feat(core): add generic type for transformed value in decorators factory

packages/core/services/reflector.service.ts

@@ -5,7 +5,7 @@ import { uid } from 'uid';
 /**
  * @publicApi
  */
-export interface CreateDecoratorOptions<T = any> {
+export interface CreateDecoratorOptions<TParam = any, TTransformed = TParam> {
   /**
    * The key for the metadata.
    * @default uid(21)
@@ -16,13 +16,21 @@ export interface CreateDecoratorOptions<T = any> {
    * The transform function to apply to the metadata value.
    * @default value => value
    */
-  transform?: (value: T) => T;
+  transform?: (value: TParam) => TTransformed;
 }
 
+type CreateDecoratorWithTransformOptions<
+  TParam,
+  TTransformed = TParam,
+> = CreateDecoratorOptions<TParam, TTransformed> &
+  Required<Pick<CreateDecoratorOptions<TParam, TTransformed>, 'transform'>>;
+
 /**
  * @publicApi
  */
-export type ReflectableDecorator<T> = ((opts?: T) => CustomDecorator) & {
+export type ReflectableDecorator<TParam, TTransformedValue = TParam> = ((
+  opts?: TParam,
+) => CustomDecorator) & {
   KEY: string;
 };
 
@@ -40,12 +48,18 @@ export class Reflector {
    * @param options Decorator options.
    * @returns A decorator function.
    */
-  static createDecorator<T>(
-    options: CreateDecoratorOptions = {},
-  ): ReflectableDecorator<T> {
+  static createDecorator<TParam>(
+    options?: CreateDecoratorOptions<TParam>,
+  ): ReflectableDecorator<TParam>;
+  static createDecorator<TParam, TTransformed>(
+    options: CreateDecoratorWithTransformOptions<TParam, TTransformed>,
+  ): ReflectableDecorator<TParam, TTransformed>;
+  static createDecorator<TParam, TTransformed = TParam>(
+    options: CreateDecoratorOptions<TParam, TTransformed> = {},
+  ): ReflectableDecorator<TParam, TTransformed> {
     const metadataKey = options.key ?? uid(21);
     const decoratorFn =
-      (metadataValue: T) =>
+      (metadataValue: TParam) =>
       (target: object | Function, key?: string | symbol, descriptor?: any) => {
         const value = options.transform
           ? options.transform(metadataValue)
@@ -54,7 +68,7 @@ export class Reflector {
       };
 
     decoratorFn.KEY = metadataKey;
-    return decoratorFn as ReflectableDecorator<T>;
+    return decoratorFn as ReflectableDecorator<TParam, TTransformed>;
   }
 
   /**
@@ -70,7 +84,7 @@ export class Reflector {
   public get<T extends ReflectableDecorator<any>>(
     decorator: T,
     target: Type<any> | Function,
-  ): T extends ReflectableDecorator<infer R> ? R : unknown;
+  ): T extends ReflectableDecorator<any, infer R> ? R : unknown;
   /**
    * Retrieve metadata for a specified key for a specified target.
    *

packages/core/test/services/reflector.service.spec.ts

@@ -1,19 +1,30 @@
 import { expect } from 'chai';
 import { Reflector } from '../../services/reflector.service';
 
+const transformDecorator = Reflector.createDecorator<string[], number>({
+  transform: value => value.length,
+});
+
 describe('Reflector', () => {
   let reflector: Reflector;
+
   class Test {}
+
+  @transformDecorator(['a', 'b', 'c'])
+  class TestTransform {}
+
   beforeEach(() => {
     reflector = new Reflector();
   });
+
   describe('get', () => {
     it('should reflect metadata by key', () => {
       const key = 'key';
       const value = 'value';
       Reflect.defineMetadata(key, value, Test);
       expect(reflector.get(key, Test)).to.eql(value);
     });
+
     it('should reflect metadata by decorator', () => {
       const decorator = Reflector.createDecorator<string>();
       const value = 'value';
@@ -37,6 +48,19 @@ describe('Reflector', () => {
       // @ts-expect-error 'value' is not assignable to parameter of type 'string[]'
       reflectedValue = true;
     });
+
+    it('should reflect metadata by decorator (with transform option)', () => {
+      let reflectedValue = reflector.get(transformDecorator, TestTransform);
+      expect(reflectedValue).to.eql(3);
+
+      // @ts-expect-error 'value' is not assignable to type 'number'
+      reflectedValue = [];
+    });
+
+    it('should require transform option when second generic type is provided', () => {
+      // @ts-expect-error Property 'transform' is missing in type {} but required in type
+      const decorator = Reflector.createDecorator<string[], number>({});
+    });
   });
 
   describe('getAll', () => {