decorator-patterns

name

decorator-patterns

description

NestJS decorator usage: built-in route/param/class decorators, custom decorators via `createParamDecorator` and `SetMetadata`, metadata reflection via `Reflector`. Apply when designing controllers, guards, interceptors, or custom decorators. Use this skill to: - Pick the right built-in decorator for routes, params, and DI. - Compose decorators (`@UseGuards(A, B) @UseInterceptors(C)`). - Build custom param decorators (e.g., `@CurrentUser()`). - Use metadata for role-based logic (Reflector + SetMetadata). - Avoid common decorator mistakes. Do NOT use this skill for: - General module/DI patterns (see nest-conventions). - GraphQL-specific decorators (see nest-advanced). - ORM entity decorators (see nest-data-layer).

NestJS Decorator Patterns

NestJS is decorator-driven. Knowing which decorator does what — and how they compose — prevents most "weird DI / weird routing" bugs.

Required tsconfig

{
  "compilerOptions": {
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true
  }
}

emitDecoratorMetadata is what makes runtime introspection (and class-validator/class-transformer) work. Never disable.

reflect-metadata polyfill must be imported once at the top of main.ts:

import 'reflect-metadata';

(NestJS does this implicitly via @nestjs/core, but if you write a script that uses decorators outside the Nest app, import it manually.)

Class-level decorators

@Controller(prefix)

@Controller('users')                            // routes prefixed /users
@Controller({ path: 'users', version: '1' })    // versioned: /v1/users (with VersioningType.URI)
export class UsersController {}

@Injectable(options?)

@Injectable()                                   // singleton (default)
@Injectable({ scope: Scope.REQUEST })           // per-request
export class MyService {}

@Module(...)

@Module({ imports, controllers, providers, exports })
export class UsersModule {}

@Global()

@Global()
@Module({ providers: [...], exports: [...] })
export class LoggerModule {}

Use sparingly — see nest-conventions.

Method-level: HTTP routing

@Get()                          // GET /users
@Get(':id')                     // GET /users/:id
@Post()
@Put(':id')
@Patch(':id')
@Delete(':id')
@All('debug')                   // any method
@Head()
@Options()

Route options

@HttpCode(204)                  // override default status (200/201)
@Header('Cache-Control', 'no-store')
@Redirect('https://example.com', 301)
@Render('user-profile')         // for view engines (rare in API-only)

Versioning

@Controller({ path: 'users', version: ['1', '2'] })
@Version('2')                   // override at method level
@Get()
findV2() {}

Param decorators

@Get(':id')
async findOne(
  @Param('id') id: string,                          // route param
  @Query('include') include: string,                // ?include=...
  @Headers('authorization') authHeader: string,     // single header
  @Headers() allHeaders: Record<string, string>,    // all headers
  @Body() dto: UpdateUserDto,                       // request body (validated by ValidationPipe)
  @Body('email') email: string,                     // single field (no DTO validation — avoid)
  @Req() req: Request,                              // raw request (escape hatch)
  @Res() res: Response,                             // raw response (rare; breaks NestJS lifecycle)
  @Ip() ip: string,
  @HostParam() hosts: Record<string, string>,
  @Session() : <, >,      
) {}

@Res() warning

When you inject @Res() and call res.send() directly, NestJS skips its response-handling pipeline (interceptors, exception filters that produce a response). Either:

• Don't inject @Res() — return data and let Nest handle the response.
• Or inject @Res({ passthrough: true }) and still return data.

@Get()
async getStream(@Res({ passthrough: true }) res: Response): Promise<StreamableFile> {
  res.set({ 'Content-Type': 'application/octet-stream' });
  return new StreamableFile(buffer);
}

Pipe binding

@Get(':id')
findOne(
  @Param('id', ParseIntPipe) id: number,                    // built-in pipe
  @Query('limit', new DefaultValuePipe(20), ParseIntPipe) limit: number,
  @Body(new ValidationPipe({ whitelist: true })) dto: CreateUserDto,
) {}

Pipes run in order, left-to-right. Combine for type-safe + validated input.

