index.js

// Use your own namespace to keep global scope clean
var myNS = myNS || Object.create(null);

// Let say we have a model with an entity
myNS.Entity = function Entity(data) {
  this.id = data.id;
  this.value = data.value;
};

// We give it some behavior
myNS.Entity.prototype.update = function update(value) {
  this.value += value;
  return this.value;
};

// Extend it to have an observable entity
myNS.ObservableEntity = function ObservableEntity(data) {
  myNS.Entity.call(this, data);
  this.observers = [];
};

myNS.ObservableEntity.prototype = Object.create(myNS.Entity.prototype);

myNS.ObservableEntity.prototype.attach = function attach(observer) {
  this.observers.push(observer);
};

myNS.ObservableEntity.prototype.notify = function notify(context) {
  this.observers.forEach(o => {
    o.notify(context);
  });
};

// Override the behavior of the entity
myNS.ObservableEntity.prototype.update = function update(value) {
  myNS.Entity.prototype.update.call(this, value);
  this.notify({
    subject: this.id,
    payload: this.value
  });
};

// Let say you have an observer service accepting callbacks
myNS.Observer = function Observer(cb) {
  this.cb = cb;
};

myNS.Observer.prototype.notify = function notify(context) {
  this.cb(context);
};

// Create some observer services
let o1 = new myNS.Observer((context) => {
  console.log(`Subject ${context.subject} updated at ${new Date()}`);
});

let o2 = new myNS.Observer((context) => {
  console.log(`Subject ${context.subject} updated to ${context.payload}`);
});

// Attach the observers to an observable object
let e = new myNS.ObservableEntity({
  id: '1',
  value: 1
});

e.attach(o1);
e.attach(o2);

e.update(10);