Javascript Observer pattern can not access private property

The following code below is javascript obeserver pattern, but when it execute "subject.subscribeObserver(observer1);", error occurs that "can not read property push of undefined", it seems that I can not access subject.observers. Why does this happen? In subscribeObserver function I keep this.observers referrence, but subject instance still not access to it. When I use prototype pattern but not return object pattern it works,that is really puzzle me.

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

        return {
            subscribeObserver: function(observer) {
                console.log(this);
                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);