原生js实现on emit

let obj = {};
const $on = (name,fn)=>{
    if(!obj[name]){
        obj[name] = [];
    }
    obj[name].push(fn);
}

const $emit = (name,val)=>{
    if(obj[name]){
        obj[name].map((fn)=>{
            fn(val);
        });
    }
}

const $off = (name,fn)=>{
    if(obj[name]){
        if(fn){
            let index = obj[name].indexOf(fn);
            if(index > -1){
                obj[name].splice(index,1);
            }           
        }else{
            obj[name].length = 0;
            //设长度为0比obj[name] = []更优，因为如果是空数组则又开辟了一个新空间，设长度为0则不必开辟新空间
        }  
    }
}

export default {
    $on,
    $emit,
    $off
}