Yeah, obviously. Think about it -- when you write the code you still think about the "types", you're just not writing them down.
Typescript's virtues are completely related to developer productivity. Documenting your code with types, better autocompletion, catching silly mistakes before needing to run it in a browser, getting new people up to speed, etc.
For example, if you know that a function takes a generic list and returns an integer, then the list’s length is pretty much the only non-trivial computation it can perform.
This is patently wrong. You _are_ thinking about types. Every object, function, variable you write has a type you store in your head. The question is about how easy it is to remember those types in 6 months, or how hard it would be for a new developer to figure them out.
> JS has always had a test driven approach
Yes, and you should keep that test-driven approach with TypeScript. I'm not sure why anyone would assert otherwise.
const len = x => x.length;