Events in Javascript

One of the features of C# that I really enjoy is events. For the many folks out there who have never used C#, an event is an implementation of the observer pattern that allows objects to watch for certain notifications within other objects. For example in the Windows.Forms library the Button object has a Click event that you can bind an action to:

  var button = new Button();
  button.Click += (sender, ev) => {
    MessageBox.Show("Hello, world!");
  };

You can define events within your own classes and fire them easily:

  public delegate void MyEventHandler();
  public event MyEventHandler MyEvent;

  //  ... lots of code ...

  // ... in some method that triggers the event:
    if (MyEvent != null){
      MyEvent();
    }

Turns out this is fairly easily implemented in Javascript. This code here should do the trick:

function createEvent(eventName, obj){
  var event = function() {
    var ob;
    for (var i = 0; i < obj[eventName].observers.length; i++){
      ob = obj[eventName].observers[i];
      ob.apply(ob, arguments);
    }
  };

  event.observers = [];

  event.add = function(observer) {
    event.observers.push(observer);
  };

  event.remove = function(observer) {
    var i = event.observers.indexOf(observer);
    if (i >= 0){
      event.observers.splice(i, 1);
    }
  };

  obj[eventName] = event;
}

Now you can go like this:

var obj = {};

createEvent("test", obj);

obj.test.add(function(){
  alert("Hello, world!");
});

obj.test();

You can try it by clicking here.