自己实现一个eventBus

function EventBus() {}
EventBus.prototype.on = function (name, callback) {
 //如果没有事件对象，新增一个
 if(!this._events){
  //创建一个干净的没有原型链的对象
  this._events = Object.create(null);
 }
 //如果没有这个事件的订阅，新增一个，如果有，push进去
 if(!this._events[name]){
  this._events[name] = [callback];
 }else{
  this._events[name].push(callback);
 }
}
EventBus.prototype.emit = function (name, ...args) {
 //发布的时候，如果有这个事件，循环执行所有这个订阅的方法
 if(this._events[name]){
  this._events[name].forEach(callback => {
   callback(...args);
  })
 }
}
EventBus.prototype.off = function (name) {
 //如果有这个事件的订阅，清除所有订阅
 if(this._events[name]){
  delete this._events[name];
 }
}
EventBus.prototype.once = function (name, callback) {
 let once = (...args) => {
  callback(...args);
  this.off(name);
 };
 this.on(name, once);
}
let eventBus = new EventBus();
eventBus.on('on', function (msg) {
 console.log(msg);
})
eventBus.once('once', function (msg) {
 console.log(msg);
})
eventBus.on('off', function (msg) {
 console.log(msg);
})
eventBus.emit('on', '发布on1')//发布on1
eventBus.emit('on', '发布on2')//发布on2
eventBus.emit('once', '发布once')//发布once
eventBus.emit('once', '发布once')
eventBus.emit('off', '发布off')//发布off
eventBus.off('off')
eventBus.emit('off', '发布off')
```

EventBus.prototype.once = function (name, callback) {
 let once = (...args) => {
  callback(...args);
  this.off(name);
 };
 this.on(name, once);
}
```