.8 release for below packages
 
-        // The enhanced component has no static method
-        typeof EnhancedComponent.staticMethod === 'undefined' // true
-        ```
-        You can overcome this by copying the methods onto the container before returning it,
+     1. React DOM
+     2. React DOM Server
+     3. React Test Renderer
+     4. React Shallow Renderer
 
-        ```javascript
-        function enhance(WrappedComponent) {
-          class Enhance extends React.Component {/*...*/}
-          // Must know exactly which method(s) to copy :(
-          Enhance.staticMethod = WrappedComponent.staticMethod;
-          return Enhance;
-        }
-        ```
-     3. **Refs aren’t passed through:**
-        For HOCs you need to pass through all props to the wrapped component but this does not work for refs. This is because ref is not really a prop similar to key. In this case you need to use the React.forwardRef API
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-255. ### How to debug forwardRefs in DevTools?
+220. ### Why do we use array destructuring (square brackets notation) in `useState`?
 
-     **React.forwardRef** accepts a render function as parameter and DevTools uses this function to determine what to display for the ref forwarding component.
+     When we declare a state variable with `useState`, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
 
-     For example, If you don't name the render function or not using displayName property then it will appear as ”ForwardRef” in the DevTools,
+     For example, the array index access would look as follows:
 
      ```javascript
-     const WrappedComponent = React.forwardRef((props, ref) => {
-       return <LogProps {...props} forwardedRef={ref} />;
-     });
+     var userStateVariable = useState("userProfile"); // Returns an array pair
+     var user = userStateVariable[0]; // Access first item
+     var setUser = userStateVariable[1]; // Access second item
      ```
 
-     But If you name the render function then it will appear as **”ForwardRef(myFunction)”**
+     Whereas with array destructuring the variables can be accessed as follows:
 
      ```javascript
-     const WrappedComponent = React.forwardRef(
-       function myFunction(props, ref) {
-         return <LogProps {...props} forwardedRef={ref} />;
-       }
-     );
+     const [user, setUser] = useState("userProfile");
      ```
 
-     As an alternative, You can also set displayName property for forwardRef function,
+     **[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     function logProps(Component) {
-       class LogProps extends React.Component {
-         // ...
-       }
+221. ### What are the sources used for introducing hooks?
 
-       function forwardRef(props, ref) {
-         return <LogProps {...props} forwardedRef={ref} />;
-       }
+     Hooks got the ideas from several different sources. Below are some of them,
 
-       // Give this component a more helpful display name in DevTools.
-       // e.g. "ForwardRef(logProps(MyComponent))"
-       const name = Component.displayName || Component.name;
-       forwardRef.displayName = `logProps(${name})`;
+     1. Previous experiments with functional APIs in the react-future repository
+     2. Community experiments with render prop APIs such as Reactions Component
+     3. State variables and state cells in DisplayScript.
+     4. Subscriptions in Rx.
+     5. Reducer components in ReasonReact.
 
-       return React.forwardRef(forwardRef);
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-256. ### When component props defaults to true?
-     If you pass no value for a prop, it defaults to true. This behavior is available so that it matches the behavior of HTML.
+222. ### How do you access imperative API of web components?
+     Web Components often expose an imperative API to implement its functions. You will need to use a **ref** to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a **wrapper** for your Web Component.
 
-     For example, below expressions are equivalent,
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     <MyInput autocomplete />
+223. ### What is formik?
 
-     <MyInput autocomplete={true} />
-     ```
-     **Note:** It is not recommended to use this approach because it can be confused with the ES6 object shorthand (example, `{name}` which is short for `{name: name}`)
+     Formik is a small react form library that helps you with the three major problems,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-257. ### What is NextJS and major features of it?
-     Next.js is a popular and lightweight framework for static and server‑rendered applications built with React. It also provides styling and routing solutions. Below are the major features provided by NextJS,
+     1. Getting values in and out of form state
+     2. Validation and error messages
+     3. Handling form submission
 
-     1. Server-rendered by default
-     2. Automatic code splitting for faster page loads
-     3. Simple client-side routing (page based)
-     4. Webpack-based dev environment which supports (HMR)
-     5. Able to implement with Express or any other Node.js HTTP server
-     6. Customizable with your own Babel and Webpack configurations
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-258. ### How do you pass an event handler to a component?
-     You can pass event handlers and other functions as props to child components. It can be used in child component as  below,
+224. ### What are typical middleware choices for handling asynchronous calls in Redux?
+     Some of the popular middleware choices for handling asynchronous calls in Redux eco system are `Redux Thunk, Redux Promise, Redux Saga`.
 
-     ```html
-     <button onClick={this.handleClick}>
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-259. ### Is it good to use arrow functions in render methods?
-     Yes, You can use. It is often the easiest way to pass parameters to callback functions. But you need to optimize the performance while using it.
+225. ### Do browsers understand JSX code?
+     No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.
 
-     ```javascript
-     class Foo extends Component {
-       handleClick() {
-         console.log('Click happened');
-       }
-       render() {
-         return <button onClick={() => this.handleClick()}>Click Me</button>;
-       }
-     }
+**[⬆ Back to Top](#table-of-contents)**
+
+226. ### Describe about data flow in react?
+     React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+227. ### What is MobX?
+     MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For ReactJS application, you need to install below packages,
+     ```bash
+     npm install mobx --save
+     npm install mobx-react --save
      ```
 
-     **Note:** Using an arrow function in render method creates a new function each time the component renders, which may have performance implications
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-260. ### How to prevent a function from being called multiple times?
-     If you use an event handler such as **onClick or onScroll** and want to prevent the callback from being fired too quickly, then you can limit the rate at which callback is executed. This can be achieved in the below possible ways,
+228. ### What are the differences between Redux and MobX?
 
-     1. **Throttling:** Changes based on a time based frequency. For example, it can be used using _.throttle lodash function
-     2. **Debouncing:** Publish changes after a period of inactivity. For example, it can be used using _.debounce lodash function
-     3. **RequestAnimationFrame throttling:** Changes based on requestAnimationFrame. For example, it can be used using raf-schd lodash function
+     Below are the main differences between Redux and MobX,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-261. ### How JSX prevents Injection Attacks?
-     React DOM escapes any values embedded in JSX before rendering them. Thus it ensures that you can never inject anything that’s not explicitly written in your application. Everything is converted to a string before being rendered.
+     | Topic         | Redux                                                         | MobX                                                                   |
+     | ------------- | ------------------------------------------------------------- | ---------------------------------------------------------------------- |
+     | Definition    | It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
+     | Programming   | It is mainly written in ES6                                   | It is written in JavaScript(ES5)                                       |
+     | Data Store    | There is only one large store exist for data storage          | There is more than one store for storage                               |
+     | Usage         | Mainly used for large and complex applications                | Used for simple applications                                           |
+     | Performance   | Need to be improved                                           | Provides better performance                                            |
+     | How it stores | Uses JS Object to store                                       | Uses observable to store the data                                      |
 
-     For example, you can embed user input as below,
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     const name = response.potentiallyMaliciousInput;
-     const element = <h1>{name}</h1>;
-     ```
-     This way you can prevent XSS(Cross-site-scripting) attacks in the application.
+229. ### Should I learn ES6 before learning ReactJS?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-262. ### How do you update rendered elements?
-     You can update UI(represented by rendered element) by passing the newly created element to ReactDOM's render method.
+     No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,
 
-     For example, lets take a ticking clock example, where it updates the time by calling render method multiple times,
+     1. **Destructuring:** To get props and use them in a component
 
-     ```javascript
-     function tick() {
-       const element = (
-         <div>
-           <h1>Hello, world!</h1>
-           <h2>It is {new Date().toLocaleTimeString()}.</h2>
-         </div>
-       );
-       ReactDOM.render(element, document.getElementById('root'));
-     }
+        ```javascript
+        // in es 5
+        var someData = this.props.someData;
+        var dispatch = this.props.dispatch;
 
-     setInterval(tick, 1000);
-     ```
+        // in es6
+        const { someData, dispatch } = this.props;
+        ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-263. ### How do you say that props are readonly?
-     When you declare a component as a function or a class, it must never modify its own props.
+     2. **Spread operator:** Helps in passing props down into a component
 
-     Let us take a below capital function,
+        ```javascript
+        // in es 5
+        <SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} />
 
-     ```javascript
-     function capital(amount, interest) {
-        return amount + interest;
-     }
-     ```
-     The above function is called “pure” because it does not attempt to change their inputs, and always return the same result for the same inputs. Hence, React has a single rule saying "All React components must act like pure functions with respect to their props."
+        // in es6
+        <SomeComponent {...this.props} />
+        ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-264. ### How do you say that state updates are merged?
-     When you call setState() in the component, React merges the object you provide into the current state.
+     3. **Arrow functions:** Makes compact syntax
+        ```javascript
+        // es 5
+        var users = usersList.map(function (user) {
+          return <li>{user.name}</li>;
+        });
+        // es 6
+        const users = usersList.map((user) => <li>{user.name}</li>);
+        ```
 
-     For example, let us take a facebook user with posts and comments details as state variables,
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-       constructor(props) {
-         super(props);
-         this.state = {
-           posts: [],
-           comments: []
-         };
-       }
-     ```
+230. ### What is Concurrent Rendering?
 
-     Now you can update them independently with separate `setState()` calls as below,
+     The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there's something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,
 
      ```javascript
-      componentDidMount() {
-         fetchPosts().then(response => {
-           this.setState({
-             posts: response.posts
-           });
-         });
+     // 1. Part of an app by wrapping with ConcurrentMode
+     <React.unstable_ConcurrentMode>
+       <Something />
+     </React.unstable_ConcurrentMode>;
 
-         fetchComments().then(response => {
-           this.setState({
-             comments: response.comments
-           });
-         });
-       }
+     // 2. Whole app using createRoot
+     ReactDOM.unstable_createRoot(domNode).render(<App />);
      ```
-     As mentioned in the above code snippets, `this.setState({comments})` updates only comments variable without modifying or replacing `posts` variable.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-265. ### How do you pass arguments to an event handler?
-     During iterations or loops, it is common to pass an extra parameter to an event handler. This can be achieved through arrow functions or bind method.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Let us take an example of user details updated in a grid,
+231. ### What is the difference between async mode and concurrent mode?
+     Both refers the same thing. Previously concurrent Mode being referred to as "Async Mode" by React team. The name has been changed to highlight React’s ability to perform work on different priority levels. So it avoids the confusion from other approaches to Async Rendering.
 
-     ```javascript
-     <button onClick={(e) => this.updateUser(userId, e)}>Update User details</button>
-     <button onClick={this.updateUser.bind(this, userId)}>Update User details</button>
-     ```
-     In the both approaches, the synthetic argument `e` is passed as a second argument. You need to pass it explicitly for arrow functions and it will be passed automatically for `bind` method.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-266. ### How to prevent component from rendering?
-     You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.
+232. ### Can I use javascript urls in react16.9?
+
+     Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like `<a href>` and create a security hole.
 
      ```javascript
-     function Greeting(props) {
-       if (!props.loggedIn) {
-         return null;
-       }
-
-       return (
-         <div className="greeting">
-           welcome, {props.name}
-         </div>
-       );
-     }
-     ```
-     ```javascript
-     class User extends React.Component {
-       constructor(props) {
-         super(props);
-         this.state = {loggedIn: false, name: 'John'};
-       }
-
-       render() {
-        return (
-            <div>
-              //Prevent component render if it is not loggedIn
-              <Greeting loggedIn={this.state.loggedIn} />
-              <UserDetails name={this.state.name}>
-            </div>
-        );
-       }
-     ```
-     In the above example, the `greeting` component skips its rendering section by applying condition and returning null value.
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-267. ### What are the conditions to safely use the index as a key?
-     There are three conditions to make sure, it is safe use the index as a key.
-
-     1. The list and items are static– they are not computed and do not change
-     2. The items in the list have no ids
-     3. The list is never reordered or filtered.
-
-
-   **[⬆ Back to Top](#table-of-contents)**
-    
-268. ### Should keys be globally unique?
-     The keys used within arrays should be unique among their siblings but they don’t need to be globally unique. i.e, You can use the same keys with two different arrays.
-
-     For example, the below `Book` component uses two arrays with different arrays,
-
-     ```javascript
-     function Book(props) {
-       const index = (
-         <ul>
-           {props.pages.map((page) =>
-             <li key={page.id}>
-               {page.title}
-             </li>
-           )}
-         </ul>
-       );
-       const content = props.pages.map((page) =>
-         <div key={page.id}>
-           <h3>{page.title}</h3>
-           <p>{page.content}</p>
-           <p>{page.pageNumber}</p>
-         </div>
-       );
-       return (
-         <div>
-           {index}
-           <hr />
-           {content}
-         </div>
-       );
-     }
+     const companyProfile = {
+       website: "javascript: alert('Your website is hacked')",
+     };
+     // It will log a warning
+     <a href={companyProfile.website}>More details</a>;
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-269. ### What is the popular choice for form handling?
-     `Formik` is a form library for react which provides solutions such as validation, keeping track of the visited fields, and handling form submission.
-
-     In detail, You can categorize them as follows,
+     Remember that the future versions will throw an error for javascript URLs.
 
-     1. Getting values in and out of form state
-     2. Validation and error messages
-     3. Handling form submission
+**[⬆ Back to Top](#table-of-contents)**
 
-     It is used to create a scalable, performant, form helper with a minimal API to solve annoying stuff.
+233. ### What is the purpose of eslint plugin for hooks?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-270. ### What are the advantages of formik over redux form library?
-     Below are the main reasons to recommend formik over redux form library,
+     The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,
 
-     1. The form state is inherently short-term and local, so tracking it in Redux (or any kind of Flux library) is unnecessary.
-     2. Redux-Form calls your entire top-level Redux reducer multiple times ON EVERY SINGLE KEYSTROKE. This way it increases input latency for large apps.
-     3. Redux-Form is 22.5 kB minified gzipped whereas Formik is 12.7 kB
+     1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
+     2. Hooks are called in the same order on every render.
 
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-271. ### Why are you not required to use inheritance?
-     In React, it is recommended to use composition over inheritance to reuse code between components. Both Props and composition give you all the flexibility you need to customize a component’s look and behavior explicitly and safely.
-     Whereas, If you want to reuse non-UI functionality between components, it is suggested to extract it into a separate JavaScript module. Later components import it and use that function, object, or class, without extending it.
+234. ### What is the difference between Imperative and Declarative in React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-272. ### Can I use web components in react application?
-     Yes, you can use web components in a react application. Even though many developers won't use this combination, it may require especially if you are using third-party UI components that are written using Web Components.
+     Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.
 
-     For example, let us use `Vaadin` date picker web component as below,
+     The imperative way of doing this would be:
 
      ```javascript
-     import React, { Component } from 'react';
-     import './App.css';
-     import '@vaadin/vaadin-date-picker';
-     class App extends Component {
-       render() {
-         return (
-           <div className="App">
-             <vaadin-date-picker label="When were you born?"></vaadin-date-picker>
-           </div>
-         );
+     if (user.likes()) {
+       if (hasBlue()) {
+         removeBlue();
+         addGrey();
+       } else {
+         removeGrey();
+         addBlue();
        }
      }
-     export default App;
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-273. ### What is dynamic import?
-     You can achieve code-splitting in your app using dynamic import.
+     Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
 
-     Let's take an example of addition,
+     In contrast, the declarative approach would be:
 
-     1. **Normal Import**
-     ```javascript
-     import { add } from './math';
-     console.log(add(10, 20));
-     ```
-     2. **Dynamic Import**
      ```javascript
-     import("./math").then(math => {
-       console.log(math.add(10, 20));
-     });
+     if (this.state.liked) {
+       return <blueLike />;
+     } else {
+       return <greyLike />;
+     }
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-274. ### What are loadable components?
-     If you want to do code-splitting in a server rendered app, it is recommend to use Loadable Components because React.lazy and Suspense is not yet available for server-side rendering. Loadable lets you render a dynamic import as a regular component.
+     Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a specific state, and is therefore much simpler to understand.
 
-     Lets take an example,
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     import loadable from '@loadable/component'
+235. ### What are the benefits of using TypeScript with ReactJS?
 
-     const OtherComponent = loadable(() => import('./OtherComponent'))
+     Below are some of the benefits of using TypeScript with ReactJS,
 
-     function MyComponent() {
-       return (
-         <div>
-           <OtherComponent />
-         </div>
-       )
-     }
-     ```
-     Now OtherComponent will be loaded in a separated bundle
+     1. It is possible to use latest JavaScript features
+     2. Use of interfaces for complex type definitions
+     3. IDEs such as VS Code was made for TypeScript
+     4. Avoid bugs with the ease of readability and Validation
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-275. ### What is suspense component?
-     If the module containing the dynamic import is not yet loaded by the time parent component renders, you must show some fallback content while you’re waiting for it to load using a loading indicator. This can be done using **Suspense** component.
+     **[⬆ Back to Top](#table-of-contents)**
 
-     For example, the below code uses suspense component,
+236. ### How do you make sure that user remains authenticated on page refresh while using Context API State Management?
 
-     ```javascript
-     const OtherComponent = React.lazy(() => import('./OtherComponent'));
+     When a user logs in and reload, to persist the state generally we add the load user action in the useEffect hooks in the main App.js. While using Redux, loadUser action can be easily accessed.
 
-     function MyComponent() {
-       return (
-         <div>
-           <Suspense fallback={<div>Loading...</div>}>
-             <OtherComponent />
-           </Suspense>
-         </div>
-       );
-     }
-     ```
-     As mentioned in the above code, Suspense is wrapped above the lazy component.
+     **App.js**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-276. ### What is route based code splitting?
-     One of the best place to do code splitting is with routes. The entire page is going to re-render at once so users are unlikely to interact with other elements in the page at the same time. Due to this, the user experience won't be disturbed.
+     ```js
+     import { loadUser } from "../actions/auth";
+     store.dispatch(loadUser());
+     ```
 
-     Let us take an example of route based website using libraries like React Router with React.lazy,
+     - But while using **Context API**, to access context in App.js, wrap the AuthState in index.js so that App.js can access the auth context. Now whenever the page reloads, no matter what route you are on, the user will be authenticated as **loadUser** action will be triggered on each re-render.
 
-     ```javascript
-     import { BrowserRouter as Router, Route, Switch } from 'react-router-dom';
-     import React, { Suspense, lazy } from 'react';
+     **index.js**
 
-     const Home = lazy(() => import('./routes/Home'));
-     const About = lazy(() => import('./routes/About'));
+     ```js
+     import React from "react";
+     import ReactDOM from "react-dom";
+     import App from "./App";
+     import AuthState from "./context/auth/AuthState";
 
-     const App = () => (
-       <Router>
-         <Suspense fallback={<div>Loading...</div>}>
-           <Switch>
-             <Route exact path="/" component={Home}/>
-             <Route path="/about" component={About}/>
-           </Switch>
-         </Suspense>
-       </Router>
+     ReactDOM.render(
+       <React.StrictMode>
+         <AuthState>
+           <App />
+         </AuthState>
+       </React.StrictMode>,
+       document.getElementById("root")
      );
      ```
-     In the above code, the code splitting will happen at each route level.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-277. ### Give an example on How to use context?
-     **Context** is designed to share data that can be considered **global** for a tree of React components.
+     **App.js**
 
-     For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.
+     ```js
+     const authContext = useContext(AuthContext);
 
-     ```javascript
-     //Lets create a context with a default theme value "luna"
-     const ThemeContext = React.createContext('luna');
-     // Create App component where it uses provider to pass theme value in the tree
-     class App extends React.Component {
-       render() {
-         return (
-           <ThemeContext.Provider value="nova">
-             <Toolbar />
-           </ThemeContext.Provider>
-         );
-       }
-     }
-     // A middle component where you don't need to pass theme prop anymore
-     function Toolbar(props) {
-       return (
-         <div>
-           <ThemedButton />
-         </div>
-       );
-     }
-     // Lets read theme value in the button component to use
-     class ThemedButton extends React.Component {
-       static contextType = ThemeContext;
-       render() {
-         return <Button theme={this.context} />;
-       }
-     }
+     const { loadUser } = authContext;
+
+     useEffect(() => {
+       loadUser();
+     }, []);
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-278. ### What is the purpose of default value in context?
-     The defaultValue argument is only used when a component does not have a matching Provider above it in the tree. This can be helpful for testing components in isolation without wrapping them.
+     **loadUser**
 
-     Below code snippet provides default theme value as Luna.
+     ```js
+     const loadUser = async () => {
+       const token = sessionStorage.getItem("token");
 
-     ```javascript
-     const MyContext = React.createContext(defaultValue);
-     ```
+       if (!token) {
+         dispatch({
+           type: ERROR,
+         });
+       }
+       setAuthToken(token);
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-279. ### How do you use contextType?
-     ContextType is used to consume the context object. The contextType property can be used in two ways,
+       try {
+         const res = await axios("/api/auth");
 
-     1. **contextType as property of class:**
-         The contextType property on a class can be assigned a Context object created by React.createContext(). After that, you can consume the nearest current value of that Context type using this.context in any of the lifecycle methods and render function.
+         dispatch({
+           type: USER_LOADED,
+           payload: res.data.data,
+         });
+       } catch (err) {
+         console.error(err);
+       }
+     };
+     ```
 
-         Lets assign contextType property on MyClass as below,
+**[⬆ Back to Top](#table-of-contents)**
 
-         ```javascript
-         class MyClass extends React.Component {
-           componentDidMount() {
-             let value = this.context;
-             /* perform a side-effect at mount using the value of MyContext */
-           }
-           componentDidUpdate() {
-             let value = this.context;
-             /* ... */
-           }
-           componentWillUnmount() {
-             let value = this.context;
-             /* ... */
-           }
-           render() {
-             let value = this.context;
-             /* render something based on the value of MyContext */
-           }
-         }
-         MyClass.contextType = MyContext;
-         ```
+237. ### What are the benefits of new JSX transform?
 
