Agent skill

nestjs-expert

Creates and configures NestJS modules, controllers, services, DTOs, guards, and interceptors for enterprise-grade TypeScript backend applications. Use when building NestJS REST APIs or GraphQL services, implementing dependency injection, scaffolding modular architecture, adding JWT/Passport authentication, integrating TypeORM or Prisma, or working with .module.ts, .controller.ts, and .service.ts files. Invoke for guards, interceptors, pipes, validation, Swagger documentation, and unit/E2E testing in NestJS projects.

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Metadata

Additional technical details for this skill

role
specialist
scope
implementation
author
https://github.com/Jeffallan
domain
backend
version
1.1.0
triggers
NestJS, Nest, Node.js backend, TypeScript backend, dependency injection, controller, service, module, guard, interceptor
output format
code
related skills
fullstack-guardian, test-master, devops-engineer

SKILL.md

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:

Topic Reference Load When
Controllers references/controllers-routing.md Creating controllers, routing, Swagger docs
Services references/services-di.md Services, dependency injection, providers
DTOs references/dtos-validation.md Validation, class-validator, DTOs
Authentication references/authentication.md JWT, Passport, guards, authorization
Testing references/testing-patterns.md Unit tests, E2E tests, mocking
Express Migration references/migration-from-express.md Migrating from Express.js to NestJS

Code Examples

Controller with DTO Validation and Swagger

typescript
// 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

typescript
// 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

typescript
// 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

typescript
// 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

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