Add this types to decorator functions (#3203)

docs/Decorators.md

@@ -117,6 +117,9 @@ upon "greet" and "log" decorators:
 fastify.decorate('utility', fn, ['greet', 'log'])
 ```
 
+Note: using an arrow function will break the binding of `this` to the
+`FastifyInstance`.
+
 If a dependency is not satisfied, the `decorate` method will throw an exception.
 The dependency check is performed before the server instance is booted. Thus,
 it cannot occur during runtime.

test/types/instance.test-d.ts

@@ -1,5 +1,11 @@
-import fastify, { FastifyBodyParser, FastifyError, FastifyInstance, ValidationResult } from '../../fastify'
-import { expectAssignable, expectError, expectType } from 'tsd'
+import fastify, {
+  FastifyBodyParser,
+  FastifyError,
+  FastifyInstance,
+  FastifyLoggerInstance,
+  ValidationResult
+} from '../../fastify'
+import { expectAssignable, expectError, expectNotAssignable, expectType } from 'tsd'
 import { FastifyRequest } from '../../types/request'
 import { FastifyReply } from '../../types/reply'
 import { HookHandlerDoneFunction } from '../../types/hooks'
@@ -130,3 +136,29 @@ expectType<string>(server.printRoutes({ includeHooks: true, commonPrefix: false,
 expectType<string>(server.printRoutes({ includeMeta: ['key1', Symbol('key2')] }))
 
 expectType<string>(server.printRoutes())
+
+server.decorate<(x: string) => void>('test', function (x: string): void {
+  expectType<FastifyInstance>(this)
+})
+server.decorate('test', function (x: string): void {
+  expectType<FastifyInstance>(this)
+})
+
+server.decorateRequest<(x: string, y: number) => void>('test', function (x: string, y: number): void {
+  expectType<FastifyRequest>(this)
+})
+server.decorateRequest('test', function (x: string, y: number): void {
+  expectType<FastifyRequest>(this)
+})
+
+server.decorateReply<(x: string) => void>('test', function (x: string): void {
+  expectType<FastifyReply>(this)
+})
+server.decorateReply('test', function (x: string): void {
+  expectType<FastifyReply>(this)
+})
+
+expectError(server.decorate<string>('test', true))
+expectError(server.decorate<(myNumber: number) => number>('test', function (myNumber: number): string {
+  return ''
+}))

types/instance.d.ts

@@ -40,10 +40,27 @@ export interface FastifyInstance<
   close(): FastifyInstance<RawServer, RawRequest, RawReply, Logger> & PromiseLike<undefined>;
   close(closeListener: () => void): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
 
-  // should be able to define something useful with the decorator getter/setter pattern using Generics to enfore the users function returns what they expect it to
-  decorate(property: string | symbol, value: any, dependencies?: string[]): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
-  decorateRequest(property: string | symbol, value: any, dependencies?: string[]): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
-  decorateReply(property: string | symbol, value: any, dependencies?: string[]): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
+  // should be able to define something useful with the decorator getter/setter pattern using Generics to enforce the users function returns what they expect it to
+  decorate<T>(property: string | symbol,
+    value: T extends (...args: any[]) => any
+      ? (this: FastifyInstance<RawServer, RawRequest, RawReply, Logger>, ...args: Parameters<T>) => ReturnType<T>
+      : T,
+    dependencies?: string[]
+  ): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
+
+  decorateRequest<T>(property: string | symbol,
+    value: T extends (...args: any[]) => any
+      ? (this: FastifyRequest, ...args: Parameters<T>) => ReturnType<T>
+      : T,
+    dependencies?: string[]
+  ): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
+
+  decorateReply<T>(property: string | symbol,
+    value: T extends (...args: any[]) => any
+      ? (this: FastifyReply, ...args: Parameters<T>) => ReturnType<T>
+      : T,
+    dependencies?: string[]
+  ): FastifyInstance<RawServer, RawRequest, RawReply, Logger>;
 
   hasDecorator(decorator: string | symbol): boolean;
   hasRequestDecorator(decorator: string | symbol): boolean;