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;   }   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); }

来源：博客园

作者：樱风凛

链接：https://www.cnblogs.com/fazero/p/11450730.html