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.
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
and removeListener
events have been removed as they
are useful only in some uncommon use-cases. - The
setMaxListeners
, getMaxListeners
, prependListener
and
prependOnceListener
methods are not available. - Support for custom context for events so there is no need to use
fn.bind
. - 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
CDN
Recommended CDN:
https://unpkg.com/eventemitter3@latest/umd/eventemitter3.min.js
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);
}
EE.once('event-name', emitted, context);
EE.on('another-event', emitted, context);
EE.removeListener('another-event', emitted, context);
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