Very interesting topic, actually it's one of my favourite ones. I've been working for the last year or so on the problem to improve type-safety for NextJS. My solution works like this:

Step 1: introspect your DataSources (Postgres, MySQL, GraphQL, REST, etc...) Step 2: combine them into a virtual Graph (virtual GraphQL API) Step 3: write GraphQL Operations and "compile" them into JSON RPC + generate a 100% type-safe client

Result? No more double declaration of types. No manual typing. More info on how this works with NextJS and Typescript here: [0]

Note, I'm the founder of WunderGraph and obsessed with creating the perfect developer experience for working with APIs. I appreciate any feedback.

[0]: https://wundergraph.com/docs/overview/features/generated_cli...