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 it's pointless memory leak warnings. If you want to add end listeners you should be able to do that without modules complaining.
• No listenerCount function. Use EE.listeners(event).length instead.
• Support for custom context for events so there is no need to use fn.bind.
• listeners method can do existence checking instead of returning only arrays.

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.

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

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

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);

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

MIT