Node.js EventEmitter: building event-driven components

EventEmitter Is the Core of Node.js

Node.js is built on the observer pattern. EventEmitter lets objects emit named events that any number of listener functions can subscribe to. Streams, HTTP servers, and many core APIs extend EventEmitter.

const { EventEmitter } = require('events');

const emitter = new EventEmitter();

emitter.on('data', (chunk) => {
  console.log('Received:', chunk);
});

emitter.once('end', () => {
  console.log('Fires only once');
});

emitter.emit('data', 'hello');
emitter.emit('end');

Extending EventEmitter

class FileWatcher extends EventEmitter {
  watch(path) {
    const fs = require('fs');
    fs.watch(path, (event) => {
      this.emit('change', { event, path });
    });
  }
}

const watcher = new FileWatcher();
watcher.on('change', ({ event, path }) => {
  console.log(path + ' changed: ' + event);
});
watcher.watch('./config.json');

Avoid Memory Leaks

Node.js warns when more than 10 listeners are attached to a single event — a common sign of a leak. Remove listeners when done with emitter.off('event', handler) or emitter.removeAllListeners('event'). Always store the handler reference if you need to remove it later.