Building event emitter in ES6

EventEmitter.js

class EventEmitter {
  constructor(){
    this.events = {};
  }
  on(eventName, cb){
    if(this.events[eventName]){
      this.events[eventName].push(cb);
    } else {
      this.events[eventName] = [cb];
    }
  }

  emit(eventName, ...rest){
    if(this.events[eventName]){
      this.events[eventName].forEach(cb => {
        cb.apply(null, rest);
      })
    }

    /* Test it  */

  const ee = new EventEmitter()
  ee.on('test', () => {
  console.log('EventEmitter works!!')
  })

  ee.emit('test');
  }
}

ES5 version would be as follows:

function EventEmitter(){
  this.events = {};
}

EventEmitter.prototype.on = function(eventName, cb) {
 if(this.events[eventName]) {
  this.events[eventName].push(cb);
 } else {
  this.events[eventName] = [cb];
 }
}

EventEmitter.prototype.emit = function(eventName) {
  if(this.events[eventName]){
  this.events[eventName].forEach((cb) => {
    cb()
  })
  }
}