What if file exists operation MUST be sync?

Then you don’t start it in a virtual thread. Your calls automatically become blocking or not blocking depending on your callsites, which is the proper place to decide that.

So it still has colors?

In TypeScript at least, async-ness is encoded in the type system, because async/await is just syntactic sugar for Promise/then/catch.

Well, JS is paired with a single-threaded execution model, so you only have concurrency to worry about. C#, Java has proper multithreading as well, that gives another dimension to the problem.

Kotlin has multithreading, and it also puts `suspend` in the type system.

Some more examples: Rust, Python (with most/all? type checkers), C# all bake this into their type systems.

This is more due to the fact that Kotlin doesn’t control its backend and is designed to work with a language that already includes this model, js.

Very simple - in Java, almost any IO operation in blocking. Exceptions are the stuff in the NIO package, thread interruption and signalling, and maybe process termination.