observer.js

// This is the object that once its state changes, 
// it will notify all the observers.
function Observable() {
	this.observersList = [];
}
Observable.prototype.addObserver = function(observer) {
	this.observersList.push(observer);
}
Observable.prototype.removeObserver = function (observer) {
	var self = this;
	this.observersList.forEach(function(el, index) {
		if (el === observer) {
			self.observersList.splice(index, 1);
		}
	})
}
Observable.prototype.notify = function(context) {
	for (var i = 0; i < this.observersList.length; i++) {
		this.observersList[i].update(context);
	}
}

// will get notified if any changes happend on Observable
function Observer(name) {
	this.name = name;
	var self = this;
	this.update = function(context) {
		console.log(self.name + " is invoked with the context as " + context);
	}
}


var observable = new Observable();

var observerA = new Observer("A");
var observerB = new Observer("B");

observable.addObserver(observerA);
observable.addObserver(observerB);
observable.notify("hello");
// should log
// A is invoked with the context as hello
// B is invoked with the context as hello

observable.removeObserver(observerA);
observable.notify("hello");
// should log
// B is invoked with the context as hello