What is wrong in this Observer Pattern Code?

I just tried out this Observer Pattern Code from http://ift.tt/1PHXozW but it doesn't work. It tells me that this.listeners is undefined, but i really don't understand why?

Here is the example code:

var Subject = function() {
this.observers = [];

return {
  subscribeObserver: function(observer) {
    this.observers.push(observer);
  },
  unsubscribeObserver: function(observer) {
    var index = this.observers.indexOf(observer);
    if(index > -1) {
      this.observers.splice(index, 1);
    }
  },
  notifyObserver: function(observer) {
    var index = this.observers.indexOf(observer);
    if(index > -1) {
      this.observers[index].notify(index);
    }
  },
  notifyAllObservers: function() {
    for(var i = 0; i < this.observers.length; i++){
      this.observers[i].notify(i);
    };
  }
};
};

var Observer = function() {
  return {
    notify: function(index) {
      console.log("Observer " + index + " is notified!");
    }
  }
}

var subject = new Subject();

var observer1 = new Observer();
var observer2 = new Observer();
var observer3 = new Observer();
var observer4 = new Observer();

subject.subscribeObserver(observer1);
subject.subscribeObserver(observer2);
subject.subscribeObserver(observer3);
subject.subscribeObserver(observer4);

subject.notifyObserver(observer2); // Observer 2 is notified!

subject.notifyAllObservers();
// Observer 1 is notified!
// Observer 2 is notified!
// Observer 3 is notified!
// Observer 4 is notified!