-     2. **Static field**
-         You can use a static class field to initialize your contextType using public class field syntax.
+     There are three major benefits of new JSX transform,
 
-         ```javascript
-         class MyClass extends React.Component {
-           static contextType = MyContext;
-           render() {
-             let value = this.context;
-             /* render something based on the value */
-           }
-         }
-         ```
+     1. It is possible to use JSX without importing React packages
+     2. The compiled output might improve the bundle size in a small amount
+     3. The future improvements provides the flexibility to reduce the number of concepts to learn React.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-280. ### What is a consumer?
-     A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Lets take a simple example,
+238. ### How is the new JSX transform different from old transform??
 
-     ```javascript
-     <MyContext.Consumer>
-       {value => /* render something based on the context value */}
-     </MyContext.Consumer>
-     ```
+     The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-281. ### How do you solve performance corner cases while using context?
-     The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders.
+     Let's take an example to look at the main differences between the old and the new transform,
 
-     For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.
+     **Old Transform:**
 
-     ```javascript
-     class App extends React.Component {
-       render() {
-         return (
-           <Provider value={{something: 'something'}}>
-             <Toolbar />
-           </Provider>
-         );
-       }
+     ```js
+     import React from "react";
+
+     function App() {
+       return <h1>Good morning!!</h1>;
      }
      ```
 
-     This can be solved by lifting up the value to parent state,
+     Now JSX transform convert the above code into regular JavaScript as below,
 
-     ```javascript
-     class App extends React.Component {
-       constructor(props) {
-         super(props);
-         this.state = {
-           value: {something: 'something'},
-         };
-       }
+     ```js
+     import React from "react";
 
-       render() {
-         return (
-           <Provider value={this.state.value}>
-             <Toolbar />
-           </Provider>
-         );
-       }
+     function App() {
+       return React.createElement("h1", null, "Good morning!!");
      }
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-282. ### What is the purpose of forward ref in HOCs?
-     Refs will not get passed through because ref is not a prop. It is handled differently by React just like **key**. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API.
+     **New Transform:**
 
-     The below HOC logs all props,
-     
-     ```javascript
-         function logProps(Component) {
-           class LogProps extends React.Component {
-             componentDidUpdate(prevProps) {
-               console.log('old props:', prevProps);
-               console.log('new props:', this.props);
-             }
-
-             render() {
-               const {forwardedRef, ...rest} = this.props;
-
-               // Assign the custom prop "forwardedRef" as a ref
-               return <Component ref={forwardedRef} {...rest} />;
-             }
-           }
+     The new JSX transform doesn't require any React imports
 
-           return React.forwardRef((props, ref) => {
-             return <LogProps {...props} forwardedRef={ref} />;
-           });
-         }
+     ```js
+     function App() {
+       return <h1>Good morning!!</h1>;
+     }
      ```
 
-     Let's use this HOC to log all props that get passed to our “fancy button” component,
+     Under the hood JSX transform compiles to below code
 
-     ```javascript
-         class FancyButton extends React.Component {
-           focus() {
-             // ...
-           }
+     ```js
+     import { jsx as _jsx } from "react/jsx-runtime";
 
-           // ...
-         }
-         export default logProps(FancyButton);
+     function App() {
+       return _jsx("h1", { children: "Good morning!!" });
+     }
      ```
 
-     Now let's create a ref and pass it to FancyButton component. In this case, you can set focus to button element.
+     **Note:** You still need to import React to use Hooks.
 
-     ```javascript
-         import FancyButton from './FancyButton';
+**[⬆ Back to Top](#table-of-contents)**
 
-         const ref = React.createRef();
-         ref.current.focus();
-         <FancyButton
-           label="Click Me"
-           handleClick={handleClick}
-           ref={ref}
-         />;
-     ```
+239. ### What are React Server components?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-283. ### Is ref argument available for all functions or class components?
-     Regular function or class components don’t receive the ref argument, and ref is not available in props either. The second ref argument only exists when you define a component with React.forwardRef call.
+     React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-284. ### Why do you need additional care for component libraries while using forward refs?
-     When you start using forwardRef in a component library, you should treat it as a breaking change and release a new major version of your library. This is because your library likely has a different behavior such as what refs get assigned to, and what types are exported. These changes can break apps and other libraries that depend on the old behavior.
+     **Note:** React Server Components is still under development and not recommended for production yet.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-285. ### How to create react class components without ES6?
-     If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     var Greeting = createReactClass({
-       getDefaultProps: function() {
-           return {
-             name: 'Jhohn'
-           };
-         },
-       getInitialState: function() {
-           return {message: this.props.message};
-         },
-       handleClick: function() {
-          console.log(this.state.message);
-       },
-       render: function() {
-         return <h1>Hello, {this.props.name}</h1>;
-       }
-     });
-     ```
-     **Note:** If you use createReactClass then auto binding is available for all methods. i.e, You don't need to use `.bind(this)` with in constructor for event handlers.
+240. ### What is prop drilling?
+     Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-286. ### Is it possible to use react without JSX?
-     Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling `React.createElement(component, props, ...children)`.
+**[⬆ Back to Top](#table-of-contents)**
 
-     For example, let us take a greeting example with JSX,
+241. ### What is the difference between useState and useRef hook?
+     1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes.
+        Essentially, useRef is like a “box” that can hold a mutable value in its (`.current`) property.
+     2. useState allows us to update the state inside components. While useRef allows referencing DOM elements and tracking values.
 
-     ```javascript
-     class Greeting extends React.Component {
-       render() {
-         return <div>Hello {this.props.message}</div>;
-       }
-     }
+**[⬆ Back to Top](#table-of-contents)**
 
-     ReactDOM.render(
-       <Greeting message="World" />,
-       document.getElementById('root')
-     );
-     ```
+242. ### What is a wrapper component?
 
-     You can write the same code without JSX as below,
+     A wrapper in React is a component that wraps or surrounds another component or group of components. It can be used for a variety of purposes such as adding additional functionality, styling, or layout to the wrapped components.
 
-     ```javascript
-     class Greeting extends React.Component {
-       render() {
-         return React.createElement('div', null, `Hello ${this.props.message}`);
-       }
-     }
+     For example, consider a simple component that displays a message:
 
-     ReactDOM.render(
-       React.createElement(Greeting, {message: 'World'}, null),
-       document.getElementById('root')
-     );
+     ```javascript
+     const Message = ({ text }) => {
+       return <p>{text}</p>;
+     };
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-287. ### What is diffing algorithm?
-     React needs to use algorithms to find out how to efficiently update the UI to match the most recent tree. The diffing algorithms is generating the minimum number of operations to transform one tree into another. However, the algorithms have a complexity in the order of O(n3) where n is the number of elements in the tree.
+     We can create a wrapper component that will add a border to the message component:
 
-     In this case, displaying 1000 elements would require in the order of one billion comparisons. This is far too expensive. Instead, React implements a heuristic O(n) algorithm based on two assumptions:
+     ```javascript
+     const MessageWrapper = (props) => {
+       return (
+         <div style={{ border: "1px solid black" }}>
+           <Message {...props} />
+         </div>
+       );
+     };
+     ```
 
-     1. Two elements of different types will produce different trees.
-     2. The developer can hint at which child elements may be stable across different renders with a key prop.
+     Now we can use the MessageWrapper component instead of the Message component and the message will be displayed with a border:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-288. ### What are the rules covered by diffing algorithm?
-     When diffing two trees, React first compares the two root elements. The behavior is different depending on the types of the root elements. It covers the below rules during reconciliation algorithm,
+     ```javascript
+     <MessageWrapper text="Hello World" />
+     ```
 
-     1. **Elements Of Different Types:**
-        Whenever the root elements have different types, React will tear down the old tree and build the new tree from scratch. For example,  elements <a> to <img>, or from <Article> to <Comment> of different types lead a full rebuild.
-     2. **DOM Elements Of The Same Type:**
-        When comparing two React DOM elements of the same type, React looks at the attributes of both, keeps the same underlying DOM node, and only updates the changed attributes. Lets take an example with same DOM elements except className attribute,
-        ```javascript
-        <div className="show" title="ReactJS" />
+     Wrapper component can also accept its own props and pass them down to the wrapped component, for example, we can create a wrapper component that will add a title to the message component:
 
-        <div className="hide" title="ReactJS" />
-        ```
-     3. **Component Elements Of The Same Type:**
-        When a component updates, the instance stays the same, so that state is maintained across renders. React updates the props of the underlying component instance to match the new element, and calls componentWillReceiveProps() and componentWillUpdate() on the underlying instance. After that, the render() method is called and the diff algorithm recurses on the previous result and the new result.
-     4. **Recursing On Children:**
-        when recursing on the children of a DOM node, React just iterates over both lists of children at the same time and generates a mutation whenever there’s a difference. For example, when adding an element at the end of the children, converting between these two trees works well.
-        ```javascript
-        <ul>
-          <li>first</li>
-          <li>second</li>
-        </ul>
+     ```javascript
+     const MessageWrapperWithTitle = ({ title, ...props }) => {
+       return (
+         <div>
+           <h3>{title}</h3>
+           <Message {...props} />
+         </div>
+       );
+     };
+     ```
 
-        <ul>
-          <li>first</li>
-          <li>second</li>
-          <li>third</li>
-        </ul>
+     Now we can use the MessageWrapperWithTitle component and pass title props:
 
-        ```
-     5. **Handling keys:**
-     React supports a key attribute. When children have keys, React uses the key to match children in the original tree with children in the subsequent tree. For example, adding a key can make the tree conversion efficient,
      ```javascript
-     <ul>
-       <li key="2015">Duke</li>
-       <li key="2016">Villanova</li>
-     </ul>
-
-     <ul>
-       <li key="2014">Connecticut</li>
-       <li key="2015">Duke</li>
-       <li key="2016">Villanova</li>
-     </ul>
+     <MessageWrapperWithTitle title="My Message" text="Hello World" />
      ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-289. ### When do you need to use refs?
-     There are few use cases to go for refs,
+     This way, the wrapper component can add additional functionality, styling, or layout to the wrapped component while keeping the wrapped component simple and reusable.
 
-     1. Managing focus, text selection, or media playback.
-     2. Triggering imperative animations.
-     3. Integrating with third-party DOM libraries.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-290. ### Must prop be named as render for render props?
-     Even though the pattern named render props, you don’t have to use a prop named render to use this pattern. i.e,  Any prop that is a function that a component uses to know what to render is technically a “render prop”. Lets take an example with the children prop for render props,
+243. ### What are the differences between useEffect and useLayoutEffect hooks?
 
-     ```javascript
-     <Mouse children={mouse => (
-       <p>The mouse position is {mouse.x}, {mouse.y}</p>
-     )}/>
-     ```
+     useEffect and useLayoutEffect are both React hooks that can be used to synchronize a component with an external system, such as a browser API or a third-party library. However, there are some key differences between the two:
 
-     Actually children prop doesn’t need to be named in the list of “attributes” in JSX element. Instead, you can keep it directly inside element,
+     - Timing: useEffect runs after the browser has finished painting, while useLayoutEffect runs synchronously before the browser paints. This means that useLayoutEffect can be used to measure and update layout in a way that feels more synchronous to the user.
 
-     ```javascript
-     <Mouse>
-       {mouse => (
-         <p>The mouse position is {mouse.x}, {mouse.y}</p>
-       )}
-     </Mouse>
-     ```
+     - Browser Paint: useEffect allows browser to paint the changes before running the effect, hence it may cause some visual flicker. useLayoutEffect synchronously runs the effect before browser paints and hence it will avoid visual flicker.
 
-     While using this above technique(without any name), explicitly state that children should be a function in your propTypes.
+     - Execution Order: The order in which multiple useEffect hooks are executed is determined by React and may not be predictable. However, the order in which multiple useLayoutEffect hooks are executed is determined by the order in which they were called.
 
-     ```javascript
-     Mouse.propTypes = {
-       children: PropTypes.func.isRequired
-     };
-     ```
+     - Error handling: useEffect has a built-in mechanism for handling errors that occur during the execution of the effect, so that it does not crash the entire application. useLayoutEffect does not have this mechanism, and errors that occur during the execution of the effect will crash the entire application.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-291. ### What are the problems of using render props with pure components?
-     If you create a function inside a render method, it negates the purpose of pure component. Because the shallow prop comparison will always return false for new props, and each render in this case will generate a new value for the render prop. You can solve this issue by defining the render function as instance method.
+     In general, it's recommended to use useEffect as much as possible, because it is more performant and less prone to errors. useLayoutEffect should only be used when you need to measure or update layout, and you can't achieve the same result using useEffect.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-292. ### How do you create HOC using render props?
-     You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a <Mouse> component, you could easily create one using a regular <Mouse> with a render prop.
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-     function withMouse(Component) {
-       return class extends React.Component {
-         render() {
-           return (
-             <Mouse render={mouse => (
-               <Component {...this.props} mouse={mouse} />
-             )}/>
-           );
-         }
-       }
-     }
-     ```
+244. ### What are the differences between Functional and Class Components?
 
-     This way render props gives the flexibility of using either pattern.
+     There are two different ways to create components in ReactJS. The main differences are listed down as below,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-293. ### What is windowing technique?
-     Windowing is a technique that only renders a small subset of your rows at any given time, and can dramatically reduce the time it takes to re-render the components as well as the number of DOM nodes created. If your application renders long lists of data then this technique is recommended. Both react-window and react-virtualized are popular windowing libraries which provides several reusable components for displaying lists, grids, and tabular data.
+     ## 1. Syntax:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-294. ### How do you print falsy values in JSX?
-     The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,
+     The class components uses ES6 classes to create the components. It uses `render` function to display the HTML content in the webpage.
 
-     ```javascript
-     <div>
-       My JavaScript variable is {String(myVariable)}.
-     </div>
-     ```
+     The syntax for class component looks like as below.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-295. ### What is the typical use case of portals?
-     React portals are very useful when a parent component has overflow: hidden or has properties that affect the stacking context (e.g. z-index, position, opacity) and you need to visually “break out” of its container.
+     ```js
+     class App extends React.Component {
+       render() {
+         return <h1>This is a class component</h1>;
+       }
+     }
+     ```
 
-     For example, dialogs, global message notifications, hovercards, and tooltips.
+     **Note:** The **Pascal Case** is the recommended approach to provide naming to a component.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-296. ### How do you set default value for uncontrolled component?
-     In React, the value attribute on form elements will override the value in the DOM. With an uncontrolled component, you might want React to specify the initial value, but leave subsequent updates uncontrolled. To handle this case, you can specify a **defaultValue** attribute instead of **value**.
+     Functional component has been improved over the years with some added features like Hooks. Here is a syntax for functional component.
 
-     ```javascript
-     render() {
+     ```js
+     function App() {
        return (
-         <form onSubmit={this.handleSubmit}>
-           <label>
-             User Name:
-             <input
-               defaultValue="John"
-               type="text"
-               ref={this.input} />
-           </label>
-           <input type="submit" value="Submit" />
-         </form>
+         <div className="App">
+           <h1>Hello, I'm a function component</h1>
+         </div>
        );
      }
      ```
-     The same applies for `select` and `textArea` inputs. But you need to use **defaultChecked** for `checkbox` and `radio` inputs.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-297. ### What is your favorite React stack?
-     Even though the tech stack varies from developer to developer, the most popular stack is used in react boilerplate project code. It mainly uses Redux and redux-saga for state management and asynchronous side-effects, react-router for routing purpose, styled-components for styling react components, axios for invoking REST api, and other supported stack such as webpack, reselect, ESNext, Babel.
-     You can clone the project https://github.com/react-boilerplate/react-boilerplate and start working on any new react project.
+     ## 2. State:
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-298. ### What is the difference between Real DOM and Virtual DOM?
-     Below are the main differences between Real DOM and Virtual DOM,
+     State contains information or data about a component which may change over time.
 
-     | Real DOM | Virtual DOM |
-     | ----- | ------- |
-     | Updates are slow | Updates are fast |
-     | DOM manipulation is very expensive. | DOM manipulation is very easy |
-     | You can update HTML directly. | You Can’t directly update HTML |
-     | It causes too much of memory wastage | 	There is no memory wastage|
-     | Creates a new DOM if element updates | It updates the JSX if element update|
+     In class component, you can update the state when a user interacts with it or server updates the data using the `setState()` method. The initial state is going to be assigned in the `Constructor()` method using the `this.state` object and it is possible to assign different data types such as string, boolean, numbers, etc.
 
+     **A simple example showing how we use the setState() and constructor():**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-299. ### How to add Bootstrap to a react application?
-     Bootstrap can be added to your React app in a three possible ways,
+     ```js
+     class App extends Component {
+       constructor() {
+         super();
+         this.state = {
+           message: "This is a class component",
+         };
+       }
+       updateMessage() {
+         this.setState({
+           message: "Updating the class component",
+         });
+       }
+       render() {
+         return (
+           <>
+             <h1>{this.state.message}</h1>
+             <button
+               onClick={() => {
+                 this.updateMessage();
+               }}>
+               Click!!
+             </button>
+           </>
+         );
+       }
+     }
 
-     1. Using the Bootstrap CDN:
-        This is the easiest way to add bootstrap. Add both bootstrap CSS and JS resources in a head tag.
-     2. Bootstrap as Dependency:
-        If you are using a build tool or a module bundler such as Webpack, then this is the preferred option for adding Bootstrap to your React application
-        ```javascript
-        npm install bootstrap
-        ```
-     3. React Bootstrap Package:
-        In this case, you can add Bootstrap to our React app is by using a package that has rebuilt Bootstrap components to work particularly as React components. Below packages are popular in this category,
-        1. react-bootstrap
-        2. reactstrap
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-300. ### Can you list down top websites or applications using react as front end framework?
-     Below are the `top 10 websites` using React as their front-end framework,
+     You didn't use state in functional components because it was only supported in class components. But over the years hooks have been implemented in functional components which enables to use state too.
 
-     1. Facebook
-     2. Uber
-     3. Instagram
-     4. WhatsApp
-     5. Khan Academy
-     6. Airbnb
-     7. Dropbox
-     8. Flipboard
-     9. Netflix
-     10. PayPal
+     The `useState()` hook can used to implement state in functional components. It returns an array with two items: the first item is current state and the next one is a function (setState) that updates the value of the current state.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-301. ### Is it recommended to use CSS In JS technique in React?
-     React does not have any opinion about how styles are defined but if you are a beginner then good starting point is to define your styles in a separate *.css file as usual and refer to them using className. This functionality is not part of React but came from third-party libraries. But If you want to try a different approach(CSS-In-JS) then styled-components library is a good option.
+     Let's see an example to demonstrate the state in functional components,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-302. ### Do I need to rewrite all my class components with hooks?
-     No. But you can try Hooks in a few components(or new components) without rewriting any existing code. Because there are no plans to remove classes in ReactJS.
+     ```js
+     function App() {
+       const [message, setMessage] = useState("This is a functional component");
+       const updateMessage = () => {
+         setMessage("Updating the functional component");
+       };
+       return (
+         <div className="App">
+           <h1>{message} </h1>
+           <button onClick={updateMessage}>Click me!!</button>
+         </div>
+       );
+     }
+     ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-303. ### How to fetch data with React Hooks?
-     The effect hook called `useEffect` is used to fetch the data with axios from the API and to set the data in the local state of the component with the state hook’s update function.
+     ## 3. Props:
 
-     Let's take an example in which it fetches list of react articles from the API
+     Props are referred to as "properties". The props are passed into React component just like arguments passed to a function. In other words, they are similar to HTML attributes.
 
-     ```javascript
-     import React, { useState, useEffect } from 'react';
-     import axios from 'axios';
+     The props are accessible in child class component using `this.props` as shown in below example,
 
-     function App() {
-       const [data, setData] = useState({ hits: [] });
+     ```js
+     class Child extends React.Component {
+       render() {
+         return (
+           <h1>
+             {" "}
+             This is a functional component and component name is {
+               this.props.name
+             }{" "}
+           </h1>
+         );
+       }
+     }
 
-       useEffect(() => {
-         (async () => {
-           const result = await axios(
-             'http://hn.algolia.com/api/v1/search?query=react',
-           );
+     class Parent extends React.Component {
+       render() {
+         return (
+           <div className="Parent">
+             <Child name="First child component" />
+             <Child name="Second child component" />
+           </div>
+         );
+       }
+     }
+     ```
 
-           setData(result.data);
-         })()
-       }, []);
+     Props in functional components are similar to that of the class components but the difference is the absence of 'this' keyword.
 
+     ```js
+     function Child(props) {
        return (
-         <ul>
-           {data.hits.map(item => (
-             <li key={item.objectID}>
-               <a href={item.url}>{item.title}</a>
-             </li>
-           ))}
-         </ul>
+         <h1>
+           This is a child component and the component name is{props.name}
+         </h1>
        );
      }
 
-     export default App;
+     function Parent() {
+       return (
+         <div className="Parent">
+           <Child name="First child component" />
+           <Child name="Second child component" />
+         </div>
+       );
+     }
      ```
 
-     Remember we provided an empty array as second argument to the effect hook to avoid activating it on component updates but only on mounting of the component. i.e, It fetches only on component mount.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-304. ### Is Hooks cover all use cases for classes?
-     Hooks doesn't cover all use cases of classes but there is a plan to add them soon. Currently there are no Hook equivalents to the uncommon **getSnapshotBeforeUpdate** and **componentDidCatch** lifecycles yet.
+245. ### What is strict mode in React?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-305. ### What is the stable release for hooks support?
-     React includes a stable implementation of React Hooks in 16.8 release for below packages
+     `React.StrictMode` is a useful component for highlighting potential problems in an application. Just like `<Fragment>`, `<StrictMode>` does not render any extra DOM elements. It activates additional checks and warnings for its descendants. These checks apply for _development mode_ only.
 
