Your are going into some hard-to-understand details (for beginners), but I think that's all right, since people can always ask :)
I wrote something about null and undefined a while ago. You may find this interesting. Also in the comments, I explained a good way to check for the existence of indices in associative arrays, which can only be done with a typeof comparison to undefined:
(answer by sebastian)
RE: The JavaScript Journey #4 - Special Values & Precise Comparisons