Except it is, because your code doesn't work:

  > [10, 1, 5, 1, 10, 10, 1, 5, 5, 5, 10].sort(byNum)
  > [10, 10, 5, 1, 10, 1, 1, 5, 5, 5, 10]

The right way is

  > function byNum(a, b) { return a - b; }