I think OP means that if your project / job doesn't support typescript, you can write .ts files on your local machine and then compile them to .js. I think it would be very challenging though to be the only person on a project using TS though.
Using tsc to type check javascript - with JSDoc type annotations - works fine. You're really not missing much from "real" typescript, and you can save yourself a transpile.
I tried the experiment of .js files driven by typescript in JSDocs for around 10k lines and there is a night and day difference in what you can express, how and reuse it at the type level.
Name one thing you can't do.
To clairfy, if I am sharing types between modules I tend to use .d.ts files. Still no transpiling needed.
The limitations are too much for complex projects at scale. I strongly believe this is why Closure types didn't take off.
That and Google's lack of developer relations around the project
And no it's not the case you may as well write typescript at that point - because you're still using the actual language the browser runs, and not transpiling.
Yes, you can. Using `import('./someFile').SomeType`
Though I do agree, JSDoc works, but isn't as nice as directly writing types in a .ts file