Both Deno and Bun still use current tsc for type checking

They just strip types and don’t do any type checking

Those are runtimes primarily, not compilers/type checkers. Likewise, TSC is not a TS runtime.

Well, of course. But TSC output (transpiled JS source code) is then run by a JS runtime like Node that has a VM like V8 that makes an internal representation for the JS code. Using Bun or Deno allows you to go to a VM IR from the TypeScript directly without a need for TSC transpilation.

But as @keturakis pointed out (thanks!), Deno/Bun still rely on TSC, which I was not aware of.

Bun doesn't even support a way to check types, just remove them.

> Note — Similar to other build tools, Bun does not typecheck the files. Use tsc (the official TypeScript CLI) if you're looking to catch static type errors.