-     1. React DOM
-     2. React DOM Server
-     3. React Test Renderer
-     4. React Shallow Renderer
+     ```jsx harmony
+     import { StrictMode } from "react";
 
-   **[⬆ Back to Top](#table-of-contents)**
+     function App() {
+       return (
+         <div>
+           <Header />
+           <StrictMode>
+             <div>
+               <ComponentOne />
+               <ComponentTwo />
+             </div>
+           </StrictMode>
+           <Header />
+         </div>
+       );
+     }
+     ```
+
+     In the example above, the _strict mode_ checks apply to `<ComponentOne>` and `<ComponentTwo>` components only. i.e., Part of the application only.
+
+     **Note:** Frameworks such as NextJS has this flag enabled by default.
+
+     **[⬆ Back to Top](#table-of-contents)**
+
+246. ### What is the benefit of strict mode?
+
+     The <StrictMode> will be helpful in the below cases,
+
+     1. To find the bugs caused by impure rendering where the components will re-render twice.
+     2. To find the bugs caused by missing cleanup of effects where the components will re-run effects one more extra time.
+     3. Identifying components with **unsafe lifecycle methods**.
+     4. Warning about **legacy string ref** API usage.
+     5. Detecting unexpected **side effects**.
+     6. Detecting **legacy context** API.
+     7. Warning about deprecated **findDOMNode** usage
+
+**[⬆ Back to Top](#table-of-contents)**
+
+247. ### Why does strict mode render twice in React?
+
+     StrictMode renders components twice in development mode(not production) in order to detect any problems with your code and warn you about those problems. This is used to detect accidental side effects in the render phase. If you used `create-react-app` development tool then it automatically enables StrictMode by default.
+
+     ```js
+     const root = createRoot(document.getElementById("root"));
+     root.render(
+       <StrictMode>
+         <App />
+       </StrictMode>
+     );
+     ```
+
+     If you want to disable this behavior then you can simply remove `strict` mode.
+
+     ```js
+     const root = createRoot(document.getElementById("root"));
+     root.render(<App />);
+     ```
+
+     To detect side effects the following functions are invoked twice:
+
+     1. Function component bodies, excluding the code inside event handlers.
+     2. Functions passed to `useState`, `useMemo`, or `useReducer` (any other Hook)
+     3. Class component's `constructor`, `render`, and `shouldComponentUpdate` methods
+     4. Class component static `getDerivedStateFromProps` method
+     5. State updater functions
+
+**[⬆ Back to Top](#table-of-contents)**
+
+248. ### What are the rules of JSX?
+
+     The below 3 rules needs to be followed while using JSX in a react application.
+
+     1. **Return a single root element**:
+        If you are returning multiple elements from a component, wrap them in a single parent element. Otherwise you will receive the below error in your browser console.
+
+        `html Adjacent JSX elements must be wrapped in an enclosing tag.`
+
+     2. **All the tags needs to be closed:**
+        Unlike HTML, all tags needs to closed explicitly with in JSX. This rule applies for self-closing tags(like hr, br and img tags) as well.
+     3. **Use camelCase naming:**
+        It is suggested to use camelCase naming for attributes in JSX. For example, the common attributes of HTML elements such as `class`, `tabindex` will be used as `className` and `tabIndex`.  
+        **Note:** There is an exception for `aria-*` and `data-*` attributes which should be lower cased all the time.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+249.  ### What is the reason behind multiple JSX tags to be wrapped?
+
+      Behind the scenes, JSX is transformed into plain javascript objects. It is not possible to return two or more objects from a function without wrapping into an array. This is the reason you can't simply return two or more JSX tags from a function without
+      wrapping them into a single parent tag or a Fragment.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+250.  ### How do you prevent mutating array variables?
+
+      The preexisting variables outside of the function scope including state, props and context leads to a mutation and they result in unpredictable bugs during the rendering stage. The below points should be taken care while working with arrays variables.
+
+      1. You need to take copy of the original array and perform array operations on it for the rendering purpose. This is called local mutation.
+      2. Avoid triggering mutation methods such as push, pop, sort and reverse methods on original array. It is safe to use filter, map and slice method because they create a new array.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+251. ### What are capture phase events?
+
+     The `onClickCapture` React event is helpful to catch all the events of child elements irrespective of event propagation logic or even if the events propagation stopped. This is useful if you need to log every click events for analytics purpose.
+
+     For example, the below code triggers the click event of parent first followed by second level child eventhough leaf child button elements stops the propagation.
+
+     ```jsx
+     <div onClickCapture={() => alert("parent")}>
+       <div onClickCapture={() => alert("child")}>
+         <button onClick={(e) => e.stopPropagation()} />
+         <button onClick={(e) => e.stopPropagation()} />
+       </div>
+     </div>
+     ```
+
+     The event propagation for the above code snippet happens in the following order:
+
+     1. It travels downwards in the DOM tree by calling all `onClickCapture` event handlers.
+     2. It executes `onClick` event handler on the target element.
+     3. It travels upwards in the DOM tree by call all `onClick` event handlers above to it.
+
+252. ### How does React updates screen in an application?
+
+     React updates UI in three steps,
+
+     1. **Triggering or initiating a render:** The component is going to triggered for render in two ways.
+
+        1. **Initial render:** When the app starts, you can trigger the initial render by calling `creatRoot` with the target DOM node followed by invoking component's `render` method. For example, the following code snippet renders `App` component on root DOM node.
+
+        ```jsx
+        import { createRoot } from "react-dom/client";
+
+        const root = createRoot(document.getElementById("root"));
+        root.render(<App />);
+        ```
+
+        2. **Re-render when the state updated:** When you update the component state using the state setter function, the componen't state automatically queues for a render.
+
+     2. **Rendering components:** After triggering a render, React will call your components to display them on the screen. React will call the root component for initial render and call the function component whose state update triggered the render. This is a recursive process for all nested components of the target component.
+
+     3. **Commit changes to DOM:** After calling components, React will modify the DOM for initial render using `appendChild()` DOM API and apply minimal necessary DOM updates for re-renders based on differences between rerenders.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+253.  ### How does React batch multiple state updates?
+
+      React prevents component from re-rendering for each and every state update by grouping multiple state updates within an event handler. This strategy improves the application performance and this process known as **batching**. The older version of React only supported batching for browser events whereas React18 supported for asynchronous actions, timeouts and intervals along with native events. This improved version of batching is called **automatic batching**.
+
+      Let's demonstrate this automatic batching feature with a below example.
+
+      ```jsx
+      import { useState } from "react";
+
+      export default function BatchingState() {
+        const [count, setCount] = useState(0);
+        const [message, setMessage] = useState("batching");
+
+        console.log("Application Rendered");
+
+        const handleUsers = () => {
+          fetch("https://jsonplaceholder.typicode.com/users/1").then(() => {
+            // Automatic Batching re-render only once
+            setCount(count + 1);
+            setMessage("users fetched");
+          });
+        };
+
+        return (
+          <>
+            <h1>{count}</h1>
+            <button onClick={handleAsyncFetch}>Click Me!</button>
+          </>
+        );
+      }
+      ```
+
+      The preceding code updated two state variables with in an event handler. However, React will perform automatic batching feature and the component will be re-rendered only once for better performance.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+254.  ### Is it possible to prevent automatic batching?
+
+      Yes, it is possible to prevent automatic batching default behavior. There might be cases where you need to re-render your component after each state update or updating one state depends on another state variable. Considering this situation, React introduced `flushSync` method from `react-dom` API for the usecases where you need to flush state updates to DOM immediately.
+
+      The usage of `flushSync` method within an `onClick` event handler will be looking like as below,
+
+      ```jsx
+      import { flushSync } from "react-dom";
+
+      const handleClick = () => {
+        flushSync(() => {
+          setClicked(!clicked); //Component will create a re-render here
+        });
+
+        setCount(count + 1); // Component will create a re-render again here
+      };
+      ```
+
+      In the above click handler, React will update DOM at first using flushSync and second time updates DOM because of the counter setter function by avoiding automatic batching.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+255.  ### What is React hydration?
+
+      React hydration is used to add client-side JavaScript interactivity to pre-rendered static HTML generated by the server. It is used only for server-side rendering(SSR) to enhance the initial rendering time and make it SEO friendly application. This hydration acts as a bridge to reduce the gap between server side and client-side rendering.
+
+      After the page loaded with generated static HTML, React will add application state and interactivity by attaching all event handlers for the respective elements. Let's demonstrate this with an example.
+
+      Consider that React DOM API(using `renderToString`) generated HTML for `<App>` component which contains `<button>` element to increment the counter.
+
+      ```jsx
+      import {useState} from 'react';
+      import { renderToString } from 'react-dom/server';
+
+      export default function App() {
+        const [count, setCount] = React.useState(0);
+
+        return (
+          <h1>Counter</h1>
+          <button onClick={() => setCount(prevCount => prevCount + 1)}>
+            {count} times
+          </button>
+          );
+      }
+
+      const html = renderToString(<App />);
+      ```
+
+      The above code generates the below HTML with a header text and button component without any interactivity.
+
+      ```html
+      <h1>Counter</h1>
+      <button>
+        <!-- -->0<!-- -->
+        times
+      </button>
+      ```
+
+      At this stage `hydrateRoot` API can be used to perform hydration by attaching `onClick` event handler.
+
+      ```jsx
+      import { hydrateRoot } from "react-dom/client";
+      import App from "./App.js";
+
+      hydrateRoot(document.getElementById("root"), <App />);
+      ```
+
+      After this step, you are able to run React application on server-side and hydrating the javascript bundle on client-side for smooth user experience and SEO purposes.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+256.  ### How do you update objects inside state?
+
+      You cannot update the objects which exists in the state directly. Instead, you should create a fresh new object (or copy from the existing object) and update the latest state using the newly created object. Eventhough JavaScript objects are mutable, you need to treat objects inside state as read-only while updating the state.
+
+      Let's see this comparison with an example. The issue with regular object mutation approach can be described by updating the user details fields of `Profile` component. The properties of `Profile` component such as firstName, lastName and age details mutated in an event handler as shown below.
+
+      ```jsx
+      import { useState } from "react";
+
+      export default function Profile() {
+        const [user, setUser] = useState({
+          firstName: "John",
+          lastName: "Abraham",
+          age: 30,
+        });
+
+        function handleFirstNameChange(e) {
+          user.firstName = e.target.value;
+        }
+
+        function handleLastNameChange(e) {
+          user.lastName = e.target.value;
+        }
+
+        function handleAgeChange(e) {
+          user.age = e.target.value;
+        }
+
+        return (
+          <>
+            <label>
+              First name:
+              <input value={user.firstName} onChange={handleFirstNameChange} />
+            </label>
+            <label>
+              Last name:
+              <input value={user.lastName} onChange={handleLastNameChange} />
+            </label>
+            <label>
+              Age:
+              <input value={user.age} onChange={handleAgeChange} />
+            </label>
+            <p>
+              Profile:
+              {person.firstName} {person.lastName} ({person.age})
+            </p>
+          </>
+        );
+      }
+      ```
+
+      Once you run the application with above user profile component, you can observe that user profile details won't be update upon entering the input fields.
+      This issue can be fixed by creating a new copy of object which includes existing properties through spread syntax(...obj) and add changed values in a single event handler itself as shown below.
+
+      ```jsx
+      handleProfileChange(e) {
+        setUser({
+        ...user,
+          [e.target.name]: e.target.value
+        });
+      }
+      ```
+
+      The above event handler is concise instead of maintaining separate event handler for each field. Now, UI displays the updated field values as expected without an issue.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+257.  ### How do you update nested objects inside state?
+
+      You cannot simply use spread syntax for all kinds of objects inside state. Because spread syntax is shallow and it copies properties for one level deep only. If the object has nested object structure, UI doesn't work as expected with regular JavaScript nested property mutation. Let's demonstrate this behavior with an example of User object which has address nested object inside of it.
+
+      ```jsx
+      const user = {
+        name: "John",
+        age: 32,
+        address: {
+          country: "Singapore",
+          postalCode: 440004,
+        },
+      };
+      ```
+
+      If you try to update the country nested field in a regular javascript fashion(as shown below) then user profile screen won't be updated with latest value.
+
+      ```js
+      user.address.country = "Germany";
+      ```
+
+      This issue can be fixed by flattening all the fields into a top-level object or create a new object for each nested object and point it to it's parent object. In this example, first you need to create copy of address object and update it with the latest value. Later, the address object should be linked to parent user object something like below.
+
+      ```js
+      setUser({
+        ...user,
+        address: {
+          ...user.address,
+          country: "Germany",
+        },
+      });
+      ```
+
+      This approach is bit verbose and not easy for deep hierarchical state updates. But this workaround can be used for few levels of nested objects without much hassle.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+258.  ### How do you update arrays inside state?
+
+      Eventhough arrays in JavaScript are mutable in nature, you need to treat them as immutable while storing them in a state. That means, similar to objects, the arrays cannot be updated directly inside state. Instead, you need to create a copy of the existing array and then set the state to use newly copied array.
+
+      To ensure that arrays are not mutated, the mutation operations like direct direct assignment(arr[1]='one'), push, pop, shift, unshift, splice etc methods should be avoided on original array. Instead, you can create a copy of existing array with help of array operations such as filter, map, slice, spread syntax etc.
+
+      For example, the below push operation doesn't add the new todo to the total todo's list in an event handler.
+
+      ```jsx
+      onClick = {
+        todos.push({
+          id: id+1,
+          name: name
+        })
+      }
+      ```
+
+      This issue is fixed by replacing push operation with spread syntax where it will create a new array and the UI updated with new todo.
+
+      ```jsx
+      onClick = {
+        [
+          ...todos,
+          { id: id+1, name: name }
+        ]
+      }
+      ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+259.  ### How do you use immer library for state updates?
+
+      Immer library enforces the immutability of state based on **copy-on-write** mechanism. It uses JavaScript proxy to keep track of updates to immutable states. Immer has 3 main states as below,
+
+      1. **Current state:** It refers to actual state
+      2. **Draft state:** All new changes will be applied to this state. In this state, draft is just a proxy of the current state.
+      3. **Next state:** It is formed after all mutations applied to the draft state
+
+      Immer can be used by following below instructions,
+
+      1. Install the dependency using `npm install use-immer` command
+      2. Replace `useState` hook with `useImmer` hook by importing at the top
+      3. The setter function of `useImmer` hook can be used to update the state.
+
+      For example, the mutation syntax of immer library simplifies the nested address object of user state as follows,
+
+      ```jsx
+      import { useImmer } from "use-immer";
+      const [user, setUser] = useImmer({
+        name: "John",
+        age: 32,
+        address: {
+          country: "Singapore",
+          postalCode: 440004,
+        },
+      });
+
+      //Update user details upon any event
+      setUser((draft) => {
+        draft.address.country = "Germany";
+      });
+      ```
+
+      The preceding code enables you to update nested objects with a conceise mutation syntax.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+260.  ### What are the benefits of preventing the direct state mutations?
+
+**[⬆ Back to Top](#table-of-contents)**
+
+261.  ### What are the preferred and non-preferred array operations for updating the state?
+
+      The below table represent preferred and non-preferred array operations for updating the component state.
+
+      | Action    | Preferred            | Non-preferred              |
+      | --------- | -------------------- | -------------------------- |
+      | Adding    | concat, [...arr]     | push, unshift              |
+      | Removing  | filter, slice        | pop, shift, splice         |
+      | Replacing | map                  | splice, arr[i] = someValue |
+      | sorting   | copying to new array | reverse, sort              |
+
+      If you use Immer library then you can able to use all array methods without any problem.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+262. ### What will happen by defining nested function components?
+
+Technically it is possible to write nested function components but it is not suggested to write nested function definitions. Because it leads to unexpected bugs and performance issues.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+263. ### Can I use keys for non-list items?
+
+     Keys are primarily used for rendering list items but they are not just for list items. You can also use them React to distinguish components. By default, React uses order of the components in
+
+**[⬆ Back to Top](#table-of-contents)**
+
+264. ### What are the guidelines to be followed for writing reducers?
+
+     There are two guidelines to be taken care while writing reducers in your code.
+
+     1. Reducers must be pure without mutating the state. That means, same input always returns the same output. These reducers run during rendering time similar to state updater functions. So these functions should not send any requests, schedule time outs and any other side effects.
+
+     2. Each action should describe a single user interaction even though there are multiple changes applied to data. For example, if you "reset" registration form which has many user input fields managed by a reducer, it is suggested to send one "reset" action instead of creating separate action for each fields. The proper ordering of actions should reflect the user interactions in the browser and it helps a lot for debugging purpose.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+265. ### How does ReactJS work behind the scenes?
+     ReactJS is a powerful JavaScript library for building user interfaces. While it appears simple on the surface, React performs a lot of complex operations behind the scenes to efficiently update the UI. Here's an overview of how it works internally:
+
+     #### **1. Virtual DOM & Component Rendering**
+
+     React doesn't manipulate the real DOM directly. Instead, it uses a **Virtual DOM** — a lightweight JavaScript representation of the UI.
+
+     When a component renders (e.g., `<App />`):
+
+        *   React **executes the component function** (e.g., `App()`).
+        *   Hooks like `useState` are registered and tracked in order.
+        *   React builds a **Virtual DOM tree** from the returned JSX.
+        *   This virtual DOM is a **plain JS object** that describes the desired UI.
+
+     This process ensures fast and efficient rendering before React decides how to update the real DOM.
+
+     #### 2. **React Fiber Architecture**
+
+     React’s core engine is called **Fiber**, introduced in React 16. Fiber is a reimplementation of the React reconciliation algorithm with the following capabilities:
+
+        *   Breaks rendering work into **units of work** (fiber nodes).
+        *   Enables **interruptible rendering** (important for responsiveness).
+        *   Supports **priority scheduling** and **concurrent rendering**.
+
+     Each Fiber node represents a component and stores:
+
+        *   The component type (function/class).
+        *   Props, state, and effects.
+        *   Links to parent, child, and sibling fibers.
+
+     #### 3. **Reconciliation (Diffing Algorithm)**
+
+     When state or props change:
+
+        *   React re-executes the component to produce a new virtual DOM.
+        *   It **compares the new virtual DOM to the previous one** using an efficient diffing algorithm.
+        *   React determines the **minimal set of DOM changes** required.
+
+     This process is known as **reconciliation**.
+
+     #### 4. **Commit Phase (Real DOM Updates)**
+
+     Once reconciliation is done:
+
+        *   React enters the **commit phase**.
+        *   It applies calculated changes to the **real DOM**.
+        *   It also runs side effects like `useEffect` or `useLayoutEffect`.
+
+     This is the only time React interacts directly with the browser DOM.
+
+     #### 5. **Hooks and State Management**
+
+     With Hooks (like `useState`, `useEffect`):
+
+        *   React keeps an **internal list of hooks per component**.
+        *   Hooks are identified by their order in the function.
+        *   When state updates occur, React re-renders the component and re-runs the hooks in the same order.
+
+     #### 6. **React Scheduler**
+
+     React uses an internal **Scheduler** to control how updates are prioritized:
+
+        *   Urgent tasks like clicks and inputs are processed immediately.
+        *   Non-urgent tasks (like data fetching) can be delayed or paused.
+        *   This improves responsiveness and allows for **time slicing** in Concurrent Mode.
+
+
+**[⬆ Back to Top](#table-of-contents)**
+
+266. ### How is `useReducer` Different from `useState`?
+     There are notable differences between `useState` and `useReducer` hooks.
+           
+        | Feature               | `useState`                           | `useReducer`                          |
+        |-----------------------|--------------------------------------|---------------------------------------|
+        | State complexity      | Simple (one variable or flat object) | Complex, multi-part or deeply nested  |
+        | Update style          | Direct (e.g. `setState(x)`)          | Through actions (e.g. `dispatch({})`) |
+        | Update logic          | In component                         | In reducer function                   |
+        | Reusability & testing | Less reusable                        | Highly reusable & testable            |
+
+**[⬆ Back to Top](#table-of-contents)**
+
+267. ### What is useContext? What are the steps to follow for useContext?
+     The `useContext` hook is a built-in React Hook that lets you access the value of a context inside a functional component without needing to wrap it in a <Context.Consumer> component.
+
+     It helps you **avoid prop drilling** (passing props through multiple levels) by allowing components to access shared data like themes, authentication status, or user preferences.
+
+     The usage of useContext involves three main steps:
+      #### **Step 1 : Create the Context**
     
-306. ### Why do we use array destructuring (square brackets notation) in `useState`?
-     When we declare a state variable with `useState`, it returns a pair — an array with two items. The first item is the current value, and the second is a function that updates the value. Using [0] and [1] to access them is a bit confusing because they have a specific meaning. This is why we use array destructuring instead.
+        Use `React.createContext()` to create a context object.
+    
+        ```jsx
+        import React, { createContext } from 'react'; 
+    
+        const ThemeContext = createContext(); // default value optional
+        ```
+    
+      You typically export this so other components can import it.
+
+      #### **Step 2: Provide the Context Value**
+    
+        Wrap your component tree (or a part of it) with the `Context.Provider` and pass a `value` prop.
+    
+        ```jsx
+        function App() {
+            return ( 
+                <ThemeContext.Provider value="dark">
+                   <MyComponent />
+                </ThemeContext.Provider>
+            ); 
+        }
+        ```
+    
+        Now any component inside `<ThemeContext.Provider>` can access the context value.
+
+        #### **Step 3: Consume the Context with** `**useContext**`
+    
+        In any functional component **inside the Provider**, use the `useContext` hook:
+    
+        ```jsx
+        import { useContext } from 'react'; 
+        function MyComponent() {
+            const theme = useContext(ThemeContext); // theme = "dark"
+            return <p>Current Theme: {theme}</p>; 
+        }
+        ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+268. ### What are the use cases of useContext hook?
+     The `useContext` hook in React is used to share data across components without having to pass props manually through each level. Here are some common and effective use cases:
+
+        1.  **Theme Customization**  
+            `useContext` can be used to manage application-wide themes, such as light and dark modes, ensuring consistent styling and enabling user-driven customization.
+        2.  **Localization and Internationalization**  
+            It supports localization by providing translated strings or locale-specific content to components, adapting the application for users in different regions.
+        3.  **User Authentication and Session Management**  
+            `useContext` allows global access to authentication status and user data. This enables conditional rendering of components and helps manage protected routes or user-specific UI elements.
+        4.  **Shared Modal or Sidebar Visibility**  
+            It's ideal for managing the visibility of shared UI components like modals, drawers, or sidebars, especially when their state needs to be controlled from various parts of the app.
+        5.  **Combining with** `**useReducer**` **for Global State Management**  
+            When combined with `useReducer`, `useContext` becomes a powerful tool for managing more complex global state logic. This pattern helps maintain cleaner, scalable state logic without introducing external libraries like Redux.
+             Some of the common use cases of useContext are listed below,
+
+**[⬆ Back to Top](#table-of-contents)**
+
+269.  ### When to use client and server components?
+
+      You can efficiently build nextjs application if you are aware about which part of the application needs to use client components and which other parts needs to use server components. The common cases of both client and server components are listed below:
+
+        **Client components:**
+        1. Whenever your need to add interactivity and event listeners such as onClick(), onChange(), etc to the pages
+        2. If you need to use State and Lifecycle Effects like useState(), useReducer(), useEffect() etc.
+        3. If there is a requirement to use browser-only APIs.
+        4. If you need to implement custom hooks that depend on state, effects, or browser-only APIs.
+        5. There are React Class components in the pages.
+
+        **Server components:**
+        1. If the component logic is about data fetching.
+        2. If you need to access backend resources directly.
+        3. When you need to keep sensitive information((access tokens, API keys, etc)	) on the server.
+        4. If you want reduce client-side JavaScript and placing large dependencies on the server.
+
+
+**[⬆ Back to Top](#table-of-contents)**
+
+270. ### What are the differences between page router and app router in nextjs?
+
+**[⬆ Back to Top](#table-of-contents)**
+
+271. ### Can you describe the useMemo() Hook?
+
+     The `useMemo()` Hook in React is used to **optimize performance** by **memoizing the result of expensive calculations**. It ensures that a function is **only re-executed when its dependencies change**, preventing unnecessary computations on every re-render.
+
+     #### Syntax
+
+     ```js
+      const memoizedValue = useMemo(() => computeExpensiveValue(arg), [dependencies]);
+     ```
+      - **`computeExpensiveValue`**:  
+      A function that returns the computed result.
+
+     - **`dependencies`**:  
+      An array of values that, when changed, will cause the memoized function to re-run.
+
+      If the dependencies haven’t changed since the last render, React returns the **cached result** instead of re-running the function.
+
+      Let's exaplain the usage of `useMemo` hook with an example of user search and its respective filtered users list.
+
+      #### Example: Memoizing a Filtered List
+      ```javascript
+      import React, { useState, useMemo } from 'react';
+
+      const users = [
+        { id: 1, name: 'Sudheer' },
+        { id: 2, name: 'Brendon' },
+        { id: 3, name: 'Charlie' },
+        { id: 4, name: 'Dary' },
+        { id: 5, name: 'Eden' }
+      ];
+
+      export default function UserSearch({ users }) {
+        const [searchTerm, setSearchTerm] = useState('');
+        const [counter, setCounter] = useState(0);
+
+        // Memoize the filtered user list based on the search term
+        const filteredUsers = useMemo(() => {
+          console.log("Filtering users...");
+          return users.filter(user =>
+            user.name.toLowerCase().includes(searchTerm.toLowerCase())
+          );
+        }, [searchTerm]);
+
+        return (
+          <div>
+            <h2>Counter: {counter}</h2>
+            <button onClick={() => setCounter(prev => prev + 1)}>Increment Counter</button>
+
+            <h2>Search Users</h2>
+            <input
+              type="text"
+              value={searchTerm}
+              onChange={(e) => setSearchTerm(e.target.value)}
+              placeholder="Enter name"
+            />
+
+            <ul>
+              {filteredUsers.map(user => (
+                <li key={user.id}>{user.name}</li>
+              ))}
+            </ul>
+          </div>
+        );
+      }
+      ```
+
+      In the above example:
+        - The filteredUsers list is only recomputed when searchTerm changes.
+        - Pressing the "Increment Counter" button does not trigger the filtering logic again, as it's not a dependency.
+        - The console will only log "Filtering users..." when the search term updates.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+272. ### Can Hooks be used in class components?
+     No, Hooks cannot be used inside class components. They are specially designed for function components. This is because hooks depend on the sequence in which they are called during a component’s render, something that's only guaranteed in functional components. However, both class and function components can coexist in the same application.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+273. ### What is an updater function? Should an updater function be used in all cases?
+     An **updater function** is a form of `setState` where you pass a **function** instead of a direct value. This function receives the **previous state** as an argument and returns the **next state**.
+     
+     The updater function expression looks like below,
+     ```js
+     setCount(prevCount => prevCount + 1); // Safe and predictable
+     ```
+     Here, `prevCount => prevCount + 1` is the updater function.
+
+     In the React community, there's often a recommendation to use updater functions when updating state that depends on its previous value. This helps prevent unexpected behaviors that can arise from working with outdated or "stale" state.
+
+     While using an updater function is a good habit, it's not always necessary. In most cases, React batches updates and ensures that the state is up-to-date at the beginning of the event handler, so you typically don’t run into stale state issues during a single synchronous event.
+     However, if you’re doing multiple updates to the same state variable within a single handler, using the updater form ensures that each update correctly uses the latest state value, rather than a potentially outdated one.
+
+     **Example: Multiple Updates in One Handler**
+     ```js
+     function handleCount() {
+        setCounter(a => a + 1);
+        setCounter(a => a + 1);
+        setCounter(a => a + 1);
+     }
+     ```
+
+     In this example, `a => a + 1` is an **updater function**. React queues these updater functions and applies them sequentially, each using the most recent state value. As a result, the counter will correctly increment by 3.
+
+     In many cases, such as setting state based on user input or assigning static values, you don’t need the updater function:
+     ```js
+     setName('Sudheer');
+     ```
+     
+**[⬆ Back to Top](#table-of-contents)**
+
+274. ### Can useState take a function as an initial value?
+     Yes, `useState` can take a function as an initial value, and this is a useful feature in React called **lazy initialization**. This function is also known as **initializer function**.
+
+     When you call useState(initialValue), you normally pass in a value directly:
+     
+     ```js
+     const [count, setCount] = useState(0);  // initial value is 0
+     ```
+
+     But if calculating that initial value is expensive or involves logic, you can pass a function that returns the value:
+     ```js
+        const [count, setCount] = useState(() => {
+        // This function only runs once — when the component first renders
+        return expensiveComputation();
+        });
+     ```
+     
+     This function avoids doing heavy computation on every render. If you don't use this function form and invokes it directly, the function will run everytime the component renders and impact the performance.
+     For example, the below usage is not recommended. 
+     ```js
+     const [count, setCount] = useState(expensiveComputation());
+     ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+275. ### What types of values can `useState` hold?
+
+     The `useState` hook accepts different types of values.
+
+     *   Primitives: `number`, `string`, `boolean`
+     *   Arrays
+     *   Objects
+     *   Functions
+     *   `null` or `undefined`
+
+     But you needs to be cautious with **reference types (objects/arrays)** because React compares old and new values **by reference**, so direct mutations won't trigger a re-render.
+     For example, the correct and wrong ways of state updates as shown below, 
+     ```js
+     user.name = "Sudheer"; //wrong way
+     setUser(prev => ({ ...prev, name: 'Sudheer' })); //correct way
+     ```
+**[⬆ Back to Top](#table-of-contents)**
+
+276. ### What happens if you call `useState` conditionally?
+     As per rules of React Hooks, hooks must be called unconditionally. For example, if you conditionally call it:
+     ```js
+     if (someCondition) { 
+        const [state, setState] = useState(0); 
+     }
+     ```
+
+     React will throw a runtime error because it **relies on the order of Hook calls**, and conditional logic breaks that order.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+277. ### Is useState Synchronous or Asynchronous?
+     The `useState` hook is synchronous, but state updates are asynchronous. When you call `useState()`, it runs synchronously and returns the state variable and setter function as tuple.
+     ```js
+     const [count, setCount] = useState(0);
+     ```
+     This happens immediately during rendering.
+     However, the state update function (**setState**) is asynchronous in the sense that it doesn't update the state immediately.
+     React **batches** updates and applies them before the next render. You won’t see the updated value immediately after calling `setState`.
+     
+     **Example:**
+     ```js
+     const [count, setCount] = useState(0);
+    
+     function handleClick() {
+       setCount(count + 1);
+       console.log(count); // ❗️Still logs the old value
+     }
+     ```
+     The > `console.log(count)` prints the **old value**, because the update hasn’t happened yet.
+     
+     To see the updated state value, you can use `useEffect()` hook. It runs **after the component has re-rendered.**  By the time `useEffect` runs:
+
+        *   The component has been updated.
+        *   The **state contains the new value**.
+     
+        ```js
+        import React, { useState, useEffect } from 'react';
+
+        function Counter() {
+        const [count, setCount] = useState(0);
+        
+        const handleClick = () => {
+        setCount(count + 1);
+        console.log('Clicked count (old):', count); // Old value
+        };
+        
+        useEffect(() => {
+        console.log('Updated count:', count); // New value
+        }, [count]); // Only runs when `count` changes
+        
+        return <button onClick={handleClick}>Count: {count}</button>;
+        }
+        ```
+**[⬆ Back to Top](#table-of-contents)**
+
+278.  ### Can you explain how useState works internally?
+      React’s hooks, including `useState`, rely on some internal machinery that keeps track of state **per component** and **per hook call** during rendering. Here's a simplified explanation of the internal mechanics:
+
+      #### 1. **Hook List / Linked List**
+
+      *   React maintains a linked list or array of "hook states" for each component.
+      *   When a component renders, React keeps track of which hook it is currently processing via a cursor/index.
+      *   Each call to `useState()` corresponds to one "slot" in this list.
+
+      #### 2. **State Storage**
+
+      *   Each slot stores:
+        *   The current state value.
+        *   A queue of pending state updates.
+
+      #### 3. **Initial Render**
+        
+      *   When the component first renders, React:
+         *   Creates a new slot for `useState` with the initial state (e.g., `0`).
+         *   Returns `[state, updaterFunction]`.
+
+      #### 4. **Updater Function**
+    
+      *   The updater function (`setCount`) is a closure that, when called:
+        *   Enqueues a state update to React's internal queue.
+        *   Schedules a re-render of the component.
+    
+      #### 5. **Re-render and State Update**
+    
+      *   On the next render:
+        *   React processes all queued updates for each hook slot.
+        *   Updates the stored state value accordingly.
+        *   Returns the new state to the component.
+
+      #### 6. **Important: Hook Order**
+
+      *   Hooks must be called in the same order on every render so React can match hook calls to their internal slots.
+      *   That’s why you can’t call hooks conditionally.
+      
+      The pseudocode for internal implementation of `useState` looks like below,
+      ```js
+        let hookIndex = 0;
+        const hooks = [];
+        
+        function useState(initialValue) {
+            const currentIndex = hookIndex;
+        
+            if (!hooks[currentIndex]) {
+                // First render: initialize state
+                hooks[currentIndex] = {
+                    state: initialValue,
+                    queue: [],
+                };
+            }
+        
+            const hook = hooks[currentIndex];
+        
+            // Process queued updates
+            hook.queue.forEach(update => {
+                hook.state = update(hook.state);
+            });
+            hook.queue = [];
+        
+            // Define updater function
+            function setState(action) {
+                // action can be new state or function(state) => new state
+                hook.queue.push(typeof action === 'function' ? action : () => action);
+                scheduleRender(); // triggers React re-render
+            }
+        
+            hookIndex++;
+            return [hook.state, setState];
+        }
+        ```
+      
+**[⬆ Back to Top](#table-of-contents)**
+
+279. ### What is `useReducer`? Why do you use useReducer?
+     The `useReducer` hook is a React hook used to manage **complex state logic** inside **functional components**. It is conceptually similar to **Redux**. i.e, Instead of directly updating state like with `useState`, you **dispatch an action** to a **reducer function**, and the reducer returns the new state.
+
+     The `useReducer` hook takes three arguments:
+
+        ```js
+        const [state, dispatch] = useReducer(reducer, initialState, initFunction);
+        ```
+
+        *   `**reducer**`: A function `(state, action) => newState` that handles how state should change based on the action.
+        *   `**initialState**`: The starting state.
+        *   `**dispatch**`: A function you call to trigger an update by passing an action.
+
+     The `useReducer` hook is used when:
+
+     *   The **state is complex**, such as nested structures or multiple related values.
+     *   State updates depend on the **previous state** and **logic**.
+     *   You want to **separate state update logic** from UI code to make it cleaner and testable.
+     *   You’re managing features like:
+         *   Forms
+         *   Wizards / Multi-step flows
+         *   Undo/Redo functionality
+         *   Shopping cart logic
+         *   Toggle & conditional UI logic
+         
+**[⬆ Back to Top](#table-of-contents)**
+
+280. ### How does `useReducer` works? Explain with an example
+     The `useReducer` hooks works similarly to Redux, where:
+
+        *   You define a **reducer function** to handle state transitions.
+        *   You dispatch actions to update the state.
+     
+     **Counter Example with Increment, Decrement, and Reset:**
+     1. Reducer function:
+
+        Define a counter reducer function that takes the current state and an action object with a type, and returns a new state based on that type.
+     
+         ```js
+         function counterReducer(state, action) {
+            switch (action.type) {
+              case 'increment':
+                return { count: state.count + 1 };
+              case 'decrement':
+                return { count: state.count - 1 };
+              case 'reset':
+                return { count: 0 };
+              default:
+                return state;
+            }
+          }
+         ```
+     2. Using `useReducer`:
+      Invoke `useReducer` with above reducer function along with initial state. Thereafter, you can attach dispatch actions for respective button handlers.
+      ```js
+      import React, { useReducer } from 'react';
+
+        function Counter() {
+          const initialState = { count: 0 };
+          const [state, dispatch] = useReducer(counterReducer, initialState);
+
+          return (
+            <div style={{ textAlign: 'center' }}>
+              <h2>Count: {state.count}</h2>
+              <button onClick={() => dispatch({ type: 'increment' })}>Increment</button>
+              <button onClick={() => dispatch({ type: 'decrement' })}>Decrement</button>
+              <button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
+            </div>
+          );
+        }
+
+      export default Counter;
+      ```
+      Once the new state has been returned, React re-renders the component with the updated `state.count`.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+281. ### Can you combine **useReducer** with **useContext**?
+
+      Yes, it's common to combine **useReducer** with **useContext** to build a lightweight state management system similar to Redux:
+
+      ```js
+      const AppContext = React.createContext();
+
+      function AppProvider({ children }) {
+        const [state, dispatch] = useReducer(reducer, initialState);
+        return (
+          <AppContext.Provider value={{ state, dispatch }}>
+            {children}
+          </AppContext.Provider>
+        );
+      }
+      ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+282. ### Can you dispatch multiple actions in a row with useReducer?
+     Yes, you can dispatch multiple actions in a row using `useReducer` but not directly in one call. You'd have to call dispatch multiple times or create a composite action in your reducer that performs multiple updates based on the action type.
+     
+     **Example: Dispatching Multiple Actions**
+     You can define a custom function with dispatching actions one by one.
+     ```js
+     function handleMultipleActions(dispatch) {
+       dispatch({ type: 'increment' });
+       dispatch({ type: 'increment' });
+       dispatch({ type: 'reset' });
+     }
+     ```
+     After that, you need to invoke it through event handler
+     ```js
+     <button onClick={() => handleMultipleActions(dispatch)}>
+       Run Multiple Actions
+     </button>
+     ```
+     **Note:** You can also define a custom action type If you want multiple state changes to be handled in one reducer call.
+     ```js
+     case 'increment_twice':
+       return { count: state.count + 2 };
+     ```
+     Then dispatch
+     ```js
+     dispatch({ type: 'increment_twice' });
+     ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+283. ### How does useContext works? Explain with an example
+     The `useContext` hook can be used for authentication state management across multiple components and pages in a React application.
+    
+     Let's build a simple authentication flow with:
+
+        *   **Login and Logout buttons**
