test(counter): add unit tests for counter function with Jest

Author: CemWebDev

__tests__/counter.test.js

@@ -0,0 +1,17 @@
+const createCounter = require('../easy/counter');
+
+describe('createCounter', () => {
+  const testCases = [
+    { startValue: 5, expected: [5, 6, 7] },
+    { startValue: 16, expected: [16, 17, 18] },
+    { startValue: 0, expected: [0, 1, 2] },
+  ];
+  testCases.forEach(({ startValue, expected }) => {
+    test(`should start from ${startValue} and increment correctly`, () => {
+      const counter = createCounter(startValue);
+      expected.forEach((value) => {
+        expect(counter()).toBe(value);
+      });
+    });
+  });
+});

easy/counter.js

@@ -9,3 +9,5 @@ const createCounter = (n) => {
   let counter = n;
   return () => counter++;
 };
+
+module.exports = createCounter;