nestjs-expert

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;
  }
}