EventEmitter

The backbone of Node’s event-driven model

EventEmitter (from node:events) is the class that powers virtually every I/O object in Node.js. The HTTP server, TCP sockets, file streams, child processes, and many third-party libraries all extend EventEmitter. Understanding it means understanding how Node communicates between components.

🟢 EventEmitter is the backbone of Node's streams, HTTP, and more

http.Server, net.Socket, fs.ReadStream — all of these extend EventEmitter. When a request arrives, the server emits 'request'. When a socket disconnects, it emits 'close'. Learning EventEmitter is learning the fundamental communication contract of Node.js.

Basic usage: on, emit, removeListener

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import { EventEmitter } from 'node:events'; var emitter = new EventEmitter(); // Register a listener emitter.on('greet', function(name) { console.log('Hello, ' + name + '!'); }); // Emit the event — listener runs synchronously emitter.emit('greet', 'Alice'); emitter.emit('greet', 'Bob'); // Remove the listener var handler = function(msg) { console.log('message:', msg); }; emitter.on('message', handler); emitter.emit('message', 'first'); emitter.removeListener('message', handler); emitter.emit('message', 'second'); // no output — listener removed

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once — fire exactly one time

once registers a listener that automatically removes itself after the first emission.

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import { EventEmitter } from 'node:events'; var emitter = new EventEmitter(); emitter.once('connect', function() { console.log('Connected! (fires once only)'); }); emitter.emit('connect'); // prints "Connected!" emitter.emit('connect'); // nothing — listener already removed emitter.emit('connect'); // nothing console.log('listener count:', emitter.listenerCount('connect')); // 0

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Building a custom EventEmitter class

Extend EventEmitter to give your own classes event-driven capabilities.

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import { EventEmitter } from 'node:events'; class DataLoader extends EventEmitter { constructor() { super(); this._items = []; } load(item) { this._items.push(item); this.emit('data', item); if (this._items.length >= 3) { this.emit('done', this._items); } } } var loader = new DataLoader(); loader.on('data', function(item) { console.log('loaded item:', item); }); loader.once('done', function(all) { console.log('all done! total:', all.length, 'items'); console.log('items:', all.join(', ')); }); loader.load('alpha'); loader.load('beta'); loader.load('gamma');

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Listener count and off()

off is an alias for removeListener added in Node 10. listenerCount lets you inspect how many listeners are registered for an event — useful for detecting potential memory leaks.

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import { EventEmitter } from 'node:events'; var ee = new EventEmitter(); var handler1 = function() { console.log('handler1'); }; var handler2 = function() { console.log('handler2'); }; ee.on('event', handler1); ee.on('event', handler2); console.log('count before off:', ee.listenerCount('event')); // 2 ee.off('event', handler1); console.log('count after off:', ee.listenerCount('event')); // 1 ee.emit('event'); // only handler2 runs

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What is the key difference between emitter.on() and emitter.once()?

on() is synchronous; once() is asynchronous once() registers a listener that is automatically removed after the first emission on() can only listen to built-in events; once() works for custom events once() fires the listener immediately without waiting for emit()

When does an EventEmitter listener run relative to emitter.emit()?

Asynchronously — after the current call stack clears In the next microtask Synchronously — before emit() returns In the next setTimeout callback

Which Node.js built-in classes extend EventEmitter?

Only third-party libraries extend EventEmitter Buffer and process only http.Server, net.Socket, fs.ReadStream, and many more Only http.Server

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