回调函数应用 Array map 数组映射的实现

<!doctype html>
<html>
<script type="text/javascript">

Array.prototype.map = function(fn){
for(var i =0; i< this.length; i++){
this[i] = fn(this[i], i);
}
return this;
}

var a=[1,2,3,4,5];
a.map(function(v){
return v*v;
});
console.log(a);