Recursive implementation of map in ES5.

es5-map.js

function map(arr, fn) {
  var head = arr[0];
  var tail = arr.slice(1);
  if(head === undefined && tail.length === 0) return [];
  if(tail.length === 0) {
    return [ fn(head) ];
  }
  return [].concat(fn(head), map(tail, fn));
}