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)
You're right! It is a bit abstract to comprehend in the beginning. Hence, me stating for the time being to view them as interchangable. Not until next lesson, where I'll be writing about variables, was I planning to explain what the concrete diffrence is. The piece you wrote explains this difference perfectly!
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