Observer_Pattern_Blueprint.js

// Observer Pattern
// Inspired by: https://addyosmani.com/resources/essentialjsdesignpatterns/book/#observerpatternjavascript

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

  return publicAPI;
  
}

var Observer = function(num) {
  return {
    notify: function() {
      console.log(`Observer ${num} is notified!`);
    }
  }
}

// Usage: 
var subject = new Subject();
var observer1 = new Observer(1);
var observer2 = new Observer(2);
var observer3 = new Observer(3);
var observer4 = new Observer(4);

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

subject.notifyObserver(observer2);
subject.unsubscribeObserver(observer2);

subject.notifyAllObservers();