Array.prototype._map=function(callback,thisArg){
            if(typeof callback!=="function"){
                throw Error('参数必须是函数')
            }
            const mapArray=[]
            const thisContext=thisArg||globalThis
            for(let i=0;i<this.length;i++){
                mapArray.push(callback.call(thisContext,this[i],i,this))
            }
            return mapArray;
        }
        const nums=[1,2,3,4,5]
        const mapNums=nums._map(num=> num*2)
        console.log(mapNums);