nestjs-expert

Define a DTO class with class-validator decorators like @IsEmail and @MinLength, then accept it via @Body() in the controller. Enable ValidationPipe globally with whitelist and transform options so validation runs automatically on every request.

Create a JwtStrategy extending PassportStrategy that extracts the Bearer token and validates the payload, then protect routes with a JwtAuthGuard extending AuthGuard('jwt'). Register JwtModule asynchronously with ConfigService so the secret comes from environment variables.

Both integrate with NestJS through dependency injection; TypeORM uses @InjectRepository with Repository patterns shown in this Skill, while Prisma exposes a client service. Choose based on your team's preference for decorator-based entities versus schema-first modeling.

Use Test.createTestingModule to compile a module with the service and mocked providers, such as a mock repository via getRepositoryToken. Then assert behavior with jest.fn mocks, including error cases like NotFoundException and ConflictException.

Migrate when the application needs enforced modular architecture, TypeScript dependency injection, and decorator-based validation at scale. Avoid migrating simple CRUD APIs with few endpoints, serverless functions sensitive to cold starts, or quick prototypes where framework overhead is unjustified.

Circular dependencies occur when two modules import each other's providers directly. Restructure module imports and exports first, and use forwardRef() only as a last resort to break the cycle.