---
title: Customize the React SDK logger
slug: feature-experimentation/docs/customize-the-react-sdk-logger
docTags: 
createdAt: 2026-08-05T20:39:52.000Z
---

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

v4.0.0+

For versions 3.x and earlier, see [React SDK prior to v4](docId\:SFjbqXH6Khg37I46vIWWg).

In v4, logging is **disabled by default**. You must pass a `logger` to `createInstance` to enable it. The `setLogLevel` and `setLogger` functions from v3 are removed.

## Enable logging

Use the `createLogger` factory function to create a logger instance and pass it to `createInstance`:

```javascript
import {
  createInstance,
  createPollingProjectConfigManager,
  createLogger,
  DEBUG,
} from '@optimizely/react-sdk';

const optimizely = createInstance({
  projectConfigManager: createPollingProjectConfigManager({
    sdkKey: 'YOUR_SDK_KEY',
  }),
  logger: createLogger({ logLevel: DEBUG }),
});
```

## Log level constants

The following log level constants are exported from the SDK for convenience:

| Constant | Description                                                                                                                                                                                                   |
| -------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `DEBUG`  | Logs extra information related to errors that aid Optimizely in debugging, like when a feature flag is not running or a user is not included in a rollout                                                     |
| `INFO`   | Logs key events to illustrate the lifecycle of an API call, such as when a decision or tracking starts and succeeds.                                                                                          |
| `WARN`   | Logs events that do not prevent feature flags from working properly but may cause unexpected results, such as future API deprecation, improper logger or error handler settings, and null values from getters |
| `ERROR`  | Logs events that prevent feature flags from working properly, such as an invalid data file during initialization or incorrect feature keys. These issues require user intervention to fix.                    |

```javascript
import {
  createLogger,
  DEBUG,
  INFO,
  WARN,
  ERROR,
} from '@optimizely/react-sdk';

// Create a logger with INFO level
const logger = createLogger({ logLevel: INFO });
```

## Disable logging

To disable logging (the default behavior in v4), simply omit the `logger` parameter when calling `createInstance`.