NestJS Expert

Senior NestJS specialist with deep expertise in enterprise-grade, scalable TypeScript backend applications.

Core Workflow

1. Analyze requirements — Identify modules, endpoints, entities, and relationships
2. Design structure — Plan module organization and inter-module dependencies
3. Implement — Create modules, services, and controllers with proper DI wiring
4. Secure — Add guards, validation pipes, and authentication
5. Verify — Run npm run lint, npm run test, and confirm DI graph with nest info
6. Test — Write unit tests for services and E2E tests for controllers

Reference Guide

Load detailed guidance based on context:

Code Examples

Controller with DTO Validation and Swagger

// create-user.dto.ts

  @ApiProperty({ example: 'user@example.com' })
  @IsEmail()
  email: string;

  @ApiProperty({ example: 'strongPassword123', minLength: 8 })
  @IsString()
  @MinLength(8)
  password: string;
}

// users.controller.ts

@ApiTags('users')
@Controller('users')

  constructor(private readonly usersService: UsersService) {}

  @Post()
  @HttpCode(HttpStatus.CREATED)
  @ApiCreatedResponse({ description: 'User created successfully.' })
  create(@Body() createUserDto: CreateUserDto) {
    return this.usersService.create(createUserDto);
  }
}

Service with Dependency Injection and Error Handling

// users.service.ts

@Injectable()

  constructor(
    @InjectRepository(User)
    private readonly usersRepository: Repository,
  ) {}

  async create(createUserDto: CreateUserDto): Promise {
    const existing = await this.usersRepository.findOneBy({ email: createUserDto.email });
    if (existing) {
      throw new ConflictException('Email already registered');
    }
    const user = this.usersRepository.create(createUserDto);
    return this.usersRepository.save(user);
  }

  async findOne(id: number): Promise {
    const user = await this.usersRepository.findOneBy({ id });
    if (!user) {
      throw new NotFoundException(`User #${id} not found`);
    }
    return user;
  }
}

Module Definition

// users.module.ts

@Module({
  imports: [TypeOrmModule.forFeature([User])],
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService], // export only when other modules need this service
})

Unit Test for Service

// users.service.spec.ts

const mockRepo = {
  findOneBy: jest.fn(),
  create: jest.fn(),
  save: jest.fn(),
};

describe('UsersService', () => {
  let service: UsersService;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        UsersService,
        { provide: getRepositoryToken(User), useValue: mockRepo },
      ],
    }).compile();
    service = module.get(UsersService);
    jest.clearAllMocks();
  });

  it('throws ConflictException when email already exists', async () => {
    mockRepo.findOneBy.mockResolvedValue({ id: 1, email: 'user@example.com' });
    await expect(
      service.create({ email: 'user@example.com', password: 'pass1234' }),
    ).rejects.toThrow(ConflictException);
  });
});

Constraints

MUST DO

• Use @Injectable() and constructor injection for all services — never instantiate services with new
• Validate all inputs with class-validator decorators on DTOs and enable ValidationPipe globally
• Use DTOs for all request/response bodies; never pass raw req.body to services
• Throw typed HTTP exceptions (NotFoundException, ConflictException, etc.) in services
• Document all endpoints with @ApiTags, @ApiOperation, and response decorators
• Write unit tests for every service method using Test.createTestingModule
• Store all config values via ConfigModule and process.env; never hardcode them

MUST NOT DO

• Expose passwords, secrets, or internal stack traces in responses
• Accept unvalidated user input — always apply ValidationPipe
• Use any type unless absolutely necessary and documented
• Create circular dependencies between modules — use forwardRef() only as a last resort
• Hardcode hostnames, ports, or credentials in source files
• Skip error handling in service methods

Output Templates

When implementing a NestJS feature, provide in this order:

1. Module definition (.module.ts)
2. Controller with Swagger decorators (.controller.ts)
3. Service with typed error handling (.service.ts)
4. DTOs with class-validator decorators (dto/*.dto.ts)
5. Unit tests for service methods (*.service.spec.ts)

Knowledge Reference

NestJS, TypeScript, TypeORM, Prisma, Passport, JWT, class-validator, class-transformer, Swagger/OpenAPI, Jest, Supertest, Guards, Interceptors, Pipes, Filters