Guard / Interceptor / Filter binding

@UseGuards(JwtAuthGuard, RolesGuard)
@UseInterceptors(LoggingInterceptor, CacheInterceptor)
@UseFilters(AllExceptionsFilter)
@UsePipes(new ValidationPipe({ transform: true }))
@Controller('users')
export class UsersController {}

Apply at route level OR globally:

// global guard via DI (preferred — can inject deps)
@Module({
  providers: [
    { provide: APP_GUARD, useClass: JwtAuthGuard },
    { provide: APP_INTERCEPTOR, useClass: LoggingInterceptor },
    { provide: APP_FILTER, useClass: AllExceptionsFilter },
    { provide: APP_PIPE, useClass: ValidationPipe },
  ],
})

// or in main.ts (cannot inject DI)
app.useGlobalGuards(new JwtAuthGuard());

APP_GUARD etc. are imported from @nestjs/core constants.

Metadata via SetMetadata + Reflector

For role-based or feature-flag logic, attach metadata to a route, then read in a guard.

Define a metadata decorator

// src/auth/roles.decorator.ts
import { SetMetadata } from '@nestjs/common';

export const ROLES_KEY = 'roles';
export const Roles = (...roles: string[]) => SetMetadata(ROLES_KEY, roles);

Apply on route

@Controller('admin')
@UseGuards(JwtAuthGuard, RolesGuard)
export class AdminController {
  @Get('users')
  @Roles('admin', 'super-admin')
  listUsers() { ... }
}

Read in guard

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const required = this.reflector.getAllAndOverride<string[]>(ROLES_KEY, [
      context.getHandler(),
      context.getClass(),
    ]);
    if (!required) return true;
    const { user } = context.switchToHttp().getRequest();
    return required.some(r => user.roles?.includes(r));
  }
}

getAllAndOverride walks handler → class metadata; method-level wins. getAllAndMerge combines both.

Custom param decorator

// src/auth/current-user.decorator.ts
import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const CurrentUser = createParamDecorator(
  (data: keyof User | undefined, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    const user = request.user as User;
    return data ? user?.[data] : user;
  },
);

Usage:

@Get('me')
me(@CurrentUser() user: User) {}

@Get('email')
myEmail(@CurrentUser('email') email: string) {}

Works in HTTP, GraphQL, WebSocket contexts via switchToHttp / switchToWs / GraphQL execution context wrapper.

Composing decorators

For a frequently-used combo, build a meta-decorator with applyDecorators:

// src/auth/auth.decorator.ts
import { applyDecorators, UseGuards } from '@nestjs/common';
import { ApiBearerAuth, ApiUnauthorizedResponse } from '@nestjs/swagger';

export function Auth(...roles: string[]) {
  return applyDecorators(
    UseGuards(JwtAuthGuard, RolesGuard),
    Roles(...roles),
    ApiBearerAuth(),
    ApiUnauthorizedResponse({ description: 'unauthorized' }),
  );
}

Usage:

@Auth('admin')
@Get('users')
listUsers() { ... }

Reduces decorator stack noise when 4+ decorators repeat.

Anti-patterns

• ❌ Using Reflect.getMetadata(...) directly — use Reflector.get() (NestJS API). It handles inheritance correctly.
• ❌ @Body('field') instead of a typed DTO — bypasses class-validator, no type safety.
• ❌ Injecting @Res() then forgetting passthrough: true — breaks interceptors.
• ❌ Multiple class-level @UseGuards(A) @UseGuards(B) — last one wins; combine into one: @UseGuards(A, B).
• ❌ Custom decorator without preserving type info — declare return type explicitly.
• ❌ SetMetadata('key', val) inline at the route — make a named decorator (Roles(...)) so it's discoverable.
• ❌ @Controller('/users/') — no leading or trailing slash; Nest adds them.