fix typescript declaration for app.use with async functions

[tellnes]

Checklist

• run npm run test and npm run benchmark
  • npm run benchmark fails with missing script: benchmark
• tests and/or benchmarks are included
  • not relevant
• documentation is changed or added
  • not relevant
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

Btw, the test fails with an listen EADDRINUSE: address already in use :::3000 if there is something else listening on port 3000 where the tests is run.

[climba03003]

I think it really doesn't matter to the return value to be void or Promise<void>
It is valid for void itself only.
Typescript Playground

[tellnes]

It does matter if you are running linting with the @typescript-eslint/no-misused-promises rule enabled.

But I think it should be unknown instead of void. That will allow you to use promises which returns something also. That will be fine since avvio seems to ignores any return value. I will update the PR.

[tellnes]

PR updated. Changed return value from void | Promise<void> to unknown | Promise<unknown>

[mcollina]

Would you mind to add a test in https://github.com/fastify/avvio/tree/master/test/types for this?

[tellnes]

Test case added. I did also remove Promise<unknown>. That is not needed anymore after I also changed the non-async part from void to unknown in an earlier change. The change is now just to change it from void to unknown.

[mcollina]

lgtm