Observer Pattern

function ObserverList() {
    this.list = [];
};

ObserverList.prototype.add = function(obj) {
    this.list.push(obj);
};
 
ObserverList.prototype.removeAt = function( index ){
  this.list.splice( index, 1 );
};

function Subject() {
    this.observers = new ObserverList();
};

Subject.prototype.addObserver = function (observer) {
    this.observers.add(observer);
};

Subject.prototype.removeObserver = function( observer ){
  this.observers.removeAt( this.observers.list.indexOf( observer, 0 ));
};
 
Subject.prototype.notify = function( context ){
  var observerCount = this.observers.list.length;
  for(var i=0; i < observerCount; i++){
    this.observers.list[i].update( context );
  }
};


function Observer(){
  this.update = function(){
    // ...
  };
}