编写一个自定义事件类，包含on/off/emit/once方法

function Event() {
this._events = {}
}

Event.prototype.on = function(type, fn) {
if(!this._events[type]) {
  this._events[type] = []
}
this._events[type].push(fn)
}

Event.prototype.off = function(type, fn) {
if(!this._events[type]) {
  return
}
if(!fn) {
  this._events[type] = undefined
  return
}
var index = this._events[type].indexOf(fn)
this._events[type].splice(index, 1)
}

Event.prototype.emit = function(type) {
if(!this._events[type]){
  return
}
thi._events[type].forEach(fn => fn())
}

Event.prototype.once = function(type, fn) {
var _ = this;
var _fn = () => {
  fn.apply(_, arguments){
    this.off(type)
  }
}
this.on(type, _fn)
}