JavaScript EventEmitter (for node.js and browsers)

EventEmitter.js

class EventEmitter {
  constructor() { this.events = {} }

  getEventListByName(eventName){
    if(typeof this.events[eventName] === 'undefined'){
      this.events[eventName] = new Set()
    }
    return this.events[eventName]
  }

  on(eventName, fn) { this.getEventListByName(eventName).add(fn)}

  once(eventName, fn) {
    const self = this
    const onceFn = function(...args){
      self.removeListener(eventName, onceFn)
      fn.apply(self, args)
    }
    this.on(eventName, onceFn)
  }

  emit(eventName, ...args) {
    this.getEventListByName(eventName).forEach(function (fn) {
      fn.apply(this,args)
    }.bind(this))

  }

  removeListener(eventName, fn) { this.getEventListByName(eventName).delete(fn) }
}

const emitter = new EventEmitter()

// node.js
module.exports = emitter