EventDispatcher initializer-less.js

var EventDispatcher = (function() {
  var api = {
    addEventListener: function(event, callback) {
      initialize(this);
      if (!this.eventListeners[event])
        this.eventListeners[event] = [];
      this.eventListeners[event].push(callback);
    },
    dispatchEvent: function(event) {
      initialize(this);
      (this.eventListeners[event] || []).
        forEach(function(callback) { callback(event); });
    }
  };
  
  // Pass bound object in instead of binding function, slightly less overhead
  var initialize = function(o) {
    if (o.eventListeners) return;
    o.eventListeners = {};
  };
  
  return api;
})();

var MyClass = function(color) {
  this.color = color;
};

// Lifted from Prototype, used for copying references to functions onto a new sterile prototype
Object.extend = function(dst, src) {
  for (var p in src) dst[p] = src[p];
  return dst;
};

// Copy properties from EventDispatcher to our own prototype
// Not strictly necessary, but this allows for multiple behaviors to be composed
// without modifying EventDispatcher's prototype
MyClass.prototype = Object.extend({}, EventDispatcher);

var myObj = new MyClass('pink');
myObj.addEventListener('event1', function() { console.log('Event 1 fired'); });
myObj.dispatchEvent('event1');