[yesbabyyes]

I've found the perfect middleground is typescript checking with JSDoc syntax. The code is just JS, no emitting necessary, but you get all the type checking.

Together with testing of @example stanzas, you get everything for cheap-ish.

Alas, there is no good JSDoc @example test runner¹. This would be very valuable. If there were, I would let that handle my unit tests and focus only on integration testing.

Edit: runtime type checking at the boundary is also valuable! I've tried runtypes, but actually prefer compiling the ts definitions to JSON Schema with typescript-json-schema, and checking with plain JSON Schema validation.

¹I've used @supabase/doctest-js and jsdoctest, and found both lacking. If you know of a better one, please share!