+        *   Global `AuthContext` to share state
+        *   Components that can **access and update** auth status
+     
+      **1. Create the Auth Context:**
+
+       You can define `AuthProvider` which holds and provides `user`, `login()`, and `logout()` via context.
+        ```js
+        // AuthContext.js
+        import React, { createContext, useContext, useState } from 'react';
+        
+        const AuthContext = createContext();
+        
+        export function AuthProvider({ children }) {
+          const [user, setUser] = useState(null);
+        
+          const login = (username) => setUser({ name: username });
+          const logout = () => setUser(null);
+        
+          return (
+            <AuthContext.Provider value={{ user, login, logout }}>
+              {children}
+            </AuthContext.Provider>
+          );
+        }
+        
+        // Custom hook for cleaner usage
+        export const useAuth = () => useContext(AuthContext);
+        ```
+     **2. Wrap Your App with the Provider:**
+       
+        Wrap the above created provider in main `App.js` file      
+        ```js
+        // App.js
+        import React from 'react';
+        import { AuthProvider } from './AuthContext';
+        import HomePage from './HomePage';
+        import Dashboard from './Dashboard';
+        
+        function App() {
+          return (
+            <AuthProvider>
+              <HomePage />
+              <Dashboard />
+            </AuthProvider>
+          );
+        }
+        
+        export default App;
+        ```
+     **3. Home page with login:**
+      Read or access user and login details through custom useAuth hook and use it inside home page.
+
+        ```js
+         // HomePage.js
+         import React from 'react';
+         import { useAuth } from './AuthContext';
+    
+         function HomePage() {
+           const { user, login } = useAuth();
+    
+           return (
+             <div>
+               <h1>Home</h1>
+               {user ? (
+                 <p>Welcome back, {user.name}!</p>
+               ) : (
+                 <button onClick={() => login('Alice')}>Login</button>
+               )}
+             </div>
+           );
+         }
+    
+         export default HomePage;
+        ```
+     
+     **4. Dashboard with logout:**
+      Read or access user and logout details from `useAuth` custom hook and use it inside dashboard page.
+
+      ```js
+        // Dashboard.js
+        import React from 'react';
+        import { useAuth } from './AuthContext';
+        
+        function Dashboard() {
+          const { user, logout } = useAuth();
+        
+          if (!user) {
+            return <p>Please login to view the dashboard.</p>;
+          }
+        
+          return (
+            <div>
+              <h2>Dashboard</h2>
+              <p>Logged in as: {user.name}</p>
+              <button onClick={logout}>Logout</button>
+            </div>
+          );
+        }
+        
+        export default Dashboard;
+      ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+284. ### Can You Use Multiple Contexts in One Component?
+     Yes, it is possible. You can use multiple contexts inside the same component by calling useContext multiple times, once for each context.
+
+     It can be achieved with below steps,
+
+        *   Create multiple contexts using `createContext()`.
+        *   Wrap your component tree with multiple `<Provider>`s.
+        *   Call `useContext()` separately for each context in the same component.
+     
+     **Example: Using `ThemeContext` and `UserContext` Together**
+     ```js
+     import React, { createContext, useContext } from 'react';
+
+      // Step 1: Create two contexts
+      const ThemeContext = createContext();
+      const UserContext = createContext();
+
+      function Dashboard() {
+        // Step 2: Use both contexts
+        const theme = useContext(ThemeContext);
+        const user = useContext(UserContext);
+
+        return (
+          <div style={{ background: theme === 'dark' ? '#333' : '#fff' }}>
+            <h1>Welcome, {user.name}</h1>
+            <p>Current theme: {theme}</p>
+          </div>
+        );
+      }
+
+      // Step 3: Provide both contexts
+      function App() {
+        return (
+          <ThemeContext.Provider value="dark">
+            <UserContext.Provider value={{ name: 'Sudheer' }}>
+              <Dashboard />
+            </UserContext.Provider>
+          </ThemeContext.Provider>
+        );
+      }
+
+      export default App;
+     ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+285. ###  What's a common pitfall when using useContext with objects?
+     A **common pitfall** when using `useContext` with objects is **triggering unnecessary re-renders** across all consuming components — even when only part of the context value changes.
+
+     When you provide an object as the context value, React compares the entire object reference. If the object changes (even slightly), React assumes the whole context has changed, and **all components using** `useContext(MyContext)` **will re-render**, regardless of whether they use the part that changed.
+
+     **Example:**
+     ```js
+        const MyContext = React.createContext();
+
+        function MyProvider({ children }) {
+          const [user, setUser] = useState(null);
+          const [theme, setTheme] = useState('light');
+
+          // This causes all consumers to re-render on any state change
+          const contextValue = { user, setUser, theme, setTheme };
+
+          return (
+            <MyContext.Provider value={contextValue}>
+              {children}
+            </MyContext.Provider>
+          );
+        }
+     ```
+     In this case, a change in `theme` will also trigger a re-render in components that only care about `user`.
+
+     This issue can be fixed in two ways,
+
+     **1. Split Contexts**  
+     Create separate contexts for unrelated pieces of state:
+
+     ```jsx
+     const UserContext = React.createContext(); 
+     const ThemeContext = React.createContext();
+     ```
+
+     **2. Memoize Context Value**  
+     Use `useMemo` to prevent unnecessary re-renders:
+
+     ```jsx
+       const contextValue = useMemo(() => ({ user, setUser, theme, setTheme }), [user, theme]);
+     ```
+
+     However, this only helps if the object structure and dependencies are well controlled.
+    
+  **[⬆ Back to Top](#table-of-contents)**
+
+286. ### What would the context value be for no matching provider?
+    
+     When a component calls `useContext(SomeContext)` but **no matching** `<SomeContext.Provider>` **is present higher up in the component tree**, the **default value** passed to `React.createContext(defaultValue)` is returned.
+    
+     ```js
+     const ThemeContext = React.createContext('light'); // 'light' is the default value
+    
+     function ThemedComponent() {
+       const theme = useContext(ThemeContext);
+       return <div>Current theme: {theme}</div>;
+     }
+    
+     // No ThemeContext.Provider anywhere in the tree
+     ```
+     In this case, `theme` will be 'light'. It's the default value you provided when you created the context.
+    
+     **Note:** If you don’t specify a default value, the context value will be undefined when used without a provider:
+    
+     ```jsx
+     const AuthContext = React.createContext(); // No default
+    
+     function Profile() {
+       const auth = useContext(AuthContext);
+       // auth will be undefined if there's no AuthContext.Provider
+     }
+     ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+287. ### How do reactive dependencies in the useEffect dependency array affect its execution behavior?
+     The `useEffect` hook accepts an optional dependencies argument that accepts an array of reactive values. The **dependency array** determines **when** the effect runs. i.e, It makes `useEffect` _reactive_ to changes in specified values.
+
+     #### **How Dependency Array Affects Behavior**
+
+     1. **Empty Dependency Array:** `**[]**`
+
+     ```css
+     useEffect(() => {
+       // runs once after the initial render
+     }, []);
+     ```
+
+      *   Effect runs **only once** (like `componentDidMount`).
+      *   Ignores all state/prop changes.
+
+     2. **With Specific Dependencies:** `**[count, user]**`
+
+     ```css
+     useEffect(() => {
+       // runs after initial render
+       // AND whenever `count` or `user` changes
+     }, [count, user]);
+     ```
+
+       *   Effect runs on **first render**, and
+       *   Again **every time** any dependency value changes.
+
+     3. **No Dependency Array (Omitted)**
+
+          ```css
+          useEffect(() => {
+            // runs after **every** render
+          });
+          ```
+
+           *   Effect runs after **every render**, regardless of what changed.
+           *   Can lead to **performance issues** if not used carefully.
+
+       React uses shallow comparison of the dependencies. If any value has changed (!==), the effect will re-run.
+       
+       **Note:** This hook works well when dependencies are primitives or memoized objects/functions.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+288. ### When and how often does React invoke the setup and cleanup functions inside a useEffect hook?
+
+     1. **Setup Function Execution (`useEffect`)**
+
+         The setup function (or the main function) you pass to `useEffect` runs at specific points:
+
+           1.  **After the component is mounted** (if the dependency array is empty `[]`)
+           2.  **After every render** (if no dependency array is provided)
+           3.  **After a dependency value changes** (if the dependency array contains variables)
+
+     2. **Cleanup Function Execution (Returned function from `useEffect`)**
+
+        The cleanup function is called **before the effect is re-executed** and when the component **unmounts**.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+289. ### What happens if you return a Promise from useEffect??
+      You should NOT return a Promise from useEffect. React expects the function passed to useEffect to return either nothing (undefined) or a cleanup function (synchronous function). i.e, It does not expect or handle a returned Promise. If you still return a Promise, React will ignore it silently, and it may lead to bugs or warnings in strict mode.
+
+      **Incorrect:**
+      ```js
+      useEffect(async () => {
+        await fetchData(); // ❌ useEffect shouldn't be async
+      }, []);
+      ```
+      **Correct:**
+      ```jsx
+      useEffect(() => {
+        const fetchData = async () => {
+          const res = await fetch('/api');
+          const data = await res.json();
+          setData(data);
+        };
+
+        fetchData();
+      }, []);
+      ```
+**[⬆ Back to Top](#table-of-contents)**
+
+289. ### Can you have multiple useEffect hooks in a single component?
+      Yes, multiple useEffect hooks are allowed and recommended when you want to separate concerns.
+
+      ```jsx
+      useEffect(() => {
+        // Handles API fetch
+      }, []);
+
+      useEffect(() => {
+        // Handles event listeners
+      }, []);
+      ```
+      Each effect runs independently and helps make code modular and easier to debug.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+290. ### How to prevent infinite loops with useEffect?
+        Infinite loops happen when the effect updates state that’s listed in its own dependency array, which causes the effect to re-run, updating state again and so on.
+        
+        **Infinite loop scenario:**
+        ```js
+        useEffect(() => {
+          setCount(count + 1);
+        }, [count]); // Triggers again every time count updates
+        ```
+        You need to ensure that setState calls do not depend on values that cause the effect to rerun, or isolate them with a guard.
+        ```js
+        useEffect(() => {
+          if (count < 5) {
+            setCount(count + 1);
+          }
+        }, [count]);
+        ```
+**[⬆ Back to Top](#table-of-contents)**
+
+291. ### What are the usecases of useLayoutEffect?
+      You need to use `useLayoutEffect` when your effect **must run before the browser paints**, such as:
+
+      *   **Reading layout measurements** (e.g., element size, scroll position)
+      *   **Synchronously applying DOM styles** to prevent visual flicker
+      *   **Animating layout or transitions**
+      *   **Integrating with third-party libraries** that require DOM manipulation
+
+      If there's no visual or layout dependency, prefer `useEffect` — it's more performance-friendly.
+
+      ```js
+      useLayoutEffect(() => {
+        const width = divRef.current.offsetWidth;
+        if (width < 400) {
+          divRef.current.style.background = 'blue'; // prevents flicker
+        }
+      }, []);
+      ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+292. ### How does useLayoutEffect work during server-side rendering (SSR)?
+
+     The `useLayoutEffect` hook does **not run on the server**, because there is no DOM. React issues a warning in server environments like Next.js if `useLayoutEffect` is used directly.
+
+     This can be mitigated using a conditional polyfill:
+
+      ```jsx
+      const useIsomorphicLayoutEffect =
+        typeof window !== 'undefined' ? useLayoutEffect : useEffect;
+      ```
+
+      i.e, Use `useIsomorphicLayoutEffect` in components that render both on client and server.     
+
+**[⬆ Back to Top](#table-of-contents)** 
+
+293. ### What happens if you use useLayoutEffect for non-layout logic?
+      Using `useLayoutEffect` for logic **unrelated to layout or visual DOM changes** (such as logging, data fetching, or analytics) is **not recommended**. It can lead to **performance issues** or even unexpected behavior.
+
+      **Example: Anti-pattern**
+      ```js
+      useLayoutEffect(() => {
+        console.log("Tracking analytics");
+        fetch('/log-page-view');
+      }, []);
+      ```
+      The above usage delays the paint of the UI just to send a network request, which could (and should) be done after paint using useEffect.
+
+**[⬆ Back to Top](#table-of-contents)** 
+
+294. ### How does useLayoutEffect cause layout thrashing?
+      The `useLayoutEffect` can **cause layout thrashing** when you **repeatedly read and write to the DOM** in ways that force the browser to recalculate layout multiple times per frame. This is because `useLayoutEffect` runs _before the browser paints_, these reflows happen _synchronously_, blocking rendering and degrading performance.
+
+      **Example:**
+      ```js
+      function ThrashingComponent() {
+        const ref = useRef();
+
+        useLayoutEffect(() => {
+          const height = ref.current.offsetHeight; //Read
+          ref.current.style.height = height + 20 + 'px'; //Write
+          const newHeight = ref.current.offsetHeight; //Read again — forces reflow
+        }, []);
+
+        return <div ref={ref}>Hello</div>;
+      }
+      ```
+      In the above code, each read/write cycle triggers synchronous reflows, blocking the main thread and delays UI rendering.
+
+      This issue can be avoided by batching your DOM reads and writes and prevent unnecessary reads after writes.
+
+**[⬆ Back to Top](#table-of-contents)** 
+
+295. ### How Do You Use useRef to Access a DOM Element in React? Give an example.
+        The `useRef` hook is commonly used in React to directly reference and interact with DOM elements — like focusing an input, scrolling to a section, or controlling media elements.
+        
+        When you assign a ref to a DOM element using useRef, React gives you access to the underlying DOM node via the .current property of the ref object.
+        
+        **Example: Focus an input**
+
+        ```js
+        import React, { useRef } from 'react';
+        
+        function FocusInput() {
+          const inputRef = useRef(null); // create the ref
+        
+          const handleFocus = () => {
+            inputRef.current.focus(); // access DOM element and focus it
+          };
+        
+          return (
+            <div>
+              <input type="text" ref={inputRef} />
+              <button onClick={handleFocus}>Focus the input</button>
+            </div>
+          );
+        }
+        ```
+       **Note:** The DOM reference is only available **after the component has mounted** — typically accessed in `useEffect` or event handlers.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+296. ### Can you use useRef to persist values across renders??
+        Yes, you can use `useRef` to persist values across renders in React. Unlike `useState`, changing `.current` does not cause re-renders, but the value is preserved across renders.
+        
+        **Example:**
+        ```js
+        function Timer() {
+          const renderCount = useRef(0);
+          useEffect(() => {
+            renderCount.current++;
+            console.log("Render count:", renderCount.current);
+          });
+        
+          return <div>Check console for render count.</div>;
+        }
+        ```
+**[⬆ Back to Top](#table-of-contents)**
+
+297. ###  Can useRef be used to store previous values?
+        Yes, `useRef` is a common pattern when you want to compare current and previous props or state without causing re-renders.
+        
+        **Example: Storing previous state value**
+        ```js
+        import { useEffect, useRef, useState } from 'react';
+        
+        function PreviousValueExample() {
+          const [count, setCount] = useState(0);
+          const prevCountRef = useRef();
+        
+          useEffect(() => {
+            prevCountRef.current = count;
+          }, [count]);
+        
+          const prevCount = prevCountRef.current;
+        
+          return (
+            <div>
+              <p>Current: {count}</p>
+              <p>Previous: {prevCount}</p>
+              <button onClick={() => setCount(c => c + 1)}>Increment</button>
+            </div>
+          );
+        }
+        ```
+     
+**[⬆ Back to Top](#table-of-contents)**
+
+298. ### Is it possible to access a ref in the render method?
+        Yes, you can access a ref in the render method, but what you get from it depends on how you're using the ref and when in the component lifecycle you're rendering.
+        
+        For example, when using ref to access a DOM node (e.g., divRef.current), it's not immediately available on the first render.
+        ```js
+        const divRef = useRef(null);
+        
+        console.log(divRef.current); // ❌ null on initial render
+        return <div ref={divRef}>Hello</div>;
+        ```
+     
+**[⬆ Back to Top](#table-of-contents)**
+
+299. ### What are the common usecases of useRef hook?
+      Some of the common cases are:
+      *   Automatically focus an input when a component mounts.
+      *   Scroll to a specific element.
+      *   Measure element dimensions (`offsetWidth`, `clientHeight`).
+      *   Control video/audio playback.
+      *   Integrate with non-React libraries (like D3 or jQuery).
+
+**[⬆ Back to Top](#table-of-contents)**
+
+300. ### What is useImperativeHandle Hook? Give an example.
+      `useImperativeHandle` is a React Hook that allows a **child component** to expose **custom functions or properties** to its **parent component**, when using `ref`.
+      It is typically used with `forwardRef` and is very useful in cases like **modals**, **dialogs**, **custom inputs**, etc., where the parent needs to **control behavior imperatively** (e.g., open, close, reset).
+
+      **Example: Dialog component**
+      ```js
+      import React, {
+        useRef,
+        useState,
+        useImperativeHandle,
+        forwardRef,
+      } from 'react';
+      import './Dialog.css'; 
+
+      const Dialog = forwardRef((props, ref) => {
+        const [isOpen, setIsOpen] = useState(false);
+        const [formData, setFormData] = useState('');
+
+        useImperativeHandle(ref, () => ({
+          open: () => setIsOpen(true),
+          close: () => setIsOpen(false),
+          reset: () => setFormData(''),
+        }));
+
+        if (!isOpen) return null;
+
+        return (
+          <div className="dialog"> 
+            <h2>Dialog</h2>
+            <input
+              type="text"
+              value={formData}
+              placeholder="Type something..."
+              onChange={(e) => setFormData(e.target.value)}
+            />
+            <br />
+            <button onClick={() => setIsOpen(false)}>Close</button>
+          </div>
+        );
+      });
+
+      function Parent() {
+        const dialogRef = useRef();
+
+        return (
+          <div>
+            <h1>useImperativeHandle Dialog Example</h1>
+            <button onClick={() => dialogRef.current.open()}>Open Dialog</button>
+            <button onClick={() => dialogRef.current.reset()}>Reset Dialog</button>
+            <button onClick={() => dialogRef.current.close()}>Close Dialog</button>
+
+            <Dialog ref={dialogRef} />
+          </div>
+        );
+      }
+
+      export default Parent;
+      ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+301. ### When should you use useImperativeHandle?
+      The useImperativeHandler hook will be used in below cases:
+
+      *   You want to expose **imperative methods** from a child component 
+            - Custom input controls exposing `focus`, `clear`, or `validate` methods
+            - Modal components exposing `open()` and `close()` methods
+            - Scroll containers exposing `scrollToTop()` or `scrollToBottom()` methods
+      *   You want to **hide internal implementation** but provide controlled external access.
+      *   You're building **reusable component libraries** (e.g., inputs, modals, form controls).
+  
+**[⬆ Back to Top](#table-of-contents)**
+
+302. ### Is that possible to use useImperativeHandle without forwardRef?
+        **No.** `useImperativeHandle` only works when the component is wrapped in `forwardRef`. It's the combination that allows parent components to use a `ref` on a function component.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+303. ### How is useMemo different from useCallback?
+     The following table compares both useMemo and useCallback:
+
+      | Feature | `useMemo` | `useCallback` |
+      | --- | --- | --- |
+      | **Purpose** | Memoizes the **result of a computation** | Memoizes a **function reference** |
+      | **Returns** | A **value** (e.g., result of a function) | A **function** |
+      | **Usage** | `useMemo(() => computeValue(), [deps])` | `useCallback(() => doSomething(), [deps])` |
+      | **Primary Use Case** | Avoid expensive recalculations | Prevent unnecessary re-creations of functions |
+      | **Common Scenario** | Filtering, sorting, calculating derived data | Passing callbacks to child components |
+      | **When It's Useful** | When the value is expensive to compute | When referential equality matters (e.g., props) |
+      | **Recomputed When** | Dependencies change | Dependencies change |
+      | **Returned Value Type** | Any (number, object, array, etc.) | Always a function |
+      | **Overhead** | Slight (evaluates a function and caches result) | Slight (caches a function reference) |
+
+**[⬆ Back to Top](#table-of-contents)**
+
+304. ### Does useMemo prevent re-rendering of child components?
+
+        The `useMemo` hook **does not directly prevent re-rendering of child components**. Its main purpose is to memoize the result of an expensive computation so that it doesn’t get recalculated unless its dependencies change. While this can improve performance, it doesn’t inherently control whether a child component re-renders.
+        
+        However, `useMemo` **can help prevent re-renders** when the memoized value is passed as a prop to a child component that is wrapped in `React.memo`. In that case, if the memoized value doesn’t change between renders (i.e., it has the same reference), React.memo can skip re-rendering the child. So, while `useMemo` doesn’t stop renders on its own, it **works in combination** with tools like `React.memo` to optimize rendering behavior.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+305. ### What is `useCallback` and why is it used?
+
+        The `useCallback` is a React Hook used to memoize **function definitions** between renders. It returns the same function reference unless its dependencies change. This is especially useful when passing callbacks to optimized child components (e.g. those wrapped in `React.memo`) to prevent unnecessary re-renders.
+        
+        **Example:**
+        
+        ```css
+        const handleClick = useCallback(() => {
+          console.log('Button clicked');
+        }, []);
+        ```
+        
+        Without `useCallback`, a new function is created on every render, potentially causing child components to re-render unnecessarily.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+306. ### What are Custom React Hooks, and How Can You Develop One?
+
+      **Custom Hooks** in React are JavaScript functions that allow you to **extract and reuse component logic** using React’s built-in Hooks like `useState`, `useEffect`, etc.
+
+      They start with the word **"use"** and let you encapsulate logic that multiple components might share—such as fetching data, handling forms, or managing timers—without repeating code.
+
+      Let's explain the custom hook usage with `useFetchData` example. The `useFetchData` custom Hook is a reusable function in React that simplifies the process of fetching data from an API. It encapsulates common logic such as initiating the fetch request, managing loading and error states, and storing the fetched data. By using built-in Hooks like `useState` and `useEffect`, `useFetchData` provides a clean interface that returns the `data`, `loading`, and `error` values, which can be directly used in components.
+
+      ```jsx
+      import { useState, useEffect } from 'react';
+
+      function useFetchData(url) {
+        const [data, setData] = useState(null);     // Holds the response
+        const [loading, setLoading] = useState(true); // Loading state
+        const [error, setError] = useState(null);     // Error state
+
+        useEffect(() => {
+          let isMounted = true; // Prevent setting state on unmounted component
+          setLoading(true);
+
+          fetch(url)
+            .then((response) => {
+              if (!response.ok) throw new Error('Network response was not ok');
+              return response.json();
+            })
+            .then((json) => {
+              if (isMounted) {
+                setData(json);
+                setLoading(false);
+              }
+            })
+            .catch((err) => {
+              if (isMounted) {
+                setError(err.message);
+                setLoading(false);
+              }
+            });
+
+          return () => {
+            isMounted = false; // Clean-up function to avoid memory leaks
+          };
+        }, [url]);
+
+        return { data, loading, error };
+      }
+      ```
+
+      The above custom hook can be used to retrieve users data for `AuthorList`, `ReviewerList` components.
+
+      **Example: AuthorList component**
+      ```jsx
+      function AuthorList() {
+        const { data, loading, error } = useFetchData('https://api.example.com/authors');
+
+        if (loading) return <p>Loading authors...</p>;
+        if (error) return <p>Error: {error}</p>;
+
+        return (
+          <ul>
+            {data.map((author) => (
+              <li key={author.id}>{author.name}</li>
+            ))}
+          </ul>
+        );
+      }
+      ```
+     
+      Some of the benefits of custom hooks are:
+       *   Promotes **code reuse**
+       *   Keeps components **clean and focused**
+       *   Makes complex logic **easier to test and maintain**
+  
+**[⬆ Back to Top](#table-of-contents)**
+
+307. ### How does React Fiber works? Explain in detail.
+
+      React Fiber is the **core engine** that enables advanced features like **concurrent rendering**, **prioritization**, and **interruptibility** in React. Here's how it works:
+          
+      ### 1. **Fiber Tree Structure**
+          
+      Each component in your app is represented by a **Fiber node** in a tree structure. A Fiber node contains:
+      *   Component type
+      *   Props & state
+      *   Pointers to parent, child, and sibling nodes
+      *   Effect tags to track changes (e.g., update, placement)
+      *   This forms the **Fiber Tree**, a data structure React uses instead of the traditional call stack.
+          
+      ### 2. **Two Phases of Rendering**
+          
+        #### **A. Render Phase (work-in-progress)**
+          
+      *   React builds a **work-in-progress Fiber tree**.
+      *   It walks through each component (begin phase), calculates what needs to change, and collects side effects (complete phase).
+      *   This phase is **interruptible**—React can pause it and resume later.
+        #### **B. Commit Phase**
+          
+      *   React applies changes to the **Real DOM**.
+      *   Runs lifecycle methods (e.g., `componentDidMount`, `useEffect`).
+      *   This phase is **non-interruptible** but fast.
+          
+        ### 3. **Work Units and Scheduling**
+          
+      *   React breaks rendering into **units of work** (small tasks).
+      *   These units are scheduled based on **priority** using the **React Scheduler**.
+      *   If time runs out (e.g., user starts typing), React can **pause and yield** control back to the browser.
+          
+        ### 4. **Double Buffering with Two Trees**
+          
+      *   React maintains two trees:
+      *   **Current Tree** – what's visible on the screen.
+      *   **Work-In-Progress Tree** – the next version being built in memory.
+      *   Only after the new tree is fully ready, React **commits** it, making it the new current tree.
+          
+        ### 5. **Concurrency and Prioritization**
+          
+      *   React can prepare multiple versions of UI at once (e.g., during slow data loading).
+      *   Updates can be **assigned priorities**, so urgent updates (like clicks) are handled faster than background work.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+## Old Q&A
+
+1. ### Why should we not update the state directly?
+
+   If you try to update the state directly then it won't re-render the component.
+
+   ```javascript
+   //Wrong
+   this.state.message = "Hello world";
+   ```
+
+   Instead use `setState()` method. It schedules an update to a component's state object. When state changes, the component responds by re-rendering.
+
+   ```javascript
+   //Correct
+   this.setState({ message: "Hello World" });
+   ```
+
+   **Note:** You can directly assign to the state object either in _constructor_ or using latest javascript's class field declaration syntax.
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+2. ### What is the purpose of callback function as an argument of `setState()`?
+
+    The callback function provided as the second argument to `setState` is executed after the state has been updated and the component has re-rendered. Because `setState()` is asynchronous, you cannot reliably perform actions that require the updated state immediately after calling `setState`. The callback ensures your code runs only after the update and re-render are complete.
+
+    #### Example
+
+    ```jsx
+    this.setState({ name: "Sudheer" }, () => {
+      console.log("The name has been updated and the component has re-rendered.");
+    });
+    ```
+
+    #### When to use the callback?
+
+    Use the `setState` callback when you need to perform an action immediately after the DOM has been updated in response to a state change. i.e, The callback is a reliable way to perform actions after a state update and re-render, especially when the timing is critical due to the asynchronous nature of state updates in React. For example, if you need to interact with the updated DOM, trigger analytics, or perform further computations that depend on the new state or rendered output.
+
+    #### Note
+
+    - In modern React (with function components), you can achieve similar effects using the `useEffect` hook to respond to state changes.
+    - In class components, you can also use lifecycle methods like `componentDidUpdate` for broader post-update logic.
+    - The `setState` callback is still useful for one-off actions that directly follow a specific state change.
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+3. ### How to bind methods or event handlers in JSX callbacks?
+
+   There are 3 possible ways to achieve this in class components:
+
+   1. **Binding in Constructor:** In JavaScript classes, the methods are not bound by default. The same rule applies for React event handlers defined as class methods. Normally we bind them in constructor.
+
+      ```javascript
+      class User extends Component {
+        constructor(props) {
+          super(props);
+          this.handleClick = this.handleClick.bind(this);
+        }
+        handleClick() {
+          console.log("SingOut triggered");
+        }
+        render() {
+          return <button onClick={this.handleClick}>SingOut</button>;
+        }
+      }
+      ```
+
+   2. **Public class fields syntax:** If you don't like to use bind approach then _public class fields syntax_ can be used to correctly bind callbacks. The Create React App enables this syntax by default.
+
+      ```jsx harmony
+      handleClick = () => {
+        console.log("SingOut triggered", this);
+      };
+      ```
+
+      ```jsx harmony
+      <button onClick={this.handleClick}>SingOut</button>
+      ```
+
+   3. **Arrow functions in callbacks:** It is possible to use _arrow functions_ directly in the callbacks.
+
+      ```jsx harmony
+      handleClick() {
+          console.log('SingOut triggered');
+      }
+      render() {
+          return <button onClick={() => this.handleClick()}>SignOut</button>;
+      }
+      ```
+
+   **Note:** If the callback is passed as prop to child components, those components might do an extra re-rendering. In those cases, it is preferred to go with `.bind()` or _public class fields syntax_ approach considering performance.
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+4. ### How to pass a parameter to an event handler or callback?
+
+   You can use an _arrow function_ to wrap around an _event handler_ and pass parameters:
+
+   ```jsx harmony
+   <button onClick={() => this.handleClick(id)} />
+   ```
+
+   This is an equivalent to calling `.bind`:
+
+   ```jsx harmony
+   <button onClick={this.handleClick.bind(this, id)} />
+   ```
+
+   Apart from these two approaches, you can also pass arguments to a function which is defined as arrow function
+
+   ```jsx harmony
+   <button onClick={this.handleClick(id)} />;
+   handleClick = (id) => () => {
+     console.log("Hello, your ticket number is", id);
+   };
+   ```
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+5. ### What is the use of refs?
+
+   The _ref_ is used to return a reference to the element. They _should be avoided_ in most cases, however, they can be useful when you need a direct access to the DOM element or an instance of a component.
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+6. ### How to create refs?
+
+   There are two approaches
+
+   1. This is a recently added approach. _Refs_ are created using `React.createRef()` method and attached to React elements via the `ref` attribute. In order to use _refs_ throughout the component, just assign the _ref_ to the instance property within constructor.
+
+      ```jsx harmony
+      class MyComponent extends React.Component {
+        constructor(props) {
+          super(props);
+          this.myRef = React.createRef();
+        }
+        render() {
+          return <div ref={this.myRef} />;
+        }
+      }
+      ```
+
+   2. You can also use ref callbacks approach regardless of React version. For example, the search bar component's input element is accessed as follows,
+      ```jsx harmony
+      class SearchBar extends Component {
+        constructor(props) {
+          super(props);
+          this.txtSearch = null;
+          this.state = { term: "" };
+          this.setInputSearchRef = (e) => {
+            this.txtSearch = e;
+          };
+        }
+        onInputChange(event) {
+          this.setState({ term: this.txtSearch.value });
+        }
+        render() {
+          return (
+            <input
+              value={this.state.term}
+              onChange={this.onInputChange.bind(this)}
+              ref={this.setInputSearchRef}
+            />
+          );
+        }
+      }
+      ```
+
+   You can also use _refs_ in function components using **closures**.
+   **Note**: You can also use inline ref callbacks even though it is not a recommended approach.
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+7. ### What are forward refs?
+
+   _Ref forwarding_ is a feature that lets some components take a _ref_ they receive, and pass it further down to a child.
+
+   ```jsx harmony
+   const ButtonElement = React.forwardRef((props, ref) => (
+     <button ref={ref} className="CustomButton">
+       {props.children}
+     </button>
+   ));
+
+   // Create ref to the DOM button:
+   const ref = React.createRef();
+   <ButtonElement ref={ref}>{"Forward Ref"}</ButtonElement>;
+   ```
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+8. ### Which is preferred option with in callback refs and findDOMNode()?
+
+   It is preferred to use _callback refs_ over `findDOMNode()` API. Because `findDOMNode()` prevents certain improvements in React in the future.
+
+   The **legacy** approach of using `findDOMNode`:
+
+   ```javascript
+   class MyComponent extends Component {
+     componentDidMount() {
+       findDOMNode(this).scrollIntoView();
+     }
+
+     render() {
+       return <div />;
+     }
+   }
+   ```
+
+   The recommended approach is:
+
+   ```javascript
+   class MyComponent extends Component {
+     constructor(props) {
+       super(props);
+       this.node = createRef();
+     }
+     componentDidMount() {
+       this.node.current.scrollIntoView();
+     }
+
+     render() {
+       return <div ref={this.node} />;
+     }
+   }
+   ```
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+9. ### Why are String Refs legacy?
+
+   If you worked with React before, you might be familiar with an older API where the `ref` attribute is a string, like `ref={'textInput'}`, and the DOM node is accessed as `this.refs.textInput`. We advise against it because _string refs have below issues_, and are considered legacy. String refs were **removed in React v16**.
+
+   1. They _force React to keep track of currently executing component_. This is problematic because it makes react module stateful, and thus causes weird errors when react module is duplicated in the bundle.
+   2. They are _not composable_ — if a library puts a ref on the passed child, the user can't put another ref on it. Callback refs are perfectly composable.
+   3. They _don't work with static analysis_ like Flow. Flow can't guess the magic that framework does to make the string ref appear on `this.refs`, as well as its type (which could be different). Callback refs are friendlier to static analysis.
+   4. It doesn't work as most people would expect with the "render callback" pattern (e.g. <DataGrid renderRow={this.renderRow} />)
+
+      ```jsx harmony
+      class MyComponent extends Component {
+        renderRow = (index) => {
+          // This won't work. Ref will get attached to DataTable rather than MyComponent:
+          return <input ref={"input-" + index} />;
+
+          // This would work though! Callback refs are awesome.
+          return <input ref={(input) => (this["input-" + index] = input)} />;
+        };
+
+        render() {
+          return (
+            <DataTable data={this.props.data} renderRow={this.renderRow} />
+          );
+        }
+      }
+      ```
+
+   **[⬆ Back to Top](#table-of-contents)**
+
+10. ### What are the different phases of component lifecycle?
+
+    The component lifecycle has three distinct lifecycle phases:
+
+    1. **Mounting:** The component is ready to mount in the browser DOM. This phase covers initialization from `constructor()`, `getDerivedStateFromProps()`, `render()`, and `componentDidMount()` lifecycle methods.
+
+    2. **Updating:** In this phase, the component gets updated in two ways, sending the new props and updating the state either from `setState()` or `forceUpdate()`. This phase covers `getDerivedStateFromProps()`, `shouldComponentUpdate()`, `render()`, `getSnapshotBeforeUpdate()` and `componentDidUpdate()` lifecycle methods.
+
+    3. **Unmounting:** In this last phase, the component is not needed and gets unmounted from the browser DOM. This phase includes `componentWillUnmount()` lifecycle method.
+
+    It's worth mentioning that React internally has a concept of phases when applying changes to the DOM. They are separated as follows
+
+    4. **Render** The component will render without any side effects. This applies to Pure components and in this phase, React can pause, abort, or restart the render.
+
+    5. **Pre-commit** Before the component actually applies the changes to the DOM, there is a moment that allows React to read from the DOM through the `getSnapshotBeforeUpdate()`.
+
+    6. **Commit** React works with the DOM and executes the final lifecycles respectively `componentDidMount()` for mounting, `componentDidUpdate()` for updating, and `componentWillUnmount()` for unmounting.
+
+    React 16.3+ Phases (or an [interactive version](http://projects.wojtekmaj.pl/react-lifecycle-methods-diagram/))
+
+    ![phases 16.4+](images/phases16.4.png)
+
+    Before React 16.3
+
+    ![phases 16.2](images/phases.png)
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+11. ### What are the lifecycle methods of React?
+
+    Before React 16.3
+
+    - **componentWillMount:** Executed before rendering and is used for App level configuration in your root component.
+    - **componentDidMount:** Executed after first rendering and here all AJAX requests, DOM or state updates, and set up event listeners should occur.
+    - **componentWillReceiveProps:** Executed when particular prop updates to trigger state transitions.
+    - **shouldComponentUpdate:** Determines if the component will be updated or not. By default it returns `true`. If you are sure that the component doesn't need to render after state or props are updated, you can return false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives new prop.
+    - **componentWillUpdate:** Executed before re-rendering the component when there are props & state changes confirmed by `shouldComponentUpdate()` which returns true.
+    - **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes.
+    - **componentWillUnmount:** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
+
+    React 16.3+
+
+    - **getDerivedStateFromProps:** Invoked right before calling `render()` and is invoked on _every_ render. This exists for rare use cases where you need a derived state. Worth reading [if you need derived state](https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html).
+    - **componentDidMount:** Executed after first rendering and where all AJAX requests, DOM or state updates, and set up event listeners should occur.
+    - **shouldComponentUpdate:** Determines if the component will be updated or not. By default, it returns `true`. If you are sure that the component doesn't need to render after the state or props are updated, you can return a false value. It is a great place to improve performance as it allows you to prevent a re-render if component receives a new prop.
+    - **getSnapshotBeforeUpdate:** Executed right before rendered output is committed to the DOM. Any value returned by this will be passed into `componentDidUpdate()`. This is useful to capture information from the DOM i.e. scroll position.
+    - **componentDidUpdate:** Mostly it is used to update the DOM in response to prop or state changes. This will not fire if `shouldComponentUpdate()` returns `false`.
+    - **componentWillUnmount** It will be used to cancel any outgoing network requests, or remove all event listeners associated with the component.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+12. ### How to create props proxy for HOC component?
+
+    You can add/edit props passed to the component using _props proxy_ pattern like this:
+
+    ```jsx harmony
+    function HOC(WrappedComponent) {
+      return class Test extends Component {
+        render() {
+          const newProps = {
+            title: "New Header",
+            footer: false,
+            showFeatureX: false,
+            showFeatureY: true,
+          };
+
+          return <WrappedComponent {...this.props} {...newProps} />;
+        }
+      };
+    }
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+13. ### What is context?
+
+    _Context_ provides a way to pass data through the component tree without having to pass props down manually at every level.
+
+    For example, authenticated users, locale preferences, UI themes need to be accessed in the application by many components.
+
+    ```javascript
+    const { Provider, Consumer } = React.createContext(defaultValue);
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+14. ### What is the purpose of using super constructor with props argument?
+
+    A child class constructor cannot make use of `this` reference until the `super()` method has been called. The same applies to ES6 sub-classes as well. The main reason for passing props parameter to `super()` call is to access `this.props` in your child constructors.
+
+    **Passing props:**
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+
+        console.log(this.props); // prints { name: 'John', age: 42 }
+      }
+    }
+    ```
+
+    **Not passing props:**
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super();
+
+        console.log(this.props); // prints undefined
+
+        // but props parameter is still available
+        console.log(props); // prints { name: 'John', age: 42 }
+      }
+
+      render() {
+        // no difference outside constructor
+        console.log(this.props); // prints { name: 'John', age: 42 }
+      }
+    }
+    ```
+
+    The above code snippets reveals that `this.props` is different only within the constructor. It would be the same outside the constructor.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+15. ### How to set state with a dynamic key name?
+
+    If you are using ES6 or the Babel transpiler to transform your JSX code then you can accomplish this with _computed property names_.
+
+    ```javascript
+    handleInputChange(event) {
+      this.setState({ [event.target.id]: event.target.value })
+    }
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+16. ### What would be the common mistake of function being called every time the component renders?
+
+    You need to make sure that function is not being called while passing the function as a parameter.
+
+    ```jsx harmony
+    render() {
+      // Wrong: handleClick is called instead of passed as a reference!
+      return <button onClick={this.handleClick()}>{'Click Me'}</button>
+    }
+    ```
+
+    Instead, pass the function itself without parenthesis:
+
+    ```jsx harmony
+    render() {
+      // Correct: handleClick is passed as a reference!
+      return <button onClick={this.handleClick}>{'Click Me'}</button>
+    }
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+17. ### What are error boundaries in React v16?
+
+    _Error boundaries_ are components that catch JavaScript errors anywhere in their child component tree, log those errors, and display a fallback UI instead of the component tree that crashed.
+
+    A class component becomes an error boundary if it defines a new lifecycle method called `componentDidCatch(error, info)` or `static getDerivedStateFromError() `:
+
+    ```jsx harmony
