code: function map (f, arr) {
      var result = [], i, n;
      for (i = 0, n = arr.length; i < n; i++) {
        result.push(f(arr[i]));
      }
      return result;
    }
docs: Array.prototype.map is supeeeeer slow. In fact, it is slower than a custom map function, which is usually slower than an inline for loop, but is more maintainable.