Simple observer pattern implementation

observer.js

function Counter() {
  this.count = 0;
  this.observers = [];
}

Counter.prototype.increment = function () {
  this.count++;
  this.notify({ count: this.count });
};

Counter.prototype.addObserver = function (observer) {
  this.observers.push(observer);
};

Counter.prototype.notify = function (data) {
  this.observers.forEach(function (observer) {
    observer.call(null, data);  
  });
};

// Test
const counter = new Counter();
const curious = function (data) {
  console.log(`hey, count just got updated, now it's ${data.count}`);
}

counter.addObserver(curious);

counter.increment();