+    class ErrorBoundary extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = { hasError: false };
+      }
+
+      componentDidCatch(error, info) {
+        // You can also log the error to an error reporting service
+        logErrorToMyService(error, info);
+      }
+
+      static getDerivedStateFromError(error) {
+        // Update state so the next render will show the fallback UI.
+        return { hasError: true };
+      }
+
+      render() {
+        if (this.state.hasError) {
+          // You can render any custom fallback UI
+          return <h1>{"Something went wrong."}</h1>;
+        }
+        return this.props.children;
+      }
+    }
+    ```
+
+    After that use it as a regular component:
+
+    ```jsx harmony
+    <ErrorBoundary>
+      <MyWidget />
+    </ErrorBoundary>
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+18. ### How are error boundaries handled in React v15?
+
+    React v15 provided very basic support for _error boundaries_ using `unstable_handleError` method. It has been renamed to `componentDidCatch` in React v16.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+19. ### What is the purpose of render method of `react-dom`?
+
+    This method is used to render a React element into the DOM in the supplied container and return a reference to the component. If the React element was previously rendered into container, it will perform an update on it and only mutate the DOM as necessary to reflect the latest changes.
+
+    ```
+    ReactDOM.render(element, container, [callback])
+    ```
+
+    If the optional callback is provided, it will be executed after the component is rendered or updated.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+20. ### What will happen if you use `setState()` in constructor?
+
+    When you use `setState()`, then apart from assigning to the object state React also re-renders the component and all its children. You would get error like this: _Can only update a mounted or mounting component._ So we need to use `this.state` to initialize variables inside constructor.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+21. ### Is it good to use `setState()` in `componentWillMount()` method?
+
+    Yes, it is safe to use `setState()` inside `componentWillMount()` method. But at the same it is recommended to avoid async initialization in `componentWillMount()` lifecycle method. `componentWillMount()` is invoked immediately before mounting occurs. It is called before `render()`, therefore setting state in this method will not trigger a re-render. Avoid introducing any side-effects or subscriptions in this method. We need to make sure async calls for component initialization happened in `componentDidMount()` instead of `componentWillMount()`.
+
+    ```jsx harmony
+    componentDidMount() {
+      axios.get(`api/todos`)
+        .then((result) => {
+          this.setState({
+            messages: [...result.data]
+          })
+        })
+    }
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+22. ### What will happen if you use props in initial state?
+
+    If the props on the component are changed without the component being refreshed, the new prop value will never be displayed because the constructor function will never update the current state of the component. The initialization of state from props only runs when the component is first created.
+
+    The below component won't display the updated input value:
+
+    ```jsx harmony
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+
+        this.state = {
+          records: [],
+          inputValue: this.props.inputValue,
+        };
+      }
+
+      render() {
+        return <div>{this.state.inputValue}</div>;
+      }
+    }
+    ```
+
+    Using props inside render method will update the value:
+
+    ```jsx harmony
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+
+        this.state = {
+          record: [],
+        };
+      }
+
+      render() {
+        return <div>{this.props.inputValue}</div>;
+      }
+    }
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+23. ### How you use decorators in React?
+
+    You can _decorate_ your _class_ components, which is the same as passing the component into a function. **Decorators** are flexible and readable way of modifying component functionality.
+
+    ```jsx harmony
+    @setTitle("Profile")
+    class Profile extends React.Component {
+      //....
+    }
+
+    /*
+      title is a string that will be set as a document title
+      WrappedComponent is what our decorator will receive when
+      put directly above a component class as seen in the example above
+    */
+    const setTitle = (title) => (WrappedComponent) => {
+      return class extends React.Component {
+        componentDidMount() {
+          document.title = title;
+        }
+
+        render() {
+          return <WrappedComponent {...this.props} />;
+        }
+      };
+    };
+    ```
+
+    **Note:** Decorators are a feature that didn't make it into ES7, but are currently a _stage 2 proposal_.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+24. ### What is CRA and its benefits?
+
+    The `create-react-app` CLI tool allows you to quickly create & run React applications with no configuration step.
+
+    Let's create Todo App using _CRA_:
+
+    ```console
+    # Installation
+    $ npm install -g create-react-app
+
+    # Create new project
+    $ create-react-app todo-app
+    $ cd todo-app
+
+    # Build, test and run
+    $ npm run build
+    $ npm run test
+    $ npm start
+    ```
+
+    It includes everything we need to build a React app:
+
+    1. React, JSX, ES6, and Flow syntax support.
+    2. Language extras beyond ES6 like the object spread operator.
+    3. Autoprefixed CSS, so you don’t need -webkit- or other prefixes.
+    4. A fast interactive unit test runner with built-in support for coverage reporting.
+    5. A live development server that warns about common mistakes.
+    6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+25. ### What is the lifecycle methods order in mounting?
+
+    The lifecycle methods are called in the following order when an instance of a component is being created and inserted into the DOM.
+
+    1. `constructor()`
+    2. `static getDerivedStateFromProps()`
+    3. `render()`
+    4. `componentDidMount()`
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+26. ### What are the lifecycle methods going to be deprecated in React v16?
+
+    The following lifecycle methods going to be unsafe coding practices and will be more problematic with async rendering.
+
+    1. `componentWillMount()`
+    2. `componentWillReceiveProps()`
+    3. `componentWillUpdate()`
+
+    Starting with React v16.3 these methods are aliased with `UNSAFE_` prefix, and the unprefixed version will be removed in React v17.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+27. ### What is the purpose of `getDerivedStateFromProps()` lifecycle method?
+
+    The new static `getDerivedStateFromProps()` lifecycle method is invoked after a component is instantiated as well as before it is re-rendered. It can return an object to update state, or `null` to indicate that the new props do not require any state updates.
+
+    ```javascript
+    class MyComponent extends React.Component {
+      static getDerivedStateFromProps(props, state) {
+        // ...
+      }
+    }
+    ```
+
+    This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillReceiveProps()`.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+28. ### What is the purpose of `getSnapshotBeforeUpdate()` lifecycle method?
+
+    The new `getSnapshotBeforeUpdate()` lifecycle method is called right before DOM updates. The return value from this method will be passed as the third parameter to `componentDidUpdate()`.
+
+    ```javascript
+    class MyComponent extends React.Component {
+      getSnapshotBeforeUpdate(prevProps, prevState) {
+        // ...
+      }
+    }
+    ```
+
+    This lifecycle method along with `componentDidUpdate()` covers all the use cases of `componentWillUpdate()`.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+29. ### What is the recommended way for naming components?
+
+    It is recommended to name the component by reference instead of using `displayName`.
+
+    Using `displayName` for naming component:
+
+    ```javascript
+    export default React.createClass({
+      displayName: "TodoApp",
+      // ...
+    });
+    ```
+
+    The **recommended** approach:
+
+    ```javascript
+    export default class TodoApp extends React.Component {
+      // ...
+    }
+    ```
+
+    also
+
+    ```javascript
+    const TodoApp = () => {
+      //...
+    };
+    export default TodoApp;
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+30. ### What is the recommended ordering of methods in component class?
+
+    _Recommended_ ordering of methods from _mounting_ to _render stage_:
+
+    1. `static` methods
+    2. `constructor()`
+    3. `getChildContext()`
+    4. `componentWillMount()`
+    5. `componentDidMount()`
+    6. `componentWillReceiveProps()`
+    7. `shouldComponentUpdate()`
+    8. `componentWillUpdate()`
+    9. `componentDidUpdate()`
+    10. `componentWillUnmount()`
+    11. click handlers or event handlers like `onClickSubmit()` or `onChangeDescription()`
+    12. getter methods for render like `getSelectReason()` or `getFooterContent()`
+    13. optional render methods like `renderNavigation()` or `renderProfilePicture()`
+    14. `render()`
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+31. ### Why we need to pass a function to setState()?
+
+    The reason behind for this is that `setState()` is an asynchronous operation. React batches state changes for performance reasons, so the state may not change immediately after `setState()` is called. That means you should not rely on the current state when calling `setState()` since you can't be sure what that state will be. The solution is to pass a function to `setState()`, with the previous state as an argument. By doing this you can avoid issues with the user getting the old state value on access due to the asynchronous nature of `setState()`.
+
+    Let's say the initial count value is zero. After three consecutive increment operations, the value is going to be incremented only by one.
+
+    ```javascript
+    // assuming this.state.count === 0
+    this.setState({ count: this.state.count + 1 });
+    this.setState({ count: this.state.count + 1 });
+    this.setState({ count: this.state.count + 1 });
+    // this.state.count === 1, not 3
+    ```
+
+    If we pass a function to `setState()`, the count gets incremented correctly.
+
+    ```javascript
+    this.setState((prevState, props) => ({
+      count: prevState.count + props.increment,
+    }));
+    // this.state.count === 3 as expected
+    ```
+
+    **(OR)**
+
+    ### Why function is preferred over object for `setState()`?
+
+    React may batch multiple `setState()` calls into a single update for performance. Because `this.props` and `this.state` may be updated asynchronously, you should not rely on their values for calculating the next state.
+
+    This counter example will fail to update as expected:
+
+    ```javascript
+    // Wrong
+    this.setState({
+      counter: this.state.counter + this.props.increment,
+    });
+    ```
+
+    The preferred approach is to call `setState()` with function rather than object. That function will receive the previous state as the first argument, and the props at the time the update is applied as the second argument.
+
+    ```javascript
+    // Correct
+    this.setState((prevState, props) => ({
+      counter: prevState.counter + props.increment,
+    }));
+    ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+32. ### Why is `isMounted()` an anti-pattern and what is the proper solution?
+
+    The primary use case for `isMounted()` is to avoid calling `setState()` after a component has been unmounted, because it will emit a warning.
+
+    ```javascript
+    if (this.isMounted()) {
+      this.setState({...})
+    }
+    ```
+
+    Checking `isMounted()` before calling `setState()` does eliminate the warning, but it also defeats the purpose of the warning. Using `isMounted()` is a code smell because the only reason you would check is because you think you might be holding a reference after the component has unmounted.
+
+    An optimal solution would be to find places where `setState()` might be called after a component has unmounted, and fix them. Such situations most commonly occur due to callbacks, when a component is waiting for some data and gets unmounted before the data arrives. Ideally, any callbacks should be canceled in `componentWillUnmount()`, prior to unmounting.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+33. ### What is the difference between constructor and getInitialState?
+
+    You should initialize state in the constructor when using ES6 classes, and `getInitialState()` method when using `React.createClass()`.
+
+    **Using ES6 classes:**
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {
+          /* initial state */
+        };
+      }
+    }
+    ```
+
+    **Using `React.createClass()`:**
+
+    ```javascript
+    const MyComponent = React.createClass({
+      getInitialState() {
+        return {
+          /* initial state */
+        };
+      },
+    });
+    ```
+
+    **Note:** `React.createClass()` is deprecated and removed in React v16. Use plain JavaScript classes instead.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+34. ### Can you force a component to re-render without calling setState?
+
+    By default, when your component's state or props change, your component will re-render. If your `render()` method depends on some other data, you can tell React that the component needs re-rendering by calling `forceUpdate()`.
+
+    ```javascript
+    component.forceUpdate(callback);
+    ```
+
+    It is recommended to avoid all uses of `forceUpdate()` and only read from `this.props` and `this.state` in `render()`.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+35. ### What is the difference between `super()` and `super(props)` in React using ES6 classes?
+
+    When you want to access `this.props` in `constructor()` then you should pass props to `super()` method.
+
+    **Using `super(props)`:**
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        console.log(this.props); // { name: 'John', ... }
+      }
+    }
+    ```
+
+    **Using `super()`:**
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super();
+        console.log(this.props); // undefined
+      }
+    }
+    ```
+
+    Outside `constructor()` both will display same value for `this.props`.
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+36. ### What is the difference between `setState()` and `replaceState()` methods?
+
+    When you use `setState()` the current and previous states are merged. `replaceState()` throws out the current state, and replaces it with only what you provide. Usually `setState()` is used unless you really need to remove all previous keys for some reason. You can also set state to `false`/`null` in `setState()` instead of using `replaceState()`.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+37. ### How to listen to state changes?
+
+    The `componentDidUpdate` lifecycle method will be called when state changes. You can compare provided state and props values with current state and props to determine if something meaningful changed.
+
+    ```
+    componentDidUpdate(object prevProps, object prevState)
+    ```
+
+    **Note:** The previous releases of ReactJS also uses `componentWillUpdate(object nextProps, object nextState)` for state changes. It has been deprecated in latest releases.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+38. ### What is the recommended approach of removing an array element in React state?
+
+    The better approach is to use `Array.prototype.filter()` method.
+
+    For example, let's create a `removeItem()` method for updating the state.
+
+    ```javascript
+    removeItem(index) {
+      this.setState({
+        data: this.state.data.filter((item, i) => i !== index)
+      })
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+39. ### Is it possible to use React without rendering HTML?
+
+    It is possible. Below are the possible options:
+
+    ```jsx harmony
+    render() {
+      return false
+    }
+    ```
+
+    ```jsx harmony
+    render() {
+      return true
+    }
+    ```
+
+    ```jsx harmony
+    render() {
+      return null
+    }
+    ```
+
+    React version >=16.0.0:
+
+    ```jsx harmony
+    render() {
+      return []
+    }
+    ```
+
+    ```jsx harmony
+    render() {
+      return ""
+    }
+    ```
+
+    React version >=16.2.0:
+
+    ```jsx harmony
+    render() {
+      return <React.Fragment></React.Fragment>
+    }
+    ```
+
+    ```jsx harmony
+    render() {
+      return <></>
+    }
+    ```
+
+    React version >=18.0.0:
+
+    ```jsx harmony
+    render() {
+      return undefined
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+40. ### What are the possible ways of updating objects in state?
+
+    1.  **Calling `setState()` with an object to merge with state:**
+
+        - Using `Object.assign()` to create a copy of the object:
+
+          ```javascript
+          const user = Object.assign({}, this.state.user, { age: 42 });
+          this.setState({ user });
+          ```
+
+        - Using _spread operator_:
+
+          ```javascript
+          const user = { ...this.state.user, age: 42 };
+          this.setState({ user });
+          ```
+
+    2.  **Calling `setState()` with a function:**
+
+        ```javascript
+        this.setState((prevState) => ({
+          user: {
+            ...prevState.user,
+            age: 42,
+          },
+        }));
+        ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+41. ### What are the approaches to include polyfills in your `create-react-app`?
+
+    There are approaches to include polyfills in create-react-app,
+
+    1.  **Manual import from `core-js`:**
+
+        Create a file called (something like) `polyfills.js` and import it into root `index.js` file. Run `npm install core-js` or `yarn add core-js` and import your specific required features.
+
+        ```javascript
+        import "core-js/fn/array/find";
+        import "core-js/fn/array/includes";
+        import "core-js/fn/number/is-nan";
+        ```
+
+    2.  **Using Polyfill service:**
+
+        Use the polyfill.io CDN to retrieve custom, browser-specific polyfills by adding this line to `index.html`:
+
+        ```html
+        <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=default,Array.prototype.includes"></script>
+        ```
+
+        In the above script we had to explicitly request the `Array.prototype.includes` feature as it is not included in the default feature set.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+42. ### How to use https instead of http in create-react-app?
+
+    You just need to use `HTTPS=true` configuration. You can edit your `package.json` scripts section:
+
+    ```json
+    "scripts": {
+      "start": "set HTTPS=true && react-scripts start"
+    }
+    ```
+
+    or just run `set HTTPS=true && npm start`
+
+**[⬆ Back to Top](#table-of-contents)**
+
+43. ### How to avoid using relative path imports in create-react-app?
+
+    Create a file called `.env` in the project root and write the import path:
+
+    ```
+    NODE_PATH=src/app
+    ```
+
+    After that restart the development server. Now you should be able to import anything inside `src/app` without relative paths.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+44. ### How to update a component every second?
+
+    You need to use `setInterval()` to trigger the change, but you also need to clear the timer when the component unmounts to prevent errors and memory leaks.
+
+    ```javascript
+    componentDidMount() {
+      this.interval = setInterval(() => this.setState({ time: Date.now() }), 1000)
+    }
+
+    componentWillUnmount() {
+      clearInterval(this.interval)
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+45. ### Why is a component constructor called only once?
+
+    React's _reconciliation_ algorithm assumes that without any information to the contrary, if a custom component appears in the same place on subsequent renders, it's the same component as before, so reuses the previous instance rather than creating a new one.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+46. ### How to define constants in React?
+
+    You can use ES7 `static` field to define constant.
+
+    ```javascript
+    class MyComponent extends React.Component {
+      static DEFAULT_PAGINATION = 10;
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+47. ### How to programmatically trigger click event in React?
+
+    You could use the ref prop to acquire a reference to the underlying `HTMLInputElement` object through a callback, store the reference as a class property, then use that reference to later trigger a click from your event handlers using the `HTMLElement.click` method.
+
+    This can be done in two steps:
+
+    1.  Create ref in render method:
+
+        ```jsx harmony
+        <input ref={(input) => (this.inputElement = input)} />
+        ```
+
+    2.  Apply click event in your event handler:
+
+        ```javascript
+        this.inputElement.click();
+        ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+48. ### How to make AJAX call and in which component lifecycle methods should I make an AJAX call?
+
+    You can use AJAX libraries such as Axios, jQuery AJAX, and the browser built-in `fetch`. You should fetch data in the `componentDidMount()` lifecycle method. This is so you can use `setState()` to update your component when the data is retrieved.
+
+    For example, the employees list fetched from API and set local state:
+
+    ```jsx harmony
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {
+          employees: [],
+          error: null,
+        };
+      }
+
+      componentDidMount() {
+        fetch("https://api.example.com/items")
+          .then((res) => res.json())
+          .then(
+            (result) => {
+              this.setState({
+                employees: result.employees,
+              });
+            },
+            (error) => {
+              this.setState({ error });
+            }
+          );
+      }
+
+      render() {
+        const { error, employees } = this.state;
+        if (error) {
+          return <div>Error: {error.message}</div>;
+        } else {
+          return (
+            <ul>
+              {employees.map((employee) => (
+                <li key={employee.name}>
+                  {employee.name}-{employee.experience}
+                </li>
+              ))}
+            </ul>
+          );
+        }
+      }
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+49. ### What are render props?
+
+    **Render Props** is a simple technique for sharing code between components using a prop whose value is a function. The below component uses render prop which returns a React element.
+
+    ```jsx harmony
+    <DataProvider render={(data) => <h1>{`Hello ${data.target}`}</h1>} />
+    ```
+
+    Libraries such as React Router and DownShift are using this pattern.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+50. ### How to dispatch an action on load?
+
+    You can dispatch an action in `componentDidMount()` method and in `render()` method you can verify the data.
+
+    ```javascript
+    class App extends Component {
+      componentDidMount() {
+        this.props.fetchData();
+      }
+
+      render() {
+        return this.props.isLoaded ? (
+          <div>{"Loaded"}</div>
+        ) : (
+          <div>{"Not Loaded"}</div>
+        );
+      }
+    }
+
+    const mapStateToProps = (state) => ({
+      isLoaded: state.isLoaded,
+    });
+
+    const mapDispatchToProps = { fetchData };
+
+    export default connect(mapStateToProps, mapDispatchToProps)(App);
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+51. ### How to use `connect()` from React Redux?
+
+    You need to follow two steps to use your store in your container:
+
+    1.  **Use `mapStateToProps()`:** It maps the state variables from your store to the props that you specify.
+    2.  **Connect the above props to your container:** The object returned by the `mapStateToProps` function is connected to the container. You can import `connect()` from `react-redux`.
+
+        ```jsx harmony
+        import React from "react";
+        import { connect } from "react-redux";
+
+        class App extends React.Component {
+          render() {
+            return <div>{this.props.containerData}</div>;
+          }
+        }
+
+        function mapStateToProps(state) {
+          return { containerData: state.data };
+        }
+
+        export default connect(mapStateToProps)(App);
+        ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+52. ### Whats the purpose of `at` symbol in the Redux connect decorator?
+
+    The **@** symbol is in fact a JavaScript expression used to signify decorators. _Decorators_ make it possible to annotate and modify classes and properties at design time.
+
+    Let's take an example setting up Redux without and with a decorator.
+
+    - **Without decorator:**
+
+      ```javascript
+      import React from "react";
+      import * as actionCreators from "./actionCreators";
+      import { bindActionCreators } from "redux";
+      import { connect } from "react-redux";
+
+      function mapStateToProps(state) {
+        return { todos: state.todos };
+      }
+
+      function mapDispatchToProps(dispatch) {
+        return { actions: bindActionCreators(actionCreators, dispatch) };
+      }
+
+      class MyApp extends React.Component {
+        // ...define your main app here
+      }
+
+      export default connect(mapStateToProps, mapDispatchToProps)(MyApp);
+      ```
+
+    - **With decorator:**
+
+      ```javascript
+      import React from "react";
+      import * as actionCreators from "./actionCreators";
+      import { bindActionCreators } from "redux";
+      import { connect } from "react-redux";
+
+      function mapStateToProps(state) {
+        return { todos: state.todos };
+      }
+
+      function mapDispatchToProps(dispatch) {
+        return { actions: bindActionCreators(actionCreators, dispatch) };
+      }
+
+      @connect(mapStateToProps, mapDispatchToProps)
+      export default class MyApp extends React.Component {
+        // ...define your main app here
+      }
+      ```
+
+    The above examples are almost similar except the usage of decorator. The decorator syntax isn't built into any JavaScript runtimes yet, and is still experimental and subject to change. You can use babel for the decorators support.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+53. ### How to use TypeScript in `create-react-app` application?
+
+        Starting from react-scripts@3.3.0+ releases onwards, you can now optionally start a new app from a template by appending `--template [template-name]` to the creation command. If you don't select a template, it will create your project with base template. Remember that templates are always named in the format `cra-template-[template-name]`, here you only need to fill the `[template-name]` section.
+
+        The typeScript can be used in your project by appending `--template typescript` to the creation command.
+
+         ```bash
+         npx create-react-app my-app --template typescript
+         ```
+
+        But if you are using React Scripting between react-scripts@2.1.0 and react-scripts@3.2.x , there is a built-in support for TypeScript. i.e, `create-react-app` now supports TypeScript natively. You can just pass `--typescript` option as below
+
+         ```bash
+         npx create-react-app my-app --typescript
+
+         # or
+
+         yarn create react-app my-app --typescript
+         ```
+
+         Whereas for lower versions of react scripts, just supply `--scripts-version` option as `react-scripts-ts` while you create a new project. `react-scripts-ts` is a set of adjustments to take the standard `create-react-app` project pipeline and bring TypeScript into the mix.
+
+         Now the project layout should look like the following:
+
+         ```
+         my-app/
+         ├─ .gitignore
+         ├─ images.d.ts
+         ├─ node_modules/
+         ├─ public/
+         ├─ src/
+         │  └─ ...
+         ├─ package.json
+         ├─ tsconfig.json
+         ├─ tsconfig.prod.json
+         ├─ tsconfig.test.json
+         └─ tslint.json
+         ```
+
+    **[⬆ Back to Top](#table-of-contents)**
+
+54. ### Does the statics object work with ES6 classes in React?
+
+    No, `statics` only works with `React.createClass()`:
+
+    ```javascript
+    someComponent = React.createClass({
+      statics: {
+        someMethod: function () {
+          // ..
+        },
+      },
+    });
+    ```
+
+    But you can write statics inside ES6+ classes as below,
+
+    ```javascript
+    class Component extends React.Component {
+      static propTypes = {
+        // ...
+      };
+
+      static someMethod() {
+        // ...
+      }
+    }
+    ```
+
+    or writing them outside class as below,
+
+    ```javascript
+    class Component extends React.Component {
+       ....
+    }
+
+    Component.propTypes = {...}
+    Component.someMethod = function(){....}
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+55. ### Why are inline ref callbacks or functions not recommended?
+
+    If the ref callback is defined as an inline function, it will get called twice during updates, first with null and then again with the DOM element. This is because a new instance of the function is created with each render, so React needs to clear the old ref and set up the new one.
+
+    ```jsx
+    class UserForm extends Component {
+      handleSubmit = () => {
+        console.log("Input Value is: ", this.input.value);
+      };
+
+      render() {
+        return (
+          <form onSubmit={this.handleSubmit}>
+            <input type="text" ref={(input) => (this.input = input)} /> //
+            Access DOM input in handle submit
+            <button type="submit">Submit</button>
+          </form>
+        );
+      }
+    }
+    ```
+
+    But our expectation is for the ref callback to get called once, when the component mounts. One quick fix is to use the ES7 class property syntax to define the function
+
+    ```jsx
+    class UserForm extends Component {
+      handleSubmit = () => {
+        console.log("Input Value is: ", this.input.value);
+      };
+
+      setSearchInput = (input) => {
+        this.input = input;
+      };
+
+      render() {
+        return (
+          <form onSubmit={this.handleSubmit}>
+            <input type="text" ref={this.setSearchInput} /> // Access DOM input
+            in handle submit
+            <button type="submit">Submit</button>
+          </form>
+        );
+      }
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+56. ### What are HOC factory implementations?
+
+    There are two main ways of implementing HOCs in React.
+
+    1.  Props Proxy (PP) and
+    2.  Inheritance Inversion (II).
+
+    But they follow different approaches for manipulating the _WrappedComponent_.
+
+    **Props Proxy**
+
+    In this approach, the render method of the HOC returns a React Element of the type of the WrappedComponent. We also pass through the props that the HOC receives, hence the name **Props Proxy**.
+
+    ```jsx
+    function ppHOC(WrappedComponent) {
+      return class PP extends React.Component {
+        render() {
+          return <WrappedComponent {...this.props} />;
+        }
+      };
+    }
+    ```
+
+    **Inheritance Inversion**
+
+    In this approach, the returned HOC class (Enhancer) extends the WrappedComponent. It is called Inheritance Inversion because instead of the WrappedComponent extending some Enhancer class, it is passively extended by the Enhancer. In this way the relationship between them seems **inverse**.
+
+    ```jsx
+    function iiHOC(WrappedComponent) {
+      return class Enhancer extends WrappedComponent {
+        render() {
+          return super.render();
+        }
+      };
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+57. ### How to use class field declarations syntax in React classes?
+
+    React Class Components can be made much more concise using the class field declarations. You can initialize the local state without using the constructor and declare class methods by using arrow functions without the extra need to bind them.
+
+    Let's take a counter example to demonstrate class field declarations for state without using constructor and methods without binding,
+
+    ```jsx
+    class Counter extends Component {
+      state = { value: 0 };
+
+      handleIncrement = () => {
+        this.setState((prevState) => ({
+          value: prevState.value + 1,
+        }));
+      };
+
+      handleDecrement = () => {
+        this.setState((prevState) => ({
+          value: prevState.value - 1,
+        }));
+      };
+
+      render() {
+        return (
+          <div>
+            {this.state.value}
+
+            <button onClick={this.handleIncrement}>+</button>
+            <button onClick={this.handleDecrement}>-</button>
+          </div>
+        );
+      }
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+58. ### Why do you not need error boundaries for event handlers?
+
+    Error boundaries do not catch errors inside event handlers.
+
+    React doesn’t need error boundaries to recover from errors in event handlers. Unlike the render method and lifecycle methods, the event handlers don’t happen during rendering. So if they throw, React still knows what to display on the screen.
+
+    If you need to catch an error inside an event handler, use the regular JavaScript try / catch statement:
+
+    ```javascript
+    class MyComponent extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = { error: null };
+        this.handleClick = this.handleClick.bind(this);
+      }
+
+      handleClick() {
+        try {
+          // Do something that could throw
+        } catch (error) {
+          this.setState({ error });
+        }
+      }
+
+      render() {
+        if (this.state.error) {
+          return <h1>Caught an error.</h1>;
+        }
+        return <button onClick={this.handleClick}>Click Me</button>;
+      }
+    }
+    ```
+
+    Note that the above example is demonstrating regular JavaScript behavior and doesn’t use error boundaries.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+59. ### What is the difference between try catch block and error boundaries?
+
+    Try catch block works with imperative code whereas error boundaries are meant for declarative code to render on the screen.
+
+    For example, the try catch block used for below imperative code
+
+    ```javascript
+    try {
+      showButton();
+    } catch (error) {
+      // ...
+    }
+    ```
+
+    Whereas error boundaries wrap declarative code as below,
+
+    ```javascript
+    <ErrorBoundary>
+      <MyComponent />
+    </ErrorBoundary>
+    ```
+
+    So if an error occurs in a **componentDidUpdate** method caused by a **setState** somewhere deep in the tree, it will still correctly propagate to the closest error boundary.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+60. ### What is the required method to be defined for a class component?
+    The `render()` method is the only required method in a class component. i.e, All methods other than render method are optional for a class component.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+61. ### What are the possible return types of render method?
+
+    Below are the list of following types used and return from render method,
+
+    1.  **React elements:** Elements that instruct React to render a DOM node. It includes html elements such as `<div/>` and user defined elements.
+    2.  **Arrays and fragments:** Return multiple elements to render as Arrays and Fragments to wrap multiple elements
+    3.  **Portals:** Render children into a different DOM subtree.
+    4.  **String and numbers:** Render both Strings and Numbers as text nodes in the DOM
+    5.  **Booleans or null:** Doesn't render anything but these types are used to conditionally render content.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+62. ### What is the main purpose of constructor?
+
+    The constructor is mainly used for two purposes,
+
+    1.  To initialize local state by assigning object to this.state
+    2.  For binding event handler methods to the instance
+        For example, the below code covers both the above cases,
+
+    ```javascript
+    constructor(props) {
+      super(props);
+      // Don't call this.setState() here!
+      this.state = { counter: 0 };
+      this.handleClick = this.handleClick.bind(this);
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+63. ### Is it mandatory to define constructor for React component?
+    No, it is not mandatory. i.e, If you don’t initialize state and you don’t bind methods, you don’t need to implement a constructor for your React component.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+64. ### Why should not call setState in componentWillUnmount?
+    You should not call `setState()` in `componentWillUnmount()` because once a component instance is unmounted, it will never be mounted again.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+65. ### What is the purpose of getDerivedStateFromError?
+
+    This lifecycle method is invoked after an error has been thrown by a descendant component. It receives the error that was thrown as a parameter and should return a value to update state.
+
+    The signature of the lifecycle method is as follows,
+
+    ```javascript
+    static getDerivedStateFromError(error)
+    ```
+
+    Let us take error boundary use case with the above lifecycle method for demonstration purpose,
+
+    ```javascript
+    class ErrorBoundary extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = { hasError: false };
+      }
+
+      static getDerivedStateFromError(error) {
+        // Update state so the next render will show the fallback UI.
+        return { hasError: true };
+      }
+
+      render() {
+        if (this.state.hasError) {
+          // You can render any custom fallback UI
+          return <h1>Something went wrong.</h1>;
+        }
+
+        return this.props.children;
+      }
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+66. ### What is the methods order when component re-rendered?
+
+    An update can be caused by changes to props or state. The below methods are called in the following order when a component is being re-rendered.
+
+    1.  static getDerivedStateFromProps()
+    2.  shouldComponentUpdate()
+    3.  render()
+    4.  getSnapshotBeforeUpdate()
+    5.  componentDidUpdate()
+
+**[⬆ Back to Top](#table-of-contents)**
+
+67. ### What are the methods invoked during error handling?
+
+    Below methods are called when there is an error during rendering, in a lifecycle method, or in the constructor of any child component.
+
+    1.  static getDerivedStateFromError()
+    2.  componentDidCatch()
+
+**[⬆ Back to Top](#table-of-contents)**
+
+68. ### What is the purpose of unmountComponentAtNode method?
+
+    This method is available from react-dom package and it removes a mounted React component from the DOM and clean up its event handlers and state. If no component was mounted in the container, calling this function does nothing. Returns true if a component was unmounted and false if there was no component to unmount.
+
+    The method signature would be as follows,
+
+    ```javascript
+    ReactDOM.unmountComponentAtNode(container);
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+69. ### What are the limitations with HOCs?
+
+    Higher-order components come with a few caveats apart from its benefits. Below are the few listed in an order,
+
+    1.  **Don’t use HOCs inside the render method:**
+        It is not recommended to apply a HOC to a component within the render method of a component.
+
+        ```javascript
+        render() {
+          // A new version of EnhancedComponent is created on every render
+          // EnhancedComponent1 !== EnhancedComponent2
+          const EnhancedComponent = enhance(MyComponent);
+          // That causes the entire subtree to unmount/remount each time!
+          return <EnhancedComponent />;
+        }
+        ```
+
+        The above code impacts on performance by remounting a component that causes the state of that component and all of its children to be lost. Instead, apply HOCs outside the component definition so that the resulting component is created only once.
+
+    2.  **Static methods must be copied over:**
+        When you apply a HOC to a component the new component does not have any of the static methods of the original component
+
+        ```javascript
+        // Define a static method
+        WrappedComponent.staticMethod = function () {
+          /*...*/
+        };
+        // Now apply a HOC
+        const EnhancedComponent = enhance(WrappedComponent);
+
+        // The enhanced component has no static method
+        typeof EnhancedComponent.staticMethod === "undefined"; // true
+        ```
+
+        You can overcome this by copying the methods onto the container before returning it,
+
+        ```javascript
+        function enhance(WrappedComponent) {
+          class Enhance extends React.Component {
+            /*...*/
+          }
+          // Must know exactly which method(s) to copy :(
+          Enhance.staticMethod = WrappedComponent.staticMethod;
+          return Enhance;
+        }
+        ```
+
+    3.  **Refs aren’t passed through:**
+        For HOCs you need to pass through all props to the wrapped component but this does not work for refs. This is because ref is not really a prop similar to key. In this case you need to use the React.forwardRef API
+
+**[⬆ Back to Top](#table-of-contents)**
+
+70. ### How to debug forwardRefs in DevTools?
+
+    **React.forwardRef** accepts a render function as parameter and DevTools uses this function to determine what to display for the ref forwarding component.
+
+    For example, If you don't name the render function or not using displayName property then it will appear as ”ForwardRef” in the DevTools,
+
+    ```javascript
+    const WrappedComponent = React.forwardRef((props, ref) => {
+      return <LogProps {...props} forwardedRef={ref} />;
+    });
+    ```
+
+    But If you name the render function then it will appear as **”ForwardRef(myFunction)”**
+
+    ```javascript
+    const WrappedComponent = React.forwardRef(function myFunction(props, ref) {
+      return <LogProps {...props} forwardedRef={ref} />;
+    });
+    ```
+
+    As an alternative, You can also set displayName property for forwardRef function,
+
+    ```javascript
+    function logProps(Component) {
+      class LogProps extends React.Component {
+        // ...
+      }
+
+      function forwardRef(props, ref) {
+        return <LogProps {...props} forwardedRef={ref} />;
+      }
+
+      // Give this component a more helpful display name in DevTools.
+      // e.g. "ForwardRef(logProps(MyComponent))"
+      const name = Component.displayName || Component.name;
+      forwardRef.displayName = `logProps(${name})`;
+
+      return React.forwardRef(forwardRef);
+    }
+    ```
+
+**[⬆ Back to Top](#table-of-contents)**
+
+71. ### Is it good to use arrow functions in render methods?
+
+    Yes, You can use. It is often the easiest way to pass parameters to callback functions. But you need to optimize the performance while using it.
+
+    ```javascript
+    class Foo extends Component {
+      handleClick() {
+        console.log("Click happened");
+      }
+      render() {
+        return <button onClick={() => this.handleClick()}>Click Me</button>;
+      }
+    }
+    ```
+
+    **Note:** Using an arrow function in render method creates a new function each time the component renders, which may have performance implications
+
+**[⬆ Back to Top](#table-of-contents)**
+
+72. ### How do you say that state updates are merged?
+
+    When you call setState() in the component, React merges the object you provide into the current state.
+
+    For example, let us take a facebook user with posts and comments details as state variables,
+
+    ```javascript
+      constructor(props) {
+        super(props);
+        this.state = {
+          posts: [],
+          comments: []
+        };
+      }
+    ```
+
+    Now you can update them independently with separate `setState()` calls as below,
+
+    ```javascript
+     componentDidMount() {
+        fetchPosts().then(response => {
+          this.setState({
+            posts: response.posts
+          });
+        });
+
+        fetchComments().then(response => {
+          this.setState({
+            comments: response.comments
+          });
+        });
+      }
+    ```
+
+    As mentioned in the above code snippets, `this.setState({comments})` updates only comments variable without modifying or replacing `posts` variable.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+73. ### How do you pass arguments to an event handler?
+
+    During iterations or loops, it is common to pass an extra parameter to an event handler. This can be achieved through arrow functions or bind method.
+
+    Let us take an example of user details updated in a grid,
+
+    ```javascript
+    <button onClick={(e) => this.updateUser(userId, e)}>Update User details</button>
+    <button onClick={this.updateUser.bind(this, userId)}>Update User details</button>
+    ```
+
+    In the both approaches, the synthetic argument `e` is passed as a second argument. You need to pass it explicitly for arrow functions and it will be passed automatically for `bind` method.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+74. ### How to prevent component from rendering?
+
+    You can prevent component from rendering by returning null based on specific condition. This way it can conditionally render component.
+
+    ```javascript
+    function Greeting(props) {
+      if (!props.loggedIn) {
+        return null;
+      }
+
+      return <div className="greeting">welcome, {props.name}</div>;
+    }
+    ```
+
+    ```javascript
+    class User extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {loggedIn: false, name: 'John'};
+      }
+
+      render() {
+       return (
+           <div>
+             //Prevent component render if it is not loggedIn
+             <Greeting loggedIn={this.state.loggedIn} />
+             <UserDetails name={this.state.name}>
+           </div>
+       );
+      }
+    ```
+
+    In the above example, the `greeting` component skips its rendering section by applying condition and returning null value.
+
+**[⬆ Back to Top](#table-of-contents)**
+
+75. ### Give an example on How to use context?
+
+    **Context** is designed to share data that can be considered **global** for a tree of React components.
+
+    For example, in the code below lets manually thread through a “theme” prop in order to style the Button component.
+
+    ```javascript
+    //Lets create a context with a default theme value "luna"
+    const ThemeContext = React.createContext("luna");
+    // Create App component where it uses provider to pass theme value in the tree
+    class App extends React.Component {
+      render() {
+        return (
+          <ThemeContext.Provider value="nova">
+            <Toolbar />
+          </ThemeContext.Provider>
+        );
+      }
+    }
+    // A middle component where you don't need to pass theme prop anymore
+    function Toolbar(props) {
+      return (
+        <div>
+          <ThemedButton />
+        </div>
+      );
+    }
+    // Lets read theme value in the button component to use
+    class ThemedButton extends React.Component {
+      static contextType = ThemeContext;
+      render() {
+        return <Button theme={this.context} />;
+      }
+    }
+    ```
 
