eventemitter2

What is eventemitter2?

The eventemitter2 npm package is an implementation of the EventEmitter module found in Node.js. It provides an interface for implementing event-driven architecture. It allows you to create objects that can emit named events that cause function objects ('listeners') to be called. It offers several enhancements over the native EventEmitter, such as namespaces, wildcards, and the ability to listen to all events.

What are eventemitter2's main functionalities?

Event Emitting

This feature allows you to emit events and register listeners that get called when those events are emitted.

const EventEmitter2 = require('eventemitter2').EventEmitter2;
const emitter = new EventEmitter2();

emitter.on('event', function() {
  console.log('an event occurred!');
});

emitter.emit('event');

Namespaces/Wildcards

This feature enables the use of namespaces and wildcards for event names, allowing for more flexible event handling.

const EventEmitter2 = require('eventemitter2').EventEmitter2;
const emitter = new EventEmitter2({
  wildcard: true
});

emitter.on('foo.*', function() {
  console.log('foo event occurred!');
});

emitter.emit('foo.bar');

Listening to All Events

This feature allows you to listen to all events that are emitted from an EventEmitter2 instance.

const EventEmitter2 = require('eventemitter2').EventEmitter2;
const emitter = new EventEmitter2();

emitter.onAny(function(event, value) {
  console.log('All event handler:', event, value);
});

emitter.emit('randomEvent', 'with some value');

Other packages similar to eventemitter2

events

The 'events' package is the original EventEmitter that comes with Node.js. It provides the basic functionality for event emitting and listening but lacks some of the advanced features of eventemitter2, such as namespaces and wildcards.

mitt

Mitt is a tiny functional event emitter / pubsub. It provides a similar event-driven approach but with a smaller footprint and without namespaces or wildcard features, focusing on simplicity and performance.

wolfy87-eventemitter

This is another EventEmitter implementation with a similar API to eventemitter2. It offers additional features like context binding and once listeners, but it does not support namespaces or wildcards.

README

SYNOPSIS

EventEmitter2 is an implementation of the EventEmitter module found in Node.js. In addition to having a better benchmark performance than EventEmitter and being browser-compatible, it also extends the interface of EventEmitter with additional non-breaking features.

DESCRIPTION

FEATURES

• Namespaces/Wildcards.
• Times To Listen (TTL), extends the once concept with many.
• Browser environment compatibility.
• Demonstrates good performance in benchmarks

EventEmitterHeatUp x 3,728,965 ops/sec \302\2610.68% (60 runs sampled)
EventEmitter x 2,822,904 ops/sec \302\2610.74% (63 runs sampled)
EventEmitter2 x 7,251,227 ops/sec \302\2610.55% (58 runs sampled)
EventEmitter2 (wild) x 3,220,268 ops/sec \302\2610.44% (65 runs sampled)
Fastest is EventEmitter2

Differences (Non-breaking, compatible with existing EventEmitter)

• The EventEmitter2 constructor takes an optional configuration object.

var EventEmitter2 = require('eventemitter2').EventEmitter2;
var server = new EventEmitter2({

  //
  // set this to `true` to use wildcards. It defaults to `false`.
  //
  wildcard: true,

  //
  // the delimiter used to segment namespaces, defaults to `.`.
  //
  delimiter: '::', 
  
  //
  // set this to `true` if you want to emit the newListener event. The default value is `true`.
  //
  newListener: false, 

  //
  // the maximum amount of listeners that can be assigned to an event, default 10.
  //
  maxListeners: 20,
  
  //
  // show event name in memory leak message when more than maximum amount of listeners is assigned, default false
  //
  verboseMemoryLeak: false
});

• Getting the actual event that fired.

server.on('foo.*', function(value1, value2) {
  console.log(this.event, value1, value2);
});

• Fire an event N times and then remove it, an extension of the once concept.

server.many('foo', 4, function() {
  console.log('hello');
});

• Pass in a namespaced event as an array rather than a delimited string.

server.many(['foo', 'bar', 'bazz'], 4, function() {
  console.log('hello');
});

Installing

$ npm install --save eventemitter2

API

When an EventEmitter instance experiences an error, the typical action is to emit an error event. Error events are treated as a special case. If there is no listener for it, then the default action is to print a stack trace and exit the program.

All EventEmitters emit the event newListener when new listeners are added. EventEmitters also emit the event removeListener when listeners are removed, and removeListenerAny when listeners added through onAny are removed.

Namespaces with Wildcards To use namespaces/wildcards, pass the wildcard option into the EventEmitter constructor. When namespaces/wildcards are enabled, events can either be strings (foo.bar) separated by a delimiter or arrays (['foo', 'bar']). The delimiter is also configurable as a constructor option.

An event name passed to any event emitter method can contain a wild card (the * character). If the event name is a string, a wildcard may appear as foo.*. If the event name is an array, the wildcard may appear as ['foo', '*'].

