Node.js events EventEmitter

What is events EventEmitter in Node.js?

EventEmitter implements the publish-subscribe pattern: an object emits named events and any number of listeners react. Streams, HTTP servers and many libraries are built on it.

events EventEmitter example

import { EventEmitter } from 'node:events';

class JobQueue extends EventEmitter {
  add(job) {
    this.emit('added', job);
    setTimeout(() => this.emit('done', job), 1000);
  }
}

const queue = new JobQueue();
queue.on('added', job => console.log('queued', job.id));
queue.once('done', job => console.log('finished', job.id));   // fires once
queue.on('error', err => console.error(err));                  // essential

queue.add({ id: 'a1' });

How this works

An "error" event with no listener is a special case: Node throws it and the process exits. Always attach an error listener to any emitter you create or consume.

Key points to remember

• on adds a listener, once adds a one-shot listener, off removes one.
• Listeners run synchronously in the order they were registered.
• The default warning at eleven listeners usually indicates a leak.

Common mistakes with events EventEmitter

• No "error" listener, which crashes the process.
• Adding a listener inside a loop or request handler without removing it.