Observer Design Pattern

observer-design-pattern.js

const Subject = function() {
  let observers = [];

  return {
    subscribeObserver(observer) {
      observers.push(observer);
    },
    unsubscribeObserver(observer) {
      const index = observers.indexOf(observer);
      if(index > -1) {
        observers.splice(index, 1);
      }
    },
    notifyObserver(observer) {
      const index = observers.indexOf(observer);
      if(index > -1) {
        observers[index].notify(index);
      }
    },
    notifyAllObservers() {
      for(var i = 0; i < observers.length; i++){
        observers[i].notify(i);
      }
    }
  };
};

const Observer = function() {
  return {
    notify(index) {
      console.log(`Observer ${index} is notified!`);
    }
  }
};

const subject = new Subject();

const observer1 = new Observer();
const observer2 = new Observer();
const observer3 = new Observer();
const observer4 = new Observer();

subject.subscribeObserver(observer1);
subject.subscribeObserver(observer2);
subject.subscribeObserver(observer3);
subject.subscribeObserver(observer4);

subject.notifyObserver(observer2); // Observer 2 is notified!

subject.notifyAllObservers();
// Observer 1 is notified!
// Observer 2 is notified!
// Observer 3 is notified!
// Observer 4 is notified!