A simple observer implementation in JavaScript

observer.js

function createObserver(listeners = {}) {
  
  function addListener(eventType, cb) {
    if (listeners.hasOwnProperty(eventType)) {
      listeners[eventType].push(cb);
    } else {
      listeners[eventType] = [cb];
    }
    return function removeListener() {
      const index = listeners[eventType].indexOf(cb);
    }
  }

  function notify(eventType, ev) {
    if (listeners[eventType]) {
      listeners[eventType].forEach((cb) => {
        cb(ev);
      })
    }
  }
  
  function removeAllListeners() {
    listeners = [];
  }

  return {
    addListener,
    notify,
    removeAllListeners
  }
}


const obj = createObserver();

obj.addListener('fooEvent', (e) => {
  console.log(e.name);
})

obj.notify('fooEvent', { name: 'fooEvent' });