Adding map operation to javascript Array

map.js

// returns a new array where every object is the result of the passed function applied to corresponding element in the original array
Array.prototype.map = function(fn){
   var nArr=[];
   for(i=0; i<this.length; i++){
     nArr[i] = fn(this[i])
   };
   return nArr
}

// to use it, define a function taking one argument and processing it:
var plusOne = function(i){return i+1}

// applying it to some Array
var otherArr = [3,5,6,7].map(plusOne);  // returns: [4, 6, 7, 8]