js实现一个事件类，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;
      }
      this._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);
    }