An Observer pattern example in JavaScript

catAttack.js

function ObserverList(){
  this.observerList = [];
}
 
ObserverList.prototype.add = function( obj ){
  return this.observerList.push( obj );
};
 
ObserverList.prototype.count = function(){
  return this.observerList.length;
};
 
ObserverList.prototype.get = function( index ){
  if( index > -1 && index < this.observerList.length ){
    return this.observerList[ index ];
  }
};
 
ObserverList.prototype.indexOf = function( obj, startIndex ){
  var i = startIndex;
 
  while( i < this.observerList.length ){
    if( this.observerList[i] === obj ){
      return i;
    }
    i++;
  }
 
  return -1;
};
 
ObserverList.prototype.removeAt = function( index ){
  this.observerList.splice( index, 1 );
};


// cat.stalk (boolean);
function Cat(name) {
	this.name = name;
	this.update = function() {
		alert(this.name + ' ATTACKS !!!');
	}
}

var cat1 = new Cat('Misifus');
var cat2 = new Cat('Benito');
var cat3 = new Cat('Garfield');
var cat4 = new Cat('Top Cat');

function Lure() {
	this.stalkers = new ObserverList();
	this.isMoving = false;
}
Lure.prototype.move = function(){
  this.isMoving = true;
  this.notify();
}

Lure.prototype.addStalker = function( stalker ){
  this.stalkers.add( stalker );
};
 
Lure.prototype.removeStalker = function( stalker ){
  this.stalkers.removeAt( this.stalkers.indexOf( stalker, 0 ) );
};
 
Lure.prototype.notify = function(){
  var stalkerCount = this.stalkers.count();
  for(var i=0; i < stalkerCount; i++){
    this.stalkers.get(i).update();
  }
};

var lure = new Lure();
lure.addStalker(cat1);
lure.addStalker(cat2);
lure.addStalker(cat3);
lure.addStalker(cat4);

lure.move();