Trivial JavaScript event object system

event.js

var Event = (function() {
  function Event(name) {
    this.name = name;
    this.listeners = [];
  }
  Event.prototype.when = function(context, callback) {
    if (arguments.length === 1) {
      callback = context;
      context = null;
    }
    this.listeners.push({callback: callback, context: context});
  };
  Event.prototype.trigger = function() {
    var i, len, listener;
    for (i = 0, len = this.listeners.length; i < len; i++) {
      listener = this.listeners[i];
      listener.callback.apply(listener.context, arguments);
    }
  };
  return Event;
})();

amplify.js was almost what I wanted, but I needed events that could be scoped to a particular object.

Usage:

event = new Event();
event.when(function(a, b) { alert(a + b); });
event.when(object, object.method);
event.trigger('Hello ', 'world!');