[TypeScript] Allow returning `Reply` from handlers

[SimenB]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Currently, a handler is allowed to return void | Promise<RouteGeneric['Reply'] | void> , see

fastify/types/route.d.ts

Line 66 in 8a2b226

) => void | Promise<RouteGeneric['Reply'] | void>

This means that in an async handler, we can do this:

app.get('/', async (request, reply) => {
  const result = await someWork();

  if (result) {
    return reply.status(201).send();
  }

  return reply.send();
});

However, if we remove the async-await, this is now a type error. The solution is to move the return to its own line, but ideally returning the Reply object would be allowed both wrapped in a Promise and not.

(another workaround is return Promise.resolve(reply.status(201).send()) which is even weirder).

---

It seems to me this was considered in v4, but not acted upon (yet? 😀):

fastify/types/type-provider.d.ts

Lines 95 to 99 in 2168282

	> extends infer Return ?
	(void | Promise<Return | void>)
	// review: support both async and sync return types
	// (Promise<Return> | Return | Promise<void> | void)
	: unknown

Motivation

Being able to return Reply from synchronous handlers makes for more concise code, and would be consistent with asynchronous handlers.

Example

See "Feature Proposal"

[mcollina]

@fastify/typescript

[mcollina]

Would you like to send a PR?

[Akash12233]

app.get('/', (request, reply) => {
const result = someWork();

if (result) {
return Promise.resolve(reply.status(201).send());
}

return Promise.resolve(reply.send());
});
this would work as we resolve the issue

[SimenB]

It has been possible to return a Reply since May 2022