class Event {
    constructor () {
        this.handlers = {}
    }
    //   on
    addEventListener(type,handler) {
        if(!(type in this.handlers)) {
            this.handlers[type] = []
        }
        if(!handler) {
            throw `请添加${type}事件`
        }
        this.handlers[type].push(handler) ;
        return this;
    }
// emit
    dispatchEvent(type,...params) {
        if(!(type in this.handlers)){
            throw new Error('事件未注册')
        }
        this.handlers[type].forEach(handler =>{
            handler(...params)
        })
        return this;
    }
// off
    removeEventListener(type,handler){
        if(!(type in this.handlers)){
            throw new Error('事件未注册')
        }
        if(!handler){
            delete this.handlers[type] 
        }else {
            const idx = this.handlers[type].findIndex(ele => ele === handler)
          
            if(idx === -1) {
                throw new Error('无该绑定事件')
            }
            this.handlers[type].splice(idx,1) ;
            if(this.handlers[type].length === 0){
                delete this.handlers[type]
            }
        }
    }

}

const evn = new Event() ;
const l1 = ()=>{  console.log('l1');}

evn.addEventListener('load',l1)
evn.addEventListener('load',(msg)=>{
    console.log(msg);
})
evn.addEventListener('load',()=>{
    console.log('addEvent33333333Listener');
})

evn.dispatchEvent('load','emit').addEventListener('load2',()=>{
    console.log('load2');
}).dispatchEvent('load2','emit') ;