I used to dread this approach (it’s part of why I like Typescript monorepos now), but LLMs are fantastic at translating most basic types/shapes between languages. Much less tedious to do this than several years ago.

Of course, it’s still a pretty rough and dirty way to do it. But it works for small/demo projects.

So in short you don't share types. Manually writing them for both is easy, but also tedious and error prone.