events

What is events?

The 'events' npm package provides a way to implement the Observer pattern, which is a software design pattern that allows an object (known as a 'subject') to maintain a list of its dependents (known as 'observers') and automatically notify them of any state changes, usually by calling one of their methods. It is the same EventEmitters API as used by Node.js core.

What are events's main functionalities?

EventEmitter creation

This code sample shows how to import the 'events' package and create a new instance of an EventEmitter.

const EventEmitter = require('events');
const myEmitter = new EventEmitter();

Event listening

This code sample demonstrates how to listen for an 'event' and execute a callback function when the event is emitted.

myEmitter.on('event', () => {
  console.log('an event occurred!');
});

Event emitting

This code sample shows how to emit an 'event', which will trigger any associated event listeners.

myEmitter.emit('event');

Once listener

This code sample demonstrates how to set up a listener that will only be called once for the next time the event is emitted.

myEmitter.once('event', () => {
  console.log('this event will be logged only once');
});

Removing listeners

This code sample shows how to remove a specific listener from an event.

const callback = () => console.log('event occurred');
myEmitter.on('event', callback);
// ...
myEmitter.removeListener('event', callback);

Other packages similar to events

mitt

Mitt is a tiny functional event emitter / pubsub. It provides the same basic functionality as 'events' but with a smaller footprint and a more functional approach.

eventemitter3

EventEmitter3 is a high performance EventEmitter. It has a similar API to 'events' but is optimized for performance and has no dependencies.

wolfy87-eventemitter

Wolfy87's EventEmitter is an implementation of the EventEmitter module found in Node.js but for the browser. It offers similar functionality to 'events' but is designed to work in a browser environment.

README

events

Node's event emitter for all engines.

This implements the Node.js events module for environments that do not have it, like browsers.

events currently matches the Node.js 10.1 API.

Note that the events module uses ES5 features. If you need to support very old browsers like IE8, use a shim like es5-shim. You need both the shim and the sham versions of es5-shim.

This module is maintained, but only by very few people. If you'd like to help, let us know in the Maintainer Needed issue!

Install

You usually do not have to install events yourself! If your code runs in Node.js, events is built in. If your code runs in the browser, bundlers like browserify or webpack also include the events module.

But if none of those apply, with npm do:

npm install events

Usage

var EventEmitter = require('events')

var ee = new EventEmitter()
ee.on('message', function (text) {
  console.log(text)
})
ee.emit('message', 'hello world')

API

See the Node.js EventEmitter docs. events currently matches the Node.js 10.1 API.

Contributing

PRs are very welcome! The main way to contribute to events is by porting features, bugfixes and tests from Node.js. Ideally, code contributions to this module are copy-pasted from Node.js and transpiled to ES5, rather than reimplemented from scratch. Matching the Node.js code as closely as possible makes maintenance simpler when new changes land in Node.js. This module intends to provide exactly the same API as Node.js, so features that are not available in the core events module will not be accepted. Feature requests should instead be directed at nodejs/node and will be added to this module once they are implemented in Node.js.

If there is a difference in behaviour between Node.js's events module and this module, please open an issue!

License

MIT

Changelog

3.0.0 (2018-05-25)

This version drops support for IE8. events no longer includes polyfills for ES5 features. If you need to support older environments, use an ES5 shim like es5-shim. Both the shim and sham versions of es5-shim are necessary.

• Update to events code from Node.js 10.x
  • (semver major) Adds off() method
• Port more tests from Node.js
• Switch browser tests to airtap, making things more reliable