An example of the Observer pattern implemented with Javascript.

ObserverPattern.js

function Subject() {
  let context = this;
  this.observers = [];
  
  return {
    subscribeObserver(observer) {
      context.observers.push(observer);
    },
    unsubscribeObserver(observer) {
      const idx = context.observers.indexOf(observer);
      if (idx > -1) {
        context.observers.splice(idx, 1);
      }
    },
    notifyObserver(observer) {
      const idx = context.observers.indexOf(observer);
      if (idx > -1) {
        context.observers[idx].notify(idx);
      }
    },
    notifyAllObservers() {
      context.observers.forEach((observer, i) => {
        observer.notify(i);
      });
    }
  };
}

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

const subject = new Subject();
const observer0 = new Observer();
const observer1 = new Observer();
const observer2 = new Observer();

subject.subscribeObserver(observer0);
subject.subscribeObserver(observer1);
subject.subscribeObserver(observer2);

subject.notifyAllObservers();
subject.notifyObserver(observer1);
subject.unsubscribeObserver(observer1);