It’s not running TS directly, it’s just preconfigured to transpile TS to JS without the user having to bring extra tooling. Neat, but you’ll see the docs still recommend tsc for type checking at build.
I wonder what's the benefit of TS if there's no type-checking? If types are not checked that means the TS type-declarations could be totally wrong and nobody would know. In other words they could be misleading.
Why incur the type-declaration overhead if they are not used after all?
This is how typescript is run today. Typescript types never exist at runtime regardless of how typescript is run. There is no overhead defining types because they are deleted at runtime. The purpose of typescript is to make the editor experience better (autocomplete, error highlight). Typically typechecking is run in addition to tests to make sure there aren't a bunch of errors no one saw in editor.
It could be, but even today without Bun, a common approach is to do type checking in a separate step from the build. This is because tsc doesn’t parallelize well, so type checking will slow down the build a lot. So you can put the type check step in a separate CI job, and have it fail like unit tests would. Then the main build can be a lot faster since it just has to strip the annotations.
Plus, for local dev, iteration and watch/rebuild is more important than failing with invalid types on every change. Sometimes it’s helpful to circle back to fix/update types after you’ve tried a couple approaches. (TS can still be finicky at times!) On top of that, your IDE should report type errors as you work anyways.
I would still prefer though that Bun did it for me, in a separate process perhaps, so I wouldn't need to configure a separate CI job, or manually enter the tsc-command. I read that Bun has its own test-runner too so why not its own type-checker too.
On Node.js I just edit the source-code then re-start the debugger on it, and edit it while in the debugger then rinse and repeat.
I use runtime assertions to catch errors in argument-types etc. as needed.
the only time you run type-checker is on CI. For the majority of the time you only need the code to compute and your editor/IDE should already have its own bundled type-checker. Unless bun has its own type-checker which means it has to play catch-up with tsc (if that's even possible, typescript's type system is very complex), I don't find a lot of benefit for Bun to merely call tsc for me.
