写一个eventEmitter类，包括on()、off()、once()、emit()方法

[powerdong]

class EventEmitter{
        constructor(){
            this._envents = {}
        }

        on(event,callback){  //监听event事件，触发时调用callback函数
            let callbacks = this._events[event] || []
            callbacks.push(callback)
            this._events[event] = callbacks
            return this
        }
        off(event,callback){  //停止监听event事件
            let callbacks = this._events[event]
            this._events[event] = callbacks && callbacks.filter(fn => fn !== callback)
            return this
        }
        emit(...args){ //触发事件，并把参数传给事件的处理函数
            const event = args[0]
            const params = [].slice.call(args,1)
            const callbacks = this._events[event]
            callbacks.forEach(fn => fn.apply(this.params))
            return this
        }
        once(event,callback){ //为事件注册单次监听器
            let wrapFanc = (...args) => {
                callback.apply(this.args)
                this.off(event,wrapFanc)
            }
            this.on(event,wrapFanc)
            return this
        }

    }