-     For example, the array index access would look as follows:
+**[⬆ Back to Top](#table-of-contents)**
 
-     ```javascript
-      var userStateVariable = useState('userProfile'); // Returns an array pair
-      var user = userStateVariable[0]; // Access first item
-      var setUser = userStateVariable[1]; // Access second item
-     ```
-     Whereas with array destructuring the variables can be accessed as follows:
-     ```javascript
-     const [user, setUser] = useState('userProfile');
-     ```
+76. ### How do you use contextType?
 
-     **[⬆ Back to Top](#table-of-contents)**
-    
-307. ### What are the sources used for introducing hooks?
-     Hooks got the ideas from several different sources. Below are some of them,
+    ContextType is used to consume the context object. The contextType property can be used in two ways,
 
-     1. Previous experiments with functional APIs in the react-future repository
-     2. Community experiments with render prop APIs such as Reactions Component
-     3. State variables and state cells in DisplayScript.
-     4. Subscriptions in Rx.
-     5. Reducer components in ReasonReact.
+    1.  **contextType as property of class:**
+        The contextType property on a class can be assigned a Context object created by React.createContext(). After that, you can consume the nearest current value of that Context type using this.context in any of the lifecycle methods and render function.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-308. ### How do you access imperative API of web components?
-     Web Components often expose an imperative API to implement its functions. You will need to use a **ref** to interact with the DOM node directly if you want to access imperative API of a web component. But if you are using third-party Web Components, the best solution is to write a React component that behaves as a **wrapper** for your Web Component.
+        Lets assign contextType property on MyClass as below,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-309. ### What is formik?
-     Formik is a small react form library that helps you with the three major problems,
+        ```javascript
+        class MyClass extends React.Component {
+          componentDidMount() {
+            let value = this.context;
+            /* perform a side-effect at mount using the value of MyContext */
+          }
+          componentDidUpdate() {
+            let value = this.context;
+            /* ... */
+          }
+          componentWillUnmount() {
+            let value = this.context;
+            /* ... */
+          }
+          render() {
+            let value = this.context;
+            /* render something based on the value of MyContext */
+          }
+        }
+        MyClass.contextType = MyContext;
+        ```
 
