event-emitter

What is event-emitter?

The event-emitter npm package is a lightweight and flexible library for managing events in JavaScript. It allows you to create, listen to, and emit events, making it easier to handle asynchronous operations and decouple different parts of your application.

What are event-emitter's main functionalities?

Creating an Event Emitter

This feature allows you to create a new event emitter instance. The instance can be used to manage events within your application.

const EventEmitter = require('event-emitter');
const emitter = EventEmitter();

Listening to Events

This feature allows you to listen for specific events. When the event is emitted, the provided callback function is executed.

emitter.on('event', function(data) {
  console.log('Event received:', data);
});

Emitting Events

This feature allows you to emit events. When an event is emitted, all listeners for that event are called with the provided data.

emitter.emit('event', { key: 'value' });

Removing Event Listeners

This feature allows you to remove specific event listeners. This is useful for cleaning up and preventing memory leaks.

const callback = function(data) {
  console.log('Event received:', data);
};
emitter.on('event', callback);
emitter.off('event', callback);

Other packages similar to event-emitter

events

The 'events' package is a core Node.js module that provides a similar event-driven architecture. It is more robust and feature-rich compared to event-emitter, offering additional functionalities like once listeners and error handling.

eventemitter3

The 'eventemitter3' package is a high-performance event emitter for Node.js and the browser. It is similar to event-emitter but is optimized for speed and has a smaller footprint.

mitt

The 'mitt' package is a tiny, functional event emitter. It is simpler and more lightweight compared to event-emitter, making it ideal for small projects or performance-critical applications.

README

event-emitter

Environment agnostic event emitter

Installation

$ npm install event-emitter

To port it to Browser or any other (non CJS) environment, use your favorite CJS bundler. No favorite yet? Try: Browserify, Webmake or Webpack

Usage

var ee = require('event-emitter');

var MyClass = function () { /* .. */ };
ee(MyClass.prototype); // All instances of MyClass will expose event-emitter interface

var emitter = new MyClass(), listener;

emitter.on('test', listener = function (args) {
  // … react to 'test' event
});

emitter.once('test', function (args) {
  // … react to first 'test' event (invoked only once!)
});

emitter.emit('test', arg1, arg2/*…args*/); // Two above listeners invoked
emitter.emit('test', arg1, arg2/*…args*/); // Only first listener invoked

emitter.off('test', listener);              // Removed first listener
emitter.emit('test', arg1, arg2/*…args*/); // No listeners invoked

Additional utilities

allOff(obj) (event-emitter/all-off)

Removes all listeners from given event emitter object

hasListeners(obj[, name]) (event-emitter/has-listeners)

Whether object has some listeners attached to the object. When name is provided, it checks listeners for specific event name

var emitter = ee();
var hasListeners = require('event-emitter/has-listeners');
var listener = function () {};

hasListeners(emitter); // false

emitter.on('foo', listener);
hasListeners(emitter); // true
hasListeners(emitter, 'foo'); // true
hasListeners(emitter, 'bar'); // false

emitter.off('foo', listener);
hasListeners(emitter, 'foo'); // false

pipe(source, target[, emitMethodName]) (event-emitter/pipe)

Pipes all events from source emitter onto target emitter (all events from source emitter will be emitted also on target emitter, but not other way).
Returns pipe object which exposes pipe.close function. Invoke it to close configured pipe.
It works internally by redefinition of emit method, if in your interface this method is referenced differently, provide its name (or symbol) with third argument.

unify(emitter1, emitter2) (event-emitter/unify)

Unifies event handling for two objects. Events emitted on emitter1 would be also emitted on emitter2, and other way back.
Non reversible.

var eeUnify = require('event-emitter/unify');

var emitter1 = ee(), listener1, listener3;
var emitter2 = ee(), listener2, listener4;

emitter1.on('test', listener1 = function () { });
emitter2.on('test', listener2 = function () { });

emitter1.emit('test'); // Invoked listener1
emitter2.emit('test'); // Invoked listener2

var unify = eeUnify(emitter1, emitter2);

emitter1.emit('test'); // Invoked listener1 and listener2
emitter2.emit('test'); // Invoked listener1 and listener2

emitter1.on('test', listener3 = function () { });
emitter2.on('test', listener4 = function () { });

emitter1.emit('test'); // Invoked listener1, listener2, listener3 and listener4
emitter2.emit('test'); // Invoked listener1, listener2, listener3 and listener4

Tests

$ npm test