map implementations in javascript

map.js

Array.prototype.map = function(f) {
  res = new Array(this.length);
  for (i=0;i < this.length; i++) {
    res[i] = f(this[i]);
  }
  return res;
};

new_a = [1,2,3,4].map(function(ele){ return ele*2; });
console.log("new_a: "+new_a);

Array.prototype.map_in_place = function(f) {
  for (i=0;i < this.length; i++) {
    this[i] = f(this[i]);
  }
  return this;
};

a = [1,2,3,4];
console.log("a before map_in_place: "+a);
a.map_in_place(function(ele){ return ele*5; });
a;
console.log("a after map_in_place: "+a);