Constructable mixins in JavaScript

EventEmitter.js

/*
  EventEmitter is usable as constructor and as mixin.
*/
function EventEmitter() {}

// this does the trick
EventEmitter.prototype = EventEmitter;

EventEmitter.addListener = function(type, listener) {
  this.listeners(type, true).push(listener);
};

EventEmitter.emit = function(type) {
  var eventsForType = this.listeners(type);
  for (var i = 0, len = eventsForType.length; i < len; i += 1) {
    eventsForType[i].apply(this, arguments);
  }
};

EventEmitter.listeners = function(type, createIfMissing) {
  var events = this._events || (createIfMissing ? (this._events = {}) : {});
  return events[type] || (createIfMissing ? (events[type] = []) : []);
};

EventEmitter.removeListener = function(type, listener) {
  var eventsForType = this.listeners(type);
  var index = eventsForType.indexOf(listener);
  if (index !== -1) {
    this.splice(index, 1);
  }
};

// In ES5: prevent tainting EventEmitter itself
Object.freeze(EventEmitter);

used-as-constructor.js

/*
  Creation of an event emitter by using the constructor.
*/
var e = new EventEmitter();
e.addListener('foo', function(type, data) { console.log('e:', type, 'event', data) });
e.emit('foo', 12345);

used-as-mixin.js

/*
  Use EventEmitter as mixin for other objects
*/
var f = mixin({/* other things here */}, EventEmitter);
f.addListener('foo', function(type, data) { console.log('f:', type, 'event', data) });
f.emit('foo', 12345);


function mixin(to, from) {
    for (var name in from) {
        to[name] = from[name];
    }
    return to;
}