fix: use TS type inference for hooks

Author: L2jLiga

test/types/hooks.test-d.ts

@@ -23,6 +23,13 @@ server.addHook('onRequest', (request, reply, done) => {
   expectType<void>(done(new Error()))
 })
 
+expectError(server.addHook('onRequest', async (request, reply, done) => {}))
+
+server.addHook('onRequest', async (request, reply) => {
+  expectType<FastifyRequest>(request)
+  expectType<FastifyReply>(reply)
+})
+
 server.addHook('preParsing', (request, reply, payload, done) => {
   expectType<FastifyRequest>(request)
   expectType<FastifyReply>(reply)

types/callback-or-promise.d.ts

@@ -0,0 +1,22 @@
+// Common type for all functions that may use callback as last argument or return promise instead
+// T - function to infer type from
+// Args - hook Args
+// DoneFn - hook callback
+export type CallbackOrPromise<T, Args extends any[], DoneFn> =
+// Ensure that TS use function as T
+  T extends (...args: [...Args, infer U]) => any
+    // Check whether last argument of given function is may be DoneFn
+    ? DoneFn extends U
+    // If yes we check whether return type is promise or not
+    ? ReturnType<T> extends Promise<any>
+      // If return type is promise we have to check is last arguments unknown
+      ? U extends unknown
+        // If it unknown then we return corresponding signature
+        ? (...args: Args) => Promise<void>
+        // If it known (for example has any type) we return never to prevent this case
+        : never
+      // Else we return sugnature with DoneFn as last argument
+      : (...args: [...Args, DoneFn]) => void
+    : ReturnType<T> extends Promise<any> ? (...args: Args) => Promise<void> : never
+    // If TS pass non function to as T then we just return T to prevent errors
+    : T

types/hooks.d.ts

@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/class-name-casing */
 
 import { Readable } from 'stream'
+import { CallbackOrPromise } from './callback-or-promise'
 import { FastifyInstance } from './instance'
 import { RouteOptions, RouteGenericInterface } from './route'
 import { RawServerBase, RawServerDefault, RawRequestDefaultExpression, RawReplyDefaultExpression, ContextConfigDefault } from './utils'
@@ -21,23 +22,20 @@ interface RequestPayload extends Readable {
  * `onRequest` is the first hook to be executed in the request lifecycle. There was no previous hook, the next hook will be `preParsing`.
  *  Notice: in the `onRequest` hook, request.body will always be null, because the body parsing happens before the `preHandler` hook.
  */
-export interface onRequestHookHandler<
+export type onRequestHookHandler<
   RawServer extends RawServerBase = RawServerDefault,
   RawRequest extends RawRequestDefaultExpression<RawServer> = RawRequestDefaultExpression<RawServer>,
   RawReply extends RawReplyDefaultExpression<RawServer> = RawReplyDefaultExpression<RawServer>,
   RouteGeneric extends RouteGenericInterface = RouteGenericInterface,
-  ContextConfig = ContextConfigDefault
-> {
-  (
-    request: FastifyRequest<RouteGeneric, RawServer, RawRequest>,
-    reply: FastifyReply<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig>,
-    done: HookHandlerDoneFunction
-  ): void;
-  (
-    request: FastifyRequest<RouteGeneric, RawServer, RawRequest>,
-    reply: FastifyReply<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig>,
-  ): Promise<unknown>;
-}
+  ContextConfig = ContextConfigDefault,
+  T extends (...args: any[]) => void = (...args: any[]) => void
+> = CallbackOrPromise<
+  T,
+  [
+    FastifyRequest<RouteGeneric, RawServer, RawRequest>,
+    FastifyReply<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig>
+  ],
+  HookHandlerDoneFunction>
 
 /**
  * `preParsing` is the second hook to be executed in the request lifecycle. The previous hook was `onRequest`, the next hook will be `preValidation`.

types/instance.d.ts

@@ -90,10 +90,11 @@ export interface FastifyInstance<
    */
   addHook<
     RouteGeneric extends RouteGenericInterface = RouteGenericInterface,
-    ContextConfig = ContextConfigDefault
+    ContextConfig = ContextConfigDefault,
+    T extends (...args: any[]) => void = (...args: any[]) => void
   >(
     name: 'onRequest',
-    hook: onRequestHookHandler<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig>
+    hook: onRequestHookHandler<RawServer, RawRequest, RawReply, RouteGeneric, ContextConfig, T>
   ): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
 
   /**