es6 event-emitter

gistfile1.js

var DEFAULT_MAX_LISTENERS = 12

function error(message, ...args){
  console.error.apply(console, [message].concat(args))
  console.trace()
}

class EventEmitter {
  constructor(){
    this._maxListeners = DEFAULT_MAX_LISTENERS
    this._events = {}
  }
  on(type, listener) {
    if(typeof listener != "function") {
      throw new TypeError()
    }
    var listeners = this._events[type] ||(this._events[type] = [])
    if(listeners.indexOf(listener) != -1) {
      return this
    }
    listeners.push(listener)
    if(listeners.length > this._maxListeners) {
      error(
        "possible memory leak, added %i %s listeners, "+
        "use EventEmitter#setMaxListeners(number) if you " +
        "want to increase the limit (%i now)",
        listeners.length,
        type,
        this._maxListeners
      )
    }
    return this
  }
  once(type, listener) {
    var eventsInstance = this
    function onceCallback(){
      eventsInstance.off(type, onceCallback)
      listener.apply(null, arguments)
    }
    return this.on(type, onceCallback)
  }
  off(type, ...args) {
    if(args.length == 0) {
      this._events[type] = null
    }
    var listener = args[0]
    if(typeof listener != "function") {
      throw new TypeError()
    }
    var listeners = this._events[type]
    if(!listeners || !listeners.length) {
      return this
    }
    var indexOfListener = listeners.indexOf(listener)
    if(indexOfListener == -1) {
      return this
    }
    listeners.splice(indexOfListener, 1)
    return this
  }
  emit(type, ...args){
    var listeners = this._events[type]
    if(!listeners || !listeners.length) {
      return false
    }
    listeners.forEach(fn => fn.apply(null, args))
    return true
  }
  setMaxListeners(newMaxListeners){
    if(parseInt(newMaxListeners) !== newMaxListeners) {
      throw new TypeError()
    }
    this._maxListeners = newMaxListeners
  }
}