name: nestjs-expert type: reference description: "Provides NestJS patterns for modules, controllers, providers, guards, interceptors, and microservices. Use when working with NestJS TypeScript files (.module.ts, .controller.ts, .service.ts) or when the user mentions NestJS, Nest.js, or NestJS modules." paths: ["**/.module.ts", "**/.controller.ts", "**/.service.ts", "/*.guard.ts", "/nest-cli.json"] effort: 3 allowed-tools: Read, Glob, Grep, Write, Edit, Bash user-invocable: true when_to_use: "When building NestJS backend APIs, microservices, or guards/interceptors"

NestJS Expert

Critical rules (non-obvious)

• Circular dependencies: use forwardRef(() => ServiceB) in both modules; better — restructure to avoid
• Global modules: use @Global() sparingly; prefer explicit imports to keep modules testable
• APP_GUARD / APP_INTERCEPTOR: registered in AppModule providers, not in individual modules
• Lifecycle hooks order: onModuleInit → onApplicationBootstrap → ready; onModuleDestroy → beforeApplicationShutdown → onApplicationShutdown
• Never use req.user without type assertion — it's any from Passport; extend Express.Request

Module structure

@Module({
  imports: [TypeOrmModule.forFeature([User]), JwtModule],
  controllers: [UserController],
  providers: [UserService, UserRepository],
  exports: [UserService],   // export what other modules need
})
export class UserModule {}

Controller with validation

@Controller("users")
@UseGuards(JwtAuthGuard)
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get(":id")
  @HttpCode(HttpStatus.OK)
  async findOne(@Param("id", ParseUUIDPipe) id: string, @CurrentUser() user: User) {
    return this.userService.findOneOrFail(id);
  }

  @Post()
  @Roles(Role.ADMIN)
  async create(@Body() dto: CreateUserDto) {
    return this.userService.create(dto);
  }
}

Service with repository pattern

@Injectable()
export class UserService {
  constructor(
    @InjectRepository(User) private readonly repo: Repository<User>,
    private readonly eventEmitter: EventEmitter2,
  ) {}

  async findOneOrFail(id: string): Promise<User> {
    const user = await this.repo.findOne({ where: { id } });
    if (!user) throw new NotFoundException(`User ${id} not found`);
    return user;
  }

  async create(dto: CreateUserDto): Promise<User> {
    const user = this.repo.create(dto);
    const saved = await this.repo.save(user);
    this.eventEmitter.emit("user.created", new UserCreatedEvent(saved));
    return saved;
  }
}

JWT auth guard pattern

@Injectable()
export class JwtAuthGuard extends AuthGuard("jwt") {
  canActivate(context: ExecutionContext) {
    return super.canActivate(context);
  }
  handleRequest(err: any, user: any) {
    if (err || !user) throw err ?? new UnauthorizedException();
    return user;
  }
}

// Custom decorator for current user
export const CurrentUser = createParamDecorator(
  (_, ctx: ExecutionContext) => ctx.switchToHttp().getRequest().user,
);

Global exception filter

@Catch()
export class AllExceptionsFilter implements ExceptionFilter {
  catch(exception: unknown, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const status = exception instanceof HttpException
      ? exception.getStatus()
      : HttpStatus.INTERNAL_SERVER_ERROR;
    ctx.getResponse().status(status).json({
      statusCode: status,
      message: exception instanceof HttpException ? exception.message : "Internal server error",
      timestamp: new Date().toISOString(),
    });
  }
}
// Register: app.useGlobalFilters(new AllExceptionsFilter())

Interceptor: response transform + timing

@Injectable()
export class TransformInterceptor implements NestInterceptor {
  intercept(ctx: ExecutionContext, next: CallHandler): Observable<any> {
    const start = Date.now();
    return next.handle().pipe(
      map(data => ({ data, duration: Date.now() - start, timestamp: new Date() })),
    );
  }
}

Validation DTO

export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsString()
  @MinLength(8)
  @Matches(/^(?=.*[A-Z])(?=.*\d)/, { message: "Must contain uppercase and digit" })
  password: string;

  @IsEnum(Role)
  @IsOptional()
  role?: Role = Role.USER;
}
// Global: app.useGlobalPipes(new ValidationPipe({ transform: true, whitelist: true }))

Config with validation

// app.module.ts
ConfigModule.forRoot({
  isGlobal: true,
  validationSchema: Joi.object({
    NODE_ENV: Joi.string().valid("development", "production", "test").required(),
    PORT: Joi.number().default(3000),
    DATABASE_URL: Joi.string().required(),
    JWT_SECRET: Joi.string().min(32).required(),
  }),
})

Common pitfalls