What is an "event emitter"?

Question

Browsing through http://microjs.com, I see lots of libraries labelled "event emitters". I like to think I know my way around the basics of the Javascript language pretty well, but I really have no idea what an "event emitter" is or does.

Anyone care to enlighten me? It sounds interesting...

Answer 1

It triggers an event to which anyone can listen. Different libraries offer different implementations and for different purposes, but the basic idea is to provide a framework for issuing events and subscribing to them.

Example from jQuery:

// Subscribe to event. $('#foo').bind('click', function() {     alert("Click!"); });  // Emit event. $('#foo').trigger('click'); 

However, with jQuery in order to emit an event you need to have a DOM object, and cannot emit events from an arbitrary object. This is where event-emitter becomes useful. Here's some pseudo-code to demo custom events (the exact same pattern as above):

// Create custom object which "inherits" from emitter. Keyword "extend" is just a pseudo-code. var myCustomObject = {}; extend(myCustomObject , EventEmitter);  // Subscribe to event. myCustomObject.on("somethingHappened", function() {      alert("something happened!"); });  // Emit event. myCustomObject.emit("somethingHappened"); 

Answer 2

In node.js an event can be described simply as a string with a corresponding callback. An event can be "emitted" (or in other words, the corresponding callback be called) multiple times or you can choose to only listen for the first time it is emitted.

Example:-

var example_emitter = new (require('events').EventEmitter); example_emitter.on("test", function () { console.log("test"); }); example_emitter.on("print", function (message) { console.log(message); }); example_emitter.emit("test"); example_emitter.emit("print", "message"); example_emitter.emit("unhandled");  > var example_emitter = new (require('events').EventEmitter); {} > example_emitter.on("test", function () { console.log("test"); }); { _events: { test: [Function] } } > example_emitter.on("print", function (message) { console.log(message); }); { _events: { test: [Function], print: [Function] } } > example_emitter.emit("test"); test //console.log'd true //return value > example_emitter.emit("print", "message"); message //console.log'd true    //return value > example_emitter.emit("unhandled"); false   //return value 

This demonstates all the basic functionality of an EventEmitter. The on or addListener method (basically the subscription method) allows you to choose the event to watch for and the callback to be called. The emit method (the publish method), on the other hand, allows you to "emit" an event, which causes all callbacks registered to the event to 'fire', (get called).

From the source What are Event Emitters?