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Do you think he should use typescript or javascript? Would typescript slow down learning or help learning?

Definitely it will slow down learning, but most of all you will be quite confused in the end, what is JavaScript and what is TypeScript.

If he's coming from typed languages he should definitely learn Typescript or at LEAST Javascript with Flow.

I wouldn’t touch TS until I had an understanding of JS. The many libraries without typings will just make things confusing IMO.