-     1. Getting values in and out of form state
-     2. Validation and error messages
-     3. Handling form submission
+    2.  **Static field**
+        You can use a static class field to initialize your contextType using public class field syntax.
 
+        ```javascript
+        class MyClass extends React.Component {
+          static contextType = MyContext;
+          render() {
+            let value = this.context;
+            /* render something based on the value */
+          }
+        }
+        ```
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-310. ### What are typical middleware choices for handling asynchronous calls in Redux?
-     Some of the popular middleware choices for handling asynchronous calls in Redux eco system are `Redux Thunk, Redux Promise, Redux Saga`.
+**[⬆ Back to Top](#table-of-contents)**
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-311. ### Do browsers understand JSX code?
-     No, browsers can't understand JSX code. You need a transpiler to convert your JSX to regular Javascript that browsers can understand. The most widely used transpiler right now is Babel.
+77. ### What is a consumer?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-312. ### Describe about data flow in react?
-     React implements one-way reactive data flow using props which reduce boilerplate and is easier to understand than traditional two-way data binding.
+    A Consumer is a React component that subscribes to context changes. It requires a function as a child which receives current context value as argument and returns a react node. The value argument passed to the function will be equal to the value prop of the closest Provider for this context above in the tree.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-313. ### What is react scripts?
-     The `react-scripts` package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The `react-scripts start` command sets up the development environment and starts a server, as well as hot module reloading.
+    Lets take a simple example,
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-314. ### What are the features of create react app?
-     Below are the list of some of the features provided by create react app.
+    ```javascript
+    <MyContext.Consumer>
+      {value => /* render something based on the context value */}
+    </MyContext.Consumer>
+    ```
 
-     1. React, JSX, ES6, Typescript and Flow syntax support.
-     2. Autoprefixed CSS
-     3. CSS Reset/Normalize  
-     4. A live development server
-     5. A fast interactive unit test runner with built-in support for coverage reporting
-     6. A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
-     7. An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.
+**[⬆ Back to Top](#table-of-contents)**
 
+78. ### How do you solve performance corner cases while using context?
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-315. ### What is the purpose of renderToNodeStream method?
-     The `ReactDOMServer#renderToNodeStream` method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes.
-     **Note:** Remember this method is not available in the browser but only server.
+    The context uses reference identity to determine when to re-render, there are some gotchas that could trigger unintentional renders in consumers when a provider’s parent re-renders.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-316. ### What is MobX?
-     MobX is a simple, scalable and battle tested state management solution for applying functional reactive programming (TFRP). For reactJs application, you need to install below packages,
-     ```bash
-     npm install mobx --save
-     npm install mobx-react --save
-     ```
+    For example, the code below will re-render all consumers every time the Provider re-renders because a new object is always created for value.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-317. ### What are the differences between Redux and MobX?
-     Below are the main differences between Redux and MobX,
+    ```javascript
+    class App extends React.Component {
+      render() {
+        return (
+          <Provider value={{ something: "something" }}>
+            <Toolbar />
+          </Provider>
+        );
+      }
+    }
+    ```
 
-     | Topic | Redux | MobX |
-     | ----- | ------- | ------- 
-     | Definition| It is a javascript library for managing the application state | It is a library for reactively managing the state of your applications |
-     | Programming | It is mainly written in ES6 | It is written in JavaScript(ES5) |
-     | Data Store | There is only one large store exist for data storage | There is more than one store for storage |
-     | Usage | Mainly used for large and complex applications | Used for simple applications |
-     | Performance | Need to be improved | Provides better performance |
-     | How it stores | Uses JS Object to store | Uses observable to store the data |
+    This can be solved by lifting up the value to parent state,
 
+    ```javascript
+    class App extends React.Component {
+      constructor(props) {
+        super(props);
+        this.state = {
+          value: { something: "something" },
+        };
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-318. ### Should I learn ES6 before learning ReactJS?
-     No, you don’t have to learn es2015/es6 to learn react. But you may find many resources or React ecosystem uses ES6 extensively. Let's see some of the frequently used ES6 features,
+      render() {
+        return (
+          <Provider value={this.state.value}>
+            <Toolbar />
+          </Provider>
+        );
+      }
+    }
+    ```
 
-     1. **Destructuring:** To get props and use them in a component
-         ```javascript
-         // in es 5
-          var someData = this.props.someData
-          var dispatch = this.props.dispatch
+**[⬆ Back to Top](#table-of-contents)**
 
-         // in es6
-         const { someData, dispatch } = this.props
-         ```
-     2. Spread operator: Helps in passing props down into a component
-         ```javascript
-         // in es 5
-         <SomeComponent someData={this.props.someData} dispatch={this.props.dispatch} />
+79. ### What is the purpose of forward ref in HOCs?
 
-         // in es6
-         <SomeComponent {...this.props} />
-         ```
-     3. Arrow functions: Makes compact syntax
-         ```javascript
-         // es 5
-         var users = usersList.map(function (user) {
-          return <li>{user.name}</li>
-         })
-         // es 6
-         const users = usersList.map(user => <li>{user.name}</li>);
-         ```
+    Refs will not get passed through because ref is not a prop. It is handled differently by React just like **key**. If you add a ref to a HOC, the ref will refer to the outermost container component, not the wrapped component. In this case, you can use Forward Ref API. For example, we can explicitly forward refs to the inner FancyButton component using the React.forwardRef API.
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-319. ### What is Concurrent Rendering?
-     The Concurrent rendering makes React apps to be more responsive by rendering component trees without blocking the main UI thread. It allows React to interrupt a long-running render to handle a high-priority event. i.e, When you enabled concurrent Mode, React will keep an eye on other tasks that need to be done, and if there's something with a higher priority it will pause what it is currently rendering and let the other task finish first. You can enable this in two ways,
+    The below HOC logs all props,
 
-     ```javascript
-     // 1. Part of an app by wrapping with ConcurrentMode
-     <React.unstable_ConcurrentMode>
-       <Something />
-     </React.unstable_ConcurrentMode>
+    ```javascript
+    function logProps(Component) {
+      class LogProps extends React.Component {
+        componentDidUpdate(prevProps) {
+          console.log("old props:", prevProps);
+          console.log("new props:", this.props);
+        }
 
-     // 2. Whole app using createRoot
-     ReactDOM.unstable_createRoot(domNode).render(<App />);
-     ```
+        render() {
+          const { forwardedRef, ...rest } = this.props;
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-320. ### What is the difference between async mode and concurrent mode?
-     Both refers the same thing. Previously concurrent Mode being referred to as "Async Mode" by React team. The name has been changed to highlight React’s ability to perform work on different priority levels. So it avoids the confusion from other approaches to Async Rendering.
+          // Assign the custom prop "forwardedRef" as a ref
+          return <Component ref={forwardedRef} {...rest} />;
+        }
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
-    
-321. ### Can I use javascript urls in react16.9?
-     Yes, you can use javascript: URLs but it will log a warning in the console. Because URLs starting with javascript: are dangerous by including unsanitized output in a tag like ```<a href>``` and create a security hole.
+      return React.forwardRef((props, ref) => {
+        return <LogProps {...props} forwardedRef={ref} />;
+      });
+    }
+    ```
 
-     ```javascript
-     const companyProfile = {
-       website: "javascript: alert('Your website is hacked')",
-     };
-     // It will log a warning
-     <a href={companyProfile.website}>More details</a>
-     ```
-     Remember that the future versions will throw an error for javascript URLs.
-  
-   **[⬆ Back to Top](#table-of-contents)**
-   
-322. ### What is the purpose of eslint plugin for hooks?
-     The ESLint plugin enforces rules of Hooks to avoid bugs. It assumes that any function starting with ”use” and a capital letter right after it is a Hook. In particular, the rule enforces that,
+    Let's use this HOC to log all props that get passed to our “fancy button” component,
 
-     1. Calls to Hooks are either inside a PascalCase function (assumed to be a component) or another useSomething function (assumed to be a custom Hook).
-     2. Hooks are called in the same order on every render.
+    ```javascript
+    class FancyButton extends React.Component {
+      focus() {
+        // ...
+      }
 
-   **[⬆ Back to Top](#table-of-contents)**
+      // ...
+    }
+    export default logProps(FancyButton);
+    ```
 
-323. ### What is the difference between Imperative and Declarative in React?
-     Imagine a simple UI component, such as a "Like" button. When you tap it, it turns blue if it was previously grey, and grey if it was previously blue.
+    Now let's create a ref and pass it to FancyButton component. In this case, you can set focus to button element.
 
