Update null-undefined.md

Author: basarat

docs/javascript/null-undefined.md

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 ## Null and Undefined
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+> [Free youtube video on the subject](https://www.youtube.com/watch?v=kaUfBNzuUAI)
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 JavaScript (and by extension TypeScript) has two bottom types : `null` and `undefined`. They are *intended* to mean different things:
 
 * Something hasn't been initialized : `undefined`.