Array.prototype._map = function (fn){
if(typeof fn !== "function") return ;
let arr = [];
for(let i = 0;i<this.length;i++){
arr.push(fn(this[i],i,this))
}
return arr;
}
Array.prototype._map = function (fn){
if(typeof fn !== "function") return ;
let arr = [];
for(let i = 0;i<this.length;i++){
arr.push(fn(this[i],i,this))
}
return arr;
}