Observer-Pattern.js

exports.RunObserver = () => {
  /**
   * Subject could be anything, Whatsapp group, Circket feed, news letter 
   * notification
   */
  var Subject = function () {
    //List of subscribers
    this.observers = [];
    return {
      //Add subscriber
      subscribeObserver: (observer) => {
        console.log(this)
        this.observers.push(observer);
      },
      //Remove Subscriber
      unsubscribeObserver: (observer) => {
        var index = this.observers.indexOf(observer);
        if (index > -1) {
          this.observers.splice(index, 1);
        }
      },
      //Notify a single suscriber
      notifyObserver: (observer) => {
        var index = this.observers.indexOf(observer);
        if (index > -1) {
          this.observers[index].notify(index);
        }
      },
      // Notify all subscribers
      notifyAllObservers: () => {
        for (var i = 0; i < this.observers.length; i++) {
          this.observers[i].notify(i);
        };
      }
    };
  };
  
  /**
   * Generic observer function 
   */
  var Observer = function () {
    return {
      notify: function (index) {
        console.log("Observer " + index + " is notified!");
      }
    }
  }

  var subject = new Subject();
  var observer1 = new Observer(); // User 1
  subject.subscribeObserver(observer1);

  var observer2 = new Observer(); // User 2
  subject.subscribeObserver(observer2);

  var observer3 = new Observer(); // User 3
  subject.subscribeObserver(observer3);

  var observer4 = new Observer(); // User 4
  subject.subscribeObserver(observer4);

  subject.notifyObserver(observer2); // Observer 2 is notification!
  
  subject.notifyAllObservers(); //Every subscribers will get the notification
}