simple_observer_example.js

//Create a function in which this demo will run.
(function() {

    //The contructor for the dog class
    function Dog() {}

    //The "defaults" for the dog
    Dog.prototype = {
        //Define a default state for the dog
        _state: "Sitting",

        //Define a getter
        get state() {
            return this._state;
        },

        //Define a setter 
        set state(value) {
            this._state = value;

            var dog = this;

            //When the dog's state changes call all the observers
            //to notify them of the change
            this.observers.forEach(function(observer) {
                //This is an example of javascript scoping,
                //Suprisingly to beginners, this is not the dog,
                //But in the case the javascript engine
                observer.call(this, dog);
            })
        },

        //Functions that care that the dog is doing
        observers: []
    }

    //Allows us to easily add observer
    function registerObserver(fn, observe) {
        observe.observers.push(fn);
    }

    //An example function we'll call when the dog is 
    function dogWatcher(dog) {
        console.log("Oh look the dog is now %s", dog.state);
    }

    //With setup out of the way, lets make a dog
    var dog = new Dog();

    //Add a function that cares about the dog
    registerObserver(dogWatcher, dog);

    //Change the dog's state
    dog.state = 'Walking';
    dog.state = 'Playing';
}).call(this);