-     The imperative way of doing this would be:
+    ```javascript
+    import FancyButton from "./FancyButton";
 
-     ```javascript
-     if( user.likes() ) {
-         if( hasBlue() ) {
-             removeBlue();
-             addGrey();
-         } else {
-             removeGrey();
-             addBlue();
-         }
-     }
-     ```
+    const ref = React.createRef();
+    ref.current.focus();
+    <FancyButton label="Click Me" handleClick={handleClick} ref={ref} />;
+    ```
 
-     Basically, you have to check what is currently on the screen and handle all the changes necessary to redraw it with the current state, including undoing the changes from the previous state. You can imagine how complex this could be in a real-world scenario.
+**[⬆ Back to Top](#table-of-contents)**
 
-     In contrast, the declarative approach would be:
+80. ### Is ref argument available for all functions or class components?
+    Regular function or class components don’t receive the ref argument, and ref is not available in props either. The second ref argument only exists when you define a component with React.forwardRef call.
 
-     ```javascript
-     if( this.state.liked ) {
-         return <blueLike />;
-     } else {
-         return <greyLike />;
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     Because the declarative approach separates concerns, this part of it only needs to handle how the UI should look in a sepecific state, and is therefore much simpler to understand.
-     
-   **[⬆ Back to Top](#table-of-contents)**
+81. ### Why do you need additional care for component libraries while using forward refs?
+    When you start using forwardRef in a component library, you should treat it as a breaking change and release a new major version of your library. This is because your library likely has a different behavior such as what refs get assigned to, and what types are exported. These changes can break apps and other libraries that depend on the old behavior.
 
-324. ### What are the benefits of using typescript with reactjs?
-     Below are some of the benefits of using typescript with Reactjs,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. It is possible to use latest JavaScript features
-     2. Use of interfaces for complex type definitions
-     3. IDEs such as VS Code was made for TypeScript
-     4. Avoid bugs with the ease of readability and Validation
+82. ### How to create react class components without ES6?
 
-     **[⬆ Back to Top](#table-of-contents)**
+    If you don’t use ES6 then you may need to use the create-react-class module instead. For default props, you need to define getDefaultProps() as a function on the passed object. Whereas for initial state, you have to provide a separate getInitialState method that returns the initial state.
 
-325. ### How do you make sure that user remains authenticated on page refresh while using Context API State Management?
-When a user logs in and reload, to persist the state generally we add the load user action in the useEffect hooks in the main App.js. While using Redux, loadUser action can be easily accessed.
+    ```javascript
+    var Greeting = createReactClass({
+      getDefaultProps: function () {
+        return {
+          name: "Jhohn",
+        };
+      },
+      getInitialState: function () {
+        return { message: this.props.message };
+      },
+      handleClick: function () {
+        console.log(this.state.message);
+      },
+      render: function () {
+        return <h1>Hello, {this.props.name}</h1>;
+      },
+    });
+    ```
 
-**App.js**
+    **Note:** If you use createReactClass then auto binding is available for all methods. i.e, You don't need to use `.bind(this)` with in constructor for event handlers.
 
-```js
-import {loadUser}  from '../actions/auth';
-store.dispatch(loadUser());
-```
+**[⬆ Back to Top](#table-of-contents)**
 
-* But while using **Context API**, to access context in App.js, wrap the AuthState in index.js so that App.js can access the auth context. Now whenever the page reloads, no matter what route you are on, the user will be authenticated as **loadUser** action will be triggered on each re-render.
+83. ### Is it possible to use react without JSX?
 
-**index.js**
+    Yes, JSX is not mandatory for using React. Actually it is convenient when you don’t want to set up compilation in your build environment. Each JSX element is just syntactic sugar for calling `React.createElement(component, props, ...children)`.
 
-```js
-import React from 'react';
-import ReactDOM from 'react-dom';
-import App from './App';
-import AuthState from './context/auth/AuthState'
+    For example, let us take a greeting example with JSX,
 
-ReactDOM.render(
-  <React.StrictMode>
-    <AuthState>
-      <App />
-    </AuthState>
-  </React.StrictMode>,
-  document.getElementById('root')
-);
-```
-**App.js**
+    ```javascript
+    class Greeting extends React.Component {
+      render() {
+        return <div>Hello {this.props.message}</div>;
+      }
+    }
 
-```js
-  const authContext = useContext(AuthContext);
+    ReactDOM.render(
+      <Greeting message="World" />,
+      document.getElementById("root")
+    );
+    ```
 
-  const { loadUser } = authContext;
+    You can write the same code without JSX as below,
 
-  useEffect(() => {
-    loadUser();
-  },[])
-```
+    ```javascript
+    class Greeting extends React.Component {
+      render() {
+        return React.createElement("div", null, `Hello ${this.props.message}`);
+      }
+    }
 
-**loadUser**
+    ReactDOM.render(
+      React.createElement(Greeting, { message: "World" }, null),
+      document.getElementById("root")
+    );
+    ```
 
-```js
-    const loadUser = async () => {
-        const token = sessionStorage.getItem('token');
+**[⬆ Back to Top](#table-of-contents)**
 
-        if(!token){
-            dispatch({
-                type: ERROR
-            })
-        }
-        setAuthToken(token);
+84. ### How do you create HOC using render props?
 
-        try {
-            const res = await axios('/api/auth'); 
+    You can implement most higher-order components (HOC) using a regular component with a render prop. For example, if you would prefer to have a withMouse HOC instead of a <Mouse> component, you could easily create one using a regular <Mouse> with a render prop.
 
-            dispatch({
-                type: USER_LOADED,
-                payload: res.data.data
-            })
-            
-        } catch (err) {
-           console.error(err); 
+    ```javascript
+    function withMouse(Component) {
+      return class extends React.Component {
+        render() {
+          return (
+            <Mouse
+              render={(mouse) => <Component {...this.props} mouse={mouse} />}
+            />
+          );
         }
+      };
     }
-```
+    ```
 
-  **[⬆ Back to Top](#table-of-contents)**
+    This way render props gives the flexibility of using either pattern.
 
-326. ### What are the benefits of new JSX transform?
-     There are three major benefits of new JSX transform,
+**[⬆ Back to Top](#table-of-contents)**
 
-     1. It is possible to use JSX without importing React packages
-     2. The compiled output might improve the bundle size in a small amount
-     3. The future improvements provides the flexibility to reduce the number of concepts to learn React.
-                 
-  **[⬆ Back to Top](#table-of-contents)**
+85. ### What is react scripts?
+    The `react-scripts` package is a set of scripts from the create-react-app starter pack which helps you kick off projects without configuring. The `react-scripts start` command sets up the development environment and starts a server, as well as hot module reloading.
 
-327. ### How does new JSX transform different from old transform?
-     The new JSX transform doesn’t require React to be in scope. i.e, You don't need to import React package for simple scenarios.
+**[⬆ Back to Top](#table-of-contents)**
 
-     Let's take an example to look at the main differences between the old and the new transform,
+86. ### What are the features of create react app?
 
-     **Old Transform:**
+    Below are the list of some of the features provided by create react app.
 
-     ```js
-     import React from 'react';
+    1.  React, JSX, ES6, Typescript and Flow syntax support.
+    2.  Autoprefixed CSS
+    3.  CSS Reset/Normalize
+    4.  A live development server
+    5.  A fast interactive unit test runner with built-in support for coverage reporting
+    6.  A build script to bundle JS, CSS, and images for production, with hashes and sourcemaps
+    7.  An offline-first service worker and a web app manifest, meeting all the Progressive Web App criteria.
 
-     function App() {
-       return <h1>Good morning!!</h1>;
-     }
-     ```
+**[⬆ Back to Top](#table-of-contents)**
 
-     Now JSX transform convert the above code into regular JavaScript as below,
+87. ### What is the purpose of renderToNodeStream method?
+    The `ReactDOMServer#renderToNodeStream` method is used to generate HTML on the server and send the markup down on the initial request for faster page loads. It also helps search engines to crawl your pages easily for SEO purposes.
+    **Note:** Remember this method is not available in the browser but only server.
 
-     ```js
-     import React from 'react';
+**[⬆ Back to Top](#table-of-contents)**
 
-     function App() {
-       return React.createElement('h1', null, 'Good morning!!');
-     }
-     ```
+88. ### How do you get redux scaffolding using create-react-app?
+    Redux team has provided official redux+js or redux+typescript templates for create-react-app project. The generated project setup includes,
+    1.  Redux Toolkit and React-Redux dependencies
+    2.  Create and configure Redux store
+    3.  React-Redux `<Provider>` passing the store to React components
+    4.  Small "counter" example to demo how to add redux logic and React-Redux hooks API to interact with the store from components
+        The below commands need to be executed along with template option as below,
+    5.  **Javascript template:**
+    ```js
+    npx create-react-app my-app --template redux
+    ```
+    2.  **Typescript template:**
+    ```js
+    npx create-react-app my-app --template redux-typescript
+    ```
 
-     **New Transform:**
+**[⬆ Back to Top](#table-of-contents)**
 
-     The new JSX transform doesn't require any React imports
+89. ### What is state mutation and how to prevent it?
 
-     ```js
-     function App() {
-       return <h1>Good morning!!</h1>;
-     }
-     ```
+    `State mutation` happens when you try to update the state of a component without actually using `setState` function. This can happen when you are trying to do some computations using a state variable and unknowingly save the result in the same state variable. This is the main reason why it is advised to return new instances of state variables from the reducers by using Object.assign({}, ...) or spread syntax.
 
-     Under the hood JSX transform compiles to below code
+    This can cause unknown issues in the UI as the value of the state variable got updated without telling React to check what all components were being affected from this update and it can cause UI bugs.
 
-     ```js
-     import {jsx as _jsx} from 'react/jsx-runtime';
+    Ex:
 
-     function App() {
-       return _jsx('h1', { children: 'Good morning!!' });
+    ```javascript
+    class A extends React.component {
+      constructor(props) {
+        super(props);
+        this.state = {
+          loading: false
+        }
      }
-     ```
-
-     **Note:** You still need to import React to use Hooks.
-                 
-  **[⬆ Back to Top](#table-of-contents)**
-                 
-328. ### How do you get redux scaffolding using create-react-app?
-     Redux team has provided official redux+js or redux+typescript templates for create-react-app project. The generated project setup includes,
-     
-     1. Redux Toolkit and React-Redux dependencies
-     2. Create and configure Redux store
-     3. React-Redux `<Provider>` passing the store to React components
-     4. Small "counter" example to demo how to add redux logic and React-Redux hooks API to interact with the store from components
-     
-     The below commands need to be executed along with template option as below,
-     
-     1. **Javascript template:**
-     ```js
-     npx create-react-app my-app --template redux
-     ```
-     2. **Typescript template:**
-     ```js
-     npx create-react-app my-app --template redux-typescript
-     ````
-  **[⬆ Back to Top](#table-of-contents)**
-                 
-329. ### What are React Server components?
-     React Server Component is a way to write React component that gets rendered in the server-side with the purpose of improving React app performance. These components allow us to load components from the backend. 
-    
-     **Note:** React Server Components is still under development and not recommended for production yet.
-                 
-  **[⬆ Back to Top](#table-of-contents)**
-     
-330. ### What is prop drilling?
-     Prop Drilling is the process by which you pass data from one component of the React Component tree to another by going through other components that do not need the data but only help in passing it around.
-                 
-  **[⬆ Back to Top](#table-of-contents)**
-
-331. ### What is state mutation and how to prevent it?
-                     
-        `State mutation` happens when you try to update the state of a component without actually using `setState` function. This can happen when you are trying to do some computations using a state variable and unknowingly save the result in the same state variable. This is the main reason why it is advised to return new instances of state variables from the reducers by using Object.assign({}, ...) or spread syntax.
 
-        This can cause unknown issues in the UI as the value of the state variable got updated without telling React to check what all components were being affected from this update and it can cause UI bugs.
-
-        Ex:
-        ```javascript
-        class A extends React.component {
-          constructor(props) {
-            super(props);
-            this.state = {
-              loading: false
-            }
-         }
+    componentDidMount() {
+      let { loading } = this.state;
+      loading = (() => true)(); // Trying to perform an operation and directly saving in a state variable
+    }
 
-        componentDidMount() {
-          let { loading } = this.state;
-          loading = (() => true)(); // Trying to perform an operation and directly saving in a state variable
-        }
+    ```
 
-        ```
+    **How to prevent it:** Make sure your state variables are immutable by either enforcing immutability by using plugins like Immutable.js, always using `setState` to make updates, and returning new instances in reducers when sending updated state values.
 
-        **How to prevent it:** Make sure your state variables are immutable by either enforcing immutability by using plugins like Immutable.js, always using `setState` to make updates, and returning new instances in reducers when sending updated state values.
-        
-  **[⬆ Back to Top](#table-of-contents)**
-                            
-332. ### What is the difference between useState and useRef hook?
-     1. useState causes components to re-render after state updates whereas useRef doesn’t cause a component to re-render when the value or state changes.
-        Essentially, useRef is like a “box” that can hold a mutable value in its (.current) property.
-     2. useState allows us to update the state inside components. While useRef allows refrencing DOM elements.
-                 
-  **[⬆ Back to Top](#table-of-contents)**
+**[⬆ Back to Top](#table-of-contents)**
 
 ## Disclaimer
 
diff --git a/coding-exercise/README.md b/coding-exercise/README.md
index 9c40dcdc..b026680b 100644
--- a/coding-exercise/README.md
+++ b/coding-exercise/README.md
@@ -1,68 +1,229 @@
-This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).
+### Coding Exercise
+
+#### 1. What is the output of below code
+
+```javascript
+import { useState } from 'react';
 
-## Available Scripts
+export default function Counter() {
+  const [counter, setCounter] = useState(5);
+
+  return (
+    <>
+      <span>{counter}</span>
+      <button onClick={() => {
+        setCounter(counter + 5);
+        setCounter(counter + 5);
+        alert(counter);
+        setCounter(counter + 5);
+        setCounter(counter + 5);
+      }}>Increment</button>
+    </>
+  )
+}
+```
+
+- 1: Alert with 5, 5
+- 2: Alert with 15, 25
+- 3: Alert with 5,  10
+- 4: Error: Cannot update the same state multiple times concurrently
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 3
+State values are fixed(i.e, default value 5) in each render and setting the state only changes it for the next render. React will wait until all the code executed with in an event handler before your state updates followed by re-rendering the UI. Also, all the 3 setter function calls are replacing the calculated value. Hence, irrespective of how many times you call `setCounter(counter + 5)` the final value is 10(5+5).
+
+This can be visuallized by substituting with state variable values in the particular render,
+```javascript
+      <button onClick={() => {
+        setCounter(5 + 5);
+        setCounter(5 + 5);
+        alert(5);
+        setCounter(5 + 5);
+        setCounter(5 + 5);
+      }}>Increment</button>
+```
+</p>
+</details>
 
-In the project directory, you can run:
+---
 
-### `yarn start`
+**[⬆ Back to Top](#table-of-contents)**
 
-Runs the app in the development mode.<br />
-Open [http://localhost:3000](http://localhost:3000) to view it in the browser.
+#### 2. What is the output of below code
 
-The page will reload if you make edits.<br />
-You will also see any lint errors in the console.
+```javascript
+import { useState } from 'react';
 
-### `yarn test`
+export default function Counter() {
+  const [counter, setCounter] = useState(5);
+
+  return (
+    <>
+      <span>{counter}</span>
+      <button onClick={() => {
+        setCounter(counter => counter + 5);
+        setCounter(counter => counter + 5);
+        alert(counter);
+        setCounter(counter => counter + 5);
+        setCounter(counter => counter + 5);
+      }}>Increment</button>
+    </>
+  )
+}
+```
+
+- 1: Alert with 5, 25
+- 2: Alert with 5, 10
+- 3: Alert with 15, 25
+- 4: Alert with 15, 10
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 1
 
-Launches the test runner in the interactive watch mode.<br />
-See the section about [running tests](https://facebook.github.io/create-react-app/docs/running-tests) for more information.
+React queues all the updater functions(e.g, counter => counter + 5) which will be processed after all the code inside event handler has been executed. During the next re-render(state update through setState), React goes through the queue and increment the counter based on the previous value in each function call. So the final value of counter becomes 25(initial value 5 + 5 + 5 + 5 + 5) whereas the alert shows default value 5 because the counter value won't be updated by that time.
 
-### `yarn build`
+</p>
+</details>
 
-Builds the app for production to the `build` folder.<br />
-It correctly bundles React in production mode and optimizes the build for the best performance.
+---
 
-The build is minified and the filenames include the hashes.<br />
-Your app is ready to be deployed!
+**[⬆ Back to Top](#table-of-contents)**
 
-See the section about [deployment](https://facebook.github.io/create-react-app/docs/deployment) for more information.
+#### 3. What is the output of span after one click?
 
-### `yarn eject`
+```javascript
+import { useRef } from 'react';
 
-**Note: this is a one-way operation. Once you `eject`, you can’t go back!**
+export default function Counter() {
+  let countRef = useRef(0);
 
-If you aren’t satisfied with the build tool and configuration choices, you can `eject` at any time. This command will remove the single build dependency from your project.
+  function handleIncrement() {
+    countRef.current = countRef.current + 1;
+  }
 
-Instead, it will copy all the configuration files and the transitive dependencies (webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except `eject` will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.
+  return 
+  <>
+    <span>Count: {countRef.current}</span>
+    <button onClick={handleIncrement}>
+      Click me
+    </button>
+  </>
+}
+```
+
+- 1: Cannot read current property of undefined
+- 2: Count: 1
+- 3: null
+- 4: Count: 0
 
-You don’t have to ever use `eject`. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.
+<details><summary><b>Answer</b></summary>
+<p>
 
-## Learn More
+##### Answer: 4
 
-You can learn more in the [Create React App documentation](https://facebook.github.io/create-react-app/docs/getting-started).
+In React, every update has two phases. 
+1. **Render:** This is where React calls the components in order to output something on the screen
+2. **Commit:** React applies changes to the DOM
 
-To learn React, check out the [React documentation](https://reactjs.org/).
+Any updates to the ref will be reflected only in the commit phase. In other words, React sets **counterRef.current** during the commit phase. Hence, **countRef.current** always holds value `0` irrespective of how many times the Increment button clicked.
+</p>
+</details>
 
-### Code Splitting
+---
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/code-splitting
+**[⬆ Back to Top](#table-of-contents)**
 
-### Analyzing the Bundle Size
+#### 4. What is the outcome of below code after button click?
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/analyzing-the-bundle-size
+```javascript
+import { useRef } from 'react';
 
-### Making a Progressive Web App
+function MyCustomInput(props) {
+  return <input {...props} />;
+}
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/making-a-progressive-web-app
+export default function MyCustomForm() {
+  const inputRef = useRef(null);
 
-### Advanced Configuration
+  function handleInputFocus() {
+    inputRef.current.focus();
+  }
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/advanced-configuration
+  return (
+    <>
+      <MyCustomInput ref={inputRef} />
+      <button onClick={handleInputFocus}>
+        Click Me
+      </button>
+    </>
+  );
+}
+```
 
-### Deployment
+- 1: Input gets the focus
+- 2: Warning: Function components cannot be given refs.
+- 3: Cannot read current property of undefined
+- 4: Warning: Missing ref on <input /> element
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/deployment
+<details><summary><b>Answer</b></summary>
+<p>
 
-### `yarn build` fails to minify
+##### Answer: 2
+By default, React does not allow a component access the DOM nodes of other components even for child components. If you still try to access the DOM nodes directly then you will receive below error:
 
-This section has moved here: https://facebook.github.io/create-react-app/docs/troubleshooting#npm-run-build-fails-to-minify
+```javascript
+Warning: Function components cannot be given refs. Attempts to access this ref will fail. Did you mean to use React.forwardRef()?
+```
+
+This issue can be fixed by wrapping the **<MyCustomInput />** component with `forwardRef` function which accepts ref as the second argument which can be used on the **<input />** element as **ref={ref}**
+
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
+
+#### 5. What is the outcome of number of clicks after 3 button clicks?
+
+```javascript
+import { useRef } from 'react';
+
+export default function Counter() {
+  let ref = useRef(0);
+
+  function handleClick() {
+    ref.current = ref.current + 1;
+  }
+
+  return (
+    <>
+		<div>Clicked + {ref.current} + times</div>
+		<button onClick={handleClick}>
+			Click me!
+		</button>
+    </>
+  );
+}
+```
+
+- 1: 3 times
+- 2: 4 times
+- 3: 2 times
+- 4: 0 times
+
+<details><summary><b>Answer</b></summary>
+<p>
+
+##### Answer: 4
+If you try to use **{ref.current}** in the render method, the number won’t be updated on click. This is because **ref.current** does not trigger a re-render unlike state. This property is mainly used to read and write the values inside event handler or outside the render method.
+</p>
+</details>
+
+---
+
+**[⬆ Back to Top](#table-of-contents)**
diff --git a/coding-exercise/src/App.js b/coding-exercise/src/App.js
index ce9cbd29..015b924f 100644
--- a/coding-exercise/src/App.js
+++ b/coding-exercise/src/App.js
@@ -8,7 +8,7 @@ function App() {
       <header className="App-header">
         <img src={logo} className="App-logo" alt="logo" />
         <p>
-          Edit <code>src/App.js</code> and save to reload.
+          Edit <code>src/App.js</code> and save to reload browser.
         </p>
         <a
           className="App-link"
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