Thanks, it started out as regular, valid JS, then I figured I might as well convert it to TypeScript, and I did so (perhaps too) quickly. I do plan to add stronger typing later.
But still, it can still save you from obvious mistakes even for a one man project.
Using GraphQL to type your in-process API would be overkilled and would require quite a bit of contortionism, and while you can share same TypeScript types between your end-points interfaces (and can be a good first step), inter-process communication is better expressed with something like GraphQL or Protobuf.