EventEmitter3 is a high performance, MIT licensed event emitter library for JavaScript that is API compatible with the Node.js built-in EventEmitter and intended for Node.js and browser developers who need faster event handling without rewriting existing event code.

What it is

EventEmitter3 is an npm package for Node.js and browser environments. It is a high performance EventEmitter micro-optimized for various code paths, and the README positions it as one of, if not the fastest EventEmitter available for Node.js and browsers. The implementation is written in EcmaScript 3 so it works in the oldest browsers and Node versions that a project needs to support. Its topic list names browser, eventemitter, and nodejs, and the module is API compatible with the EventEmitter that ships by default with Node.js. It is a drop in replacement for existing EventEmitters, but faster.

The concrete problem is that code using the Node.js built-in EventEmitter can pay avoidable overhead in hot event paths. EventEmitter3 replaces that default emitter with a micro-optimized implementation while keeping the familiar event API, so existing EventEmitter usage can switch without large rewrites. There are deliberate API differences: domain support has been removed; it does not throw an error when an error event is emitted and nobody is listening; the newListener and removeListener events have been removed; setMaxListeners, getMaxListeners, prependListener, and prependOnceListener are not available; custom context is supported for events so fn.bind is unnecessary; and removeListener removes all matching listeners, not only the first. The GitHub description notes that there is also a number 2 and that it is faster.

Key capabilities

• API compatible with Node.js built-in EventEmitter; drop in replacement for existing EventEmitters.
• Micro-optimized code paths make it one of, if not the fastest EventEmitter available for Node.js and browsers, per README.
• EcmaScript 3 implementation supports oldest browsers and Node versions.
• Contextual emits: EventEmitter.on, EventEmitter.once, and EventEmitter.removeListener accept extra context argument for this value, avoiding fn.bind.
• removeListener removes all matching listeners, not only first.
• Browser UMD build available from recommended CDN: https://unpkg.com/eventemitter3@latest/dist/eventemitter3.umd.min.js
• Documented omissions: domain support, throw on unhandled error event, newListener and removeListener events, setMaxListeners, getMaxListeners, prependListener, prependOnceListener.

Who uses it and how

• Node.js developers who want same EventEmitter API with faster event handling replace built-in emitter by requiring eventemitter3.
• Browser developers load UMD minified build from unpkg CDN for event-driven front-end code.
• Library and framework authors who need EventEmitter compatibility across Node.js and browsers, including oldest supported versions, adopt ES3 implementation.
• Projects already using on, once, emit, and removeListener with Node.js semantics can migrate without rewriting event code.
• Contributors clone GitHub repository to run npm test for tests and npm run benchmark for benchmarks; tests and benchmarks are not included in npm package, and benchmark run requires extra npm i in benchmarks folder.

Getting started

Install with npm install --save eventemitter3, then require eventemitter3 to create EventEmitter instances. For browsers, the recommended CDN is https://unpkg.com/eventemitter3@latest/dist/eventemitter3.umd.min.js.

How it compares

EventEmitter3 occupies the same niche as the Node.js built-in EventEmitter and presents itself as a drop in replacement for it. It preserves API compatibility while dropping domain support, not throwing when an error event has no listener, omitting newListener and removeListener events, omitting setMaxListeners, getMaxListeners, prependListener, and prependOnceListener, and changing removeListener to remove all matching listeners instead of only the first.

When to use it — and when not to

Use it when Node.js or browser code needs faster event handling and can accept the documented API differences. Projects that depend on domain support, automatic throw for unhandled error events, newListener or removeListener events, max listener methods, prepend methods, or remove-first-only semantics should stay with the Node.js built-in EventEmitter or another emitter. There is no database, storage, or SMTP to operate because this is a library, but tests and benchmarks are not shipped in the npm package, so contributors must clone the GitHub repository, and the benchmark run needs an extra npm i in the benchmarks folder; the repository shows 3,538 stars, 236 forks, 22 open issues, and last push on 2026-01-19.

$ npm install --save eventemitter3

https://unpkg.com/eventemitter3@latest/dist/eventemitter3.umd.min.js

var EventEmitter = require('eventemitter3');

var EE = new EventEmitter()
  , context = { foo: 'bar' };

function emitted() {
  console.log(this === context); // true
}

EE.once('event-name', emitted, context);
EE.on('another-event', emitted, context);
EE.removeListener('another-event', emitted, context);