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It’s probably better to say 3 hours to learn enough to start coding a project.

If you already know JS I really think you could learn 80% of TypeScript in three hours. But the last 20% would take a long time: generics, union types, that sort of stuff.

Yeah it's pretty intuitive, just type things with :<type> where the any's are inferred (squiggles).

The harder parts are the advanced types which you can learn middle out as you go.

And if you're already familiar with such things, I could definitely see picking it up rather quickly.

Sorry, not to learn entirely but to learn more than enough to benefit from it greatly.

Not if you already know JS and something like Ocaml or Haskell

They already knew JS.

Still. Generics. `extends` with ternaries. `infer` within conditional types. Type narrowing. Lots of the language quirks.

This is correct. I feel there are 2 distinct levels of knowledge with Typescript: the application level and the library level. The application level is fairly easy, but if you want to write libraries with good types you're going to have to dive in the more complex features of Typescript.

Still not rocket science, but it takes way more than 3 hours, even if one studies it intensively.