Customize the JavaScript SDK v6+ logger

The logger captures details about your experiments, making it easier for you to debug them. You can customize where the log information is stored and choose the types of data you want to track.

Minimum SDK version

v6.0.0+

For versions 5.3.5 and below, see JavaScript (Browser) SDK or JavaScript (Node) SDK. See the SDK compatibility matrix documentation for a list of current SDK releases and the features they support.

Description

To enable logging for the JavaScript SDK, create a logger using the createLoggerfactory function and pass the logger to createInstance.

import {
  createInstance,
  createPollingProjectConfigManager,
  createLogger,
  LogLevel,
  INFO
} from "@optimizely/optimizely-sdk";

const SDK_KEY = "YOUR_SDK_KEY";

const pollingConfigManager = createPollingProjectConfigManager({
  sdkKey: SDK_KEY,
});

const customLogHandler = {
  log: (level, message) => {
    switch (level) {
      case LogLevel.Info:
        // INFO log message
        console.info(message);
        break;
      
      case LogLevel.Debug:
        // DEBUG log message
        console.debug(message);
        break;

      case LogLevel.Warn:
        // WARNING log message
        console.warn(message);
        break;

      case LogLevel.Error:
        // ERROR log message
        console.error(message);
        break;
    }
  }
}

const logger = createLogger({
  level: INFO,
});

const loggerWithCustomHandler = createLogger({
  level: INFO,
  logHandler: customLogHandler,  
})

const optimizely = createInstance({
  projectConfigManager: pollingConfigManager,
  logger
});

To customize the logger instance, you could use following configuration:

Parameter	Type	Description
`level`	OpaqueLevelPreset	Level preset that can be used directly to create logger instance. Level presets can be imported from the package root.
`logHandler` optional	LogHandler	An interface that implements the log method. A custom `LogHandler`can be used to modify how log messages are handled (for example, storing in a file or sending to an external system). By default, a console log handler is used which prints the log message to the console.

Log level presets

The following table lists the log level presets for the JavaScript SDK v6+.

Log Level	Explanation
ERROR	Logs events that prevent feature flags from functioning correctly, such as an invalid datafile during initialization or incorrect feature keys. These issues require user intervention to fix.
WARN	Logs events that do not prevent feature flags from functioning correctly but may lead to unexpected outcomes, like future API deprecation, improper logger or error handler settings, and nil values from getters.
INFO	Logs significant events, such as when a decision or tracking starts and succeeds, to illustrate the lifecycle of an API call.
DEBUG	Logs additional information related to errors that assist Optimizely in debugging, like when a feature flag is not running or a user is not included in a rollout.

They can be imported with the following code sample:

import {
	DEBUG,
  INFO,
  WARN,
  ERROR
} from "@optimizely/optimizely-sdk";