If either of the above described events were passed to the on method, subsequent emits such as the following would be observed...

emitter.emit('foo.bazz');
emitter.emit(['foo', 'bar']);

NOTE: An event name may use more than one wildcard. For example, foo.*.bar.* is a valid event name, and would match events such as foo.x.bar.y, or ['foo', 'bazz', 'bar', 'test']

Multi-level Wildcards

A double wildcard (the string **) matches any number of levels (zero or more) of events. So if for example 'foo.**' is passed to the on method, the following events would be observed:

emitter.emit('foo');
emitter.emit('foo.bar');
emitter.emit('foo.bar.baz');

On the other hand, if the single-wildcard event name was passed to the on method, the callback would only observe the second of these events.

emitter.addListener(event, listener)

emitter.on(event, listener)

Adds a listener to the end of the listeners array for the specified event.

server.on('data', function(value1, value2, value3, ...) {
  console.log('The event was raised!');
});

server.on('data', function(value) {
  console.log('The event was raised!');
});

emitter.prependListener(event, listener)

Adds a listener to the beginning of the listeners array for the specified event.

server.prependListener('data', function(value1, value2, value3, ...) {
  console.log('The event was raised!');
});

emitter.onAny(listener)

Adds a listener that will be fired when any event is emitted. The event name is passed as the first argument to the callback.

server.onAny(function(event, value) {
  console.log('All events trigger this.');
});

emitter.prependAny(listener)

Adds a listener that will be fired when any event is emitted. The event name is passed as the first argument to the callback. The listener is added to the beginning of the listeners array

server.prependAny(function(event, value) {
  console.log('All events trigger this.');
});

emitter.offAny(listener)

Removes the listener that will be fired when any event is emitted.

server.offAny(function(value) {
  console.log('The event was raised!');
});

emitter.once(event, listener)

Adds a one time listener for the event. The listener is invoked only the first time the event is fired, after which it is removed.

server.once('get', function (value) {
  console.log('Ah, we have our first value!');
});

emitter.prependOnceListener(event, listener)

Adds a one time listener for the event. The listener is invoked only the first time the event is fired, after which it is removed. The listener is added to the beginning of the listeners array

server.prependOnceListener('get', function (value) {
  console.log('Ah, we have our first value!');
});

emitter.many(event, timesToListen, listener)

Adds a listener that will execute n times for the event before being removed. The listener is invoked only the first n times the event is fired, after which it is removed.

server.many('get', 4, function (value) {
  console.log('This event will be listened to exactly four times.');
});

emitter.prependMany(event, timesToListen, listener)

Adds a listener that will execute n times for the event before being removed. The listener is invoked only the first n times the event is fired, after which it is removed. The listener is added to the beginning of the listeners array.

server.many('get', 4, function (value) {
  console.log('This event will be listened to exactly four times.');
});

emitter.removeListener(event, listener)

emitter.off(event, listener)

Remove a listener from the listener array for the specified event. Caution: Calling this method changes the array indices in the listener array behind the listener.

var callback = function(value) {
  console.log('someone connected!');
};
server.on('get', callback);
// ...
server.removeListener('get', callback);

emitter.removeAllListeners([event])

Removes all listeners, or those of the specified event.

emitter.setMaxListeners(n)

By default EventEmitters will print a warning if more than 10 listeners are added to it. This is a useful default which helps finding memory leaks. Obviously not all Emitters should be limited to 10. This function allows that to be increased. Set to zero for unlimited.

emitter.listeners(event)

Returns an array of listeners for the specified event. This array can be manipulated, e.g. to remove listeners.

server.on('get', function(value) {
  console.log('someone connected!');
});
console.log(server.listeners('get')); // [ [Function] ]

emitter.listenersAny()

Returns an array of listeners that are listening for any event that is specified. This array can be manipulated, e.g. to remove listeners.

server.onAny(function(value) {
  console.log('someone connected!');
});
console.log(server.listenersAny()[0]); // [ [Function] ]

emitter.emit(event, [arg1], [arg2], [...])

Execute each of the listeners that may be listening for the specified event name in order with the list of arguments.

emitter.emitAsync(event, [arg1], [arg2], [...])

Return the results of the listeners via Promise.all. Only this method doesn't work IE.

emitter.on('get',function(i) {
  return new Promise(function(resolve){
    setTimeout(function(){
      resolve(i+3);
    },50);
  });
});
emitter.on('get',function(i) {
  return new Promise(function(resolve){
    resolve(i+2)
  });
});
emitter.on('get',function(i) {
  return Promise.resolve(i+1);
});
emitter.on('get',function(i) {
  return i+0;
});
emitter.on('get',function(i) {
  // noop
});

emitter.emitAsync('get',0)
.then(function(results){
  console.log(results); // [3,2,1,0,undefined]
});

emitter.eventNames()

Returns an array listing the events for which the emitter has registered listeners. The values in the array will be strings.

emitter.on('foo', () => {});
emitter.on('bar', () => {});

console.log(emitter.eventNames());
// Prints: [ 'foo', 'bar' ]