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:

https://stackoverflow.com/questions/5076944/what-is-the-difference-between-null-and-undefined-in-javascript

(answer by sebastian)