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

EventEmitter2

EventEmitter2 is a an implementation of the EventEmitter found in Node.js

Features

• Namespaced events
• Wildcards for namespaces
• Times To Listen (TTL), extends the once concept
• Browser environment compatibility
• As good or better performance for emission and listener registration as Node.js core EventEmitter
• Smaller. EventEmitter2.js (2.2K Minified) VS. events.js (2.7K Minified)

Differences (Non breaking, compatible with existing EventEmitter)

• The constructor takes a configuration object.

   var server = EventEmitter2({
     delimiter: '/', // the delimiter used to segment namespaces, defaults to `.`.
     maxListeners: 20 // the max number of listeners that can be assigned to an event, defaults to 10.
   });

• The first parameter of a listener is the actual event name that the listener reacted to (because of wildcards).

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

• A new method was adde. Times to listen, an extension of the once concept.

    server.many('foo', function(event, value1, value2) {
      console.log('a values were', value1, value2);
    }, 4);

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.

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

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

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.many(event, listener, timesToListen)

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

    server.many('get', function (value) {
      console.log('Ah, we have our first value!');
    }, 4);

emitter.removeListener(event, listener)

Remove a listener from the listener array for the specified event. Caution: changes 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(console.log(server.listeners('get')); // [ [Function] ]

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

Execute each of the listeners in order with the list of arguments.

Test coverage

There is a test suite that tries to cover each use case, it can be found here.

Licence

(The MIT License)

Copyright (c) 2011 hij1nx http://www.twitter.com/hij1nx, indexzero http://www.twitter.com/indexzero, Fedor Indutny http://www.twitter.com/indutny

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.