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
• The emitAsync method to return the results of the listeners via Promise.all
• Feature-rich waitFor method to wait for events using promises
• Extended version of the events.once method from the node events API
• Browser & Workers 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 `false`.
  //
  newListener: false, 
  
  //
  // set this to `true` if you want to emit the removeListener event. The default value is `false`.
  //
  removeListener: 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,

  //
  // disable throwing uncaughtException if an error event is emitted and it has no listeners
  //
  ignoreErrors: 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.getMaxListeners()

Returns the current max listener value for the EventEmitter which is either set by emitter.setMaxListeners(n) or defaults to EventEmitter2.defaultMaxListeners

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.waitFor(event, [options])

emitter.waitFor(event, [timeout])

emitter.waitFor(event, [filter])

Returns a thenable object (promise interface) that resolves when a specific event occurs

emitter.waitFor('event').then(function (data) { 
    console.log(data); // ['bar']
});

emitter.emit('event', 'bar');

emitter.waitFor('event', { 
    // handle first event data argument as an error (err, ...data)
    handleError: false,
    // the timeout for resolving the promise before it is rejected with an error (Error: timeout).
    timeout: 0, 
    //filter function to determine acceptable values for resolving the promise.
    filter: function(arg0, arg1){ 
        return arg0==='foo' && arg1==='bar'
    },
    Promise: Promise, // Promise constructor to use,
    overload: false // overload cancellation api in a case of external Promise class
}).then(function(data){
    console.log(data); // ['foo', 'bar']
});

emitter.emit('event', 'foo', 'bar')

var promise= emitter.waitFor('event');

promise.then(null, function(error){
    console.log(error); //Error: canceled
});

promise.cancel(); //stop listening the event and reject the promise

emitter.waitFor('event', {
    handleError: true
}).then(null, function(error){
    console.log(error); //Error: custom error
});

emitter.emit('event', new Error('custom error')); // reject the promise

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' ]

EventEmitter2.once(emitter, name, [options])

Creates a cancellable Promise that is fulfilled when the EventEmitter emits the given event or that is rejected when the EventEmitter emits 'error'. The Promise will resolve with an array of all the arguments emitted to the given event. This method is intentionally generic and works with the web platform EventTarget interface, which has no special 'error' event semantics and does not listen to the 'error' event.

Basic example:

var emitter= new EventEmitter2();

EventEmitter2.once(emitter, 'event', {
    timeout: 0,
    Promise: Promise, // a custom Promise constructor
    overload: false // overload promise cancellation api if exists with library implementation
}).then(function(data){
    console.log(data); // [1, 2, 3]
});

emitter.emit('event', 1, 2, 3);

With timeout option:

EventEmitter2.once(emitter, 'event', {
    timeout: 1000
}).then(null, function(err){
    console.log(err); // Error: timeout
});

The library promise cancellation API:

promise= EventEmitter2.once(emitter, 'event');
// notice: the cancel method exists only in the first promise chain
promise.then(null, function(err){
    console.log(err); // Error: canceled
});

promise.cancel();

Using the custom Promise class (bluebird.js):

var BBPromise = require("bluebird");

EventEmitter2.once(emitter, 'event', {
    Promise: BBPromise
}).then(function(data){
    console.log(data); // [4, 5, 6]
});

emitter.emit('event', 4, 5, 6);

var BBPromise = require("bluebird");

BBPromise.config({
    // if false or options.overload enabled, the library cancellation API will be used
    cancellation: true 
});

var promise= EventEmitter2.once(emitter, 'event', {
    Promise: BBPromise,
    overload: false // use bluebird cancellation API
}).then(function(data){
    // notice: never executed due to BlueBird cancellation logic
}, function(err){
    // notice: never executed due to BlueBird cancellation logic
});

promise.cancel();

emitter.emit('event', 'never handled');

emitter.listeners(eventName)

Returns the array of listeners for the event named eventName.

EventEmitter2.defaultMaxListeners

Sets default max listeners count globally for all instances, including those created before the change is made.