eventemitter3

What is eventemitter3?

The eventemitter3 package is a high-performance event emitter library that provides an interface for emitting and listening to events. It is a drop-in replacement for existing EventEmitter implementations with a focus on performance.

What are eventemitter3's main functionalities?

Emitting events

This feature allows you to emit events with a specified name and pass arguments to the event listeners.

const EventEmitter = require('eventemitter3');
const emitter = new EventEmitter();
emitter.on('greet', function(message) {
  console.log(message);
});
emitter.emit('greet', 'Hello World!');

Listening to events

This feature allows you to add a listener for a specific type of event. The listener will be invoked when an event with that name is emitted.

const EventEmitter = require('eventemitter3');
const emitter = new EventEmitter();
emitter.on('greet', function(message) {
  console.log(message);
});

Removing event listeners

This feature allows you to remove a specific listener from an event so that it no longer gets called when the event is emitted.

const EventEmitter = require('eventemitter3');
const emitter = new EventEmitter();
function onGreet(message) {
  console.log(message);
}
emitter.on('greet', onGreet);
emitter.removeListener('greet', onGreet);

Once listeners

This feature allows you to add a one-time listener for an event. The listener will be invoked only the first time the event is emitted, after which it is removed.

const EventEmitter = require('eventemitter3');
const emitter = new EventEmitter();
emitter.once('greet', function(message) {
  console.log('This will only be logged once:', message);
});
emitter.emit('greet', 'Hello World!');
emitter.emit('greet', 'Hello again!');

Other packages similar to eventemitter3

events

The 'events' package is Node.js's native event emitter implementation. It is very similar to eventemitter3 but may not be as optimized for performance.

mitt

Mitt is a tiny functional event emitter / pubsub. It offers the same core functionality as eventemitter3 but with a smaller footprint and a functional API.

wolfy87-eventemitter

Wolfy87's EventEmitter is an implementation of the EventEmitter module found in Node.js but can be used in the browser. It is larger in size compared to eventemitter3 and includes additional features like namespaces and wildcard listeners.

README

EventEmitter3

EventEmitter3 is a high performance EventEmitter. It has been micro-optimized for various of code paths making this, one of, if not the fastest EventEmitter available for Node.js and browsers. The module is API compatible with the EventEmitter that ships by default with Node.js but there are some slight differences:

• Domain support has been removed.
• We do not throw an error when you emit an error event and nobody is listening.
• The newListener event is removed as the use-cases for this functionality are really just edge cases.
• No setMaxListeners and its pointless memory leak warnings. If you want to add end listeners you should be able to do that without modules complaining.
• No listenerCount method. Use EE.listeners(event).length instead.
• Support for custom context for events so there is no need to use fn.bind.
• The listeners method can do existence checking instead of returning only arrays.
• The removeListener method removes all matching listeners, not only the first.

It's a drop in replacement for existing EventEmitters, but just faster. Free performance, who wouldn't want that? The EventEmitter is written in EcmaScript 3 so it will work in the oldest browsers and node versions that you need to support.

Installation

$ npm install --save eventemitter3        # npm
$ component install primus/eventemitter3  # Component
$ bower install eventemitter3             # Bower

Usage

After installation the only thing you need to do is require the module:

var EventEmitter = require('eventemitter3');

And you're ready to create your own EventEmitter instances. For the API documentation, please follow the official Node.js documentation:

http://nodejs.org/api/events.html

Contextual emits

We've upgraded the API of the EventEmitter.on, EventEmitter.once and EventEmitter.removeListener to accept an extra argument which is the context or this value that should be set for the emitted events. This means you no longer have the overhead of an event that required fn.bind in order to get a custom this value.

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);

Existence

To check if there is already a listener for a given event you can supply the listeners method with an extra boolean argument. This will transform the output from an array, to a boolean value which indicates if there are listeners in place for the given event:

var EE = new EventEmitter();
EE.once('event-name', function () {});
EE.on('another-event', function () {});

EE.listeners('event-name', true); // returns true
EE.listeners('unknown-name', true); // returns false

Tests and benchmarks

This module is well tested. You can run:

• npm test to run the tests under Node.js.
• npm run test-browser to run the tests in real browsers via Sauce Labs.

We also have a set of benchmarks to compare EventEmitter3 with some available alternatives. To run the benchmarks run npm run benchmark.

Tests and benchmarks are not included in the npm package. If you want to play with them you have to clone the GitHub repository.

License

MIT