feat: Support providing a dynamic context

README.md

@@ -32,9 +32,33 @@ Logging levels are (from lower to higher priority): `'debug'`, `'info'`, `'warn'
 The logger provides the logging functions with the following signatures: `logger.<level>(message, objectOrError)`
 
 Here is an example of how the logger can be used:
-```
+```javascript
 logger.error('An error happened', new ApiError('The api call failed', 404)); // will log the message, the error message, the stack trace, and the statusCode error property
 logger.warn('Be warned', {a: 1, b: Date.now(), c: 'some string'});
 logger.info('An event happened', {a: 1, b: Date.now(), c: 'some string'});
 logger.debug('A minor operation', {a: 1, b: Date.now(), c: 'some string'});
 ```
+
+## Fetching context dynamically
+
+In addition to providing a context object, you can also use `logger.addContext` to provide a function which will be called on every log to get a context object.
+
+This can be helpful if you are using [async_hooks](https://nodejs.org/docs/latest/api/async_hooks.html) or [cls-hooked](https://www.npmjs.com/package/cls-hooked) to store request metadata similarly to thread-local storage.
+
+For instance, you can do the following without having to explicitly pass the `correlationId` to each log:
+```javascript
+const asyncLocalStorage = new AsyncLocalStorage();
+
+expressApp.use((req, res, next) {
+  const correlationId = req.get('X-Correlation-Id') || uuidV4();
+  asyncLocalStorage.run({correlationId}, () => next());
+});
+
+logger.addContext(() => asyncLocalStorage.getStore());
+logger.info('User profile updated', {userId: 1234});
+```
+
+And the output will look like:
+```json
+{"level":"info","timestamp":"2021-02-23T17:55:43.011Z","correlationId":"12bf5b37-e0b8-56e0-8dcf-dc8c4aefc123","userId":1234}
+```