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:

references/controllers-routing.md

references/services-di.md

references/dtos-validation.md

references/authentication.md

references/testing-patterns.md

references/migration-from-express.md

Code Examples

Controller with DTO Validation and Swagger

// create-user.dto.ts
import { IsEmail, IsString, MinLength } from 'class-validator';
import { ApiProperty } from '@nestjs/swagger';

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

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

// users.controller.ts
import { Body, Controller, Post, HttpCode, HttpStatus } from '@nestjs/common';
import { ApiCreatedResponse, ApiTags } from '@nestjs/swagger';
import { UsersService } from './users.service';
import { CreateUserDto } from './dto/create-user.dto';

@ApiTags('users')
@Controller('users')
export class UsersController {
  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
import { Injectable, ConflictException, NotFoundException } from '@nestjs/common';
import { InjectRepository } from '@nestjs/typeorm';
import { Repository } from 'typeorm';
import { User } from './entities/user.entity';
import { CreateUserDto } from './dto/create-user.dto';

@Injectable()
export class UsersService {
  constructor(
    @InjectRepository(User)
    private readonly usersRepository: Repository<User>,
  ) {}

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

Module Definition

// users.module.ts
import { Module } from '@nestjs/common';
import { TypeOrmModule } from '@nestjs/typeorm';
import { UsersController } from './users.controller';
import { UsersService } from './users.service';
import { User } from './entities/user.entity';

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

Unit Test for Service

// users.service.spec.ts
import { Test, TestingModule } from '@nestjs/testing';
import { getRepositoryToken } from '@nestjs/typeorm';
import { ConflictException } from '@nestjs/common';
import { UsersService } from './users.service';
import { User } from './entities/user.entity';

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>(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