Implementation of Array.prototype.map() in Javascript

mappable.spec.js

// Run using jasmine-node.
//   e.g.: jasmine-node file_name.spec.js

Array.prototype.mymap = function (callback) {
  var obj = Object(this);

  if (obj.length === 0) return null;
  if (typeof(callback) === 'undefined') return null;

  for (var i = 0, o; o = obj[i]; i++) {
    obj[i] = callback(o);
  }

  return obj;
};

describe('An Array', function () {
  describe('mymap', function () {
    var arr;

    beforeEach(function () {
      arr = [1, 2, 3, 4, 5];
    });

    it('should return null if no arguments are used', function () {
      expect(arr.mymap()).toBe(null);
    });

    it('should return itself if the block does nothing', function () {
      expect(arr.mymap(function (a) { return a })).toEqual([1, 2, 3, 4, 5]);
    });

    it('should return +1 to each element', function () {
      expect(arr.mymap(function (a) { return a + 1 })).toEqual([2, 3, 4, 5, 6]);
    });
  });
});

Hey, I think in line 5 you should use:
var obj = new this.constructor[Symbol.species](...this);
Since in the original implementation, if some class extends Array and sets [Symbol.species] getter to return something else, Array.map should create a mapped array using the supplies constructor