nestjs-patterns

# NestJS Development Patterns

Production-grade NestJS patterns for modular TypeScript backends.

## When to Activate

- Building NestJS APIs or services
- Structuring modules, controllers, and providers
- Adding DTO validation, guards, interceptors, or exception filters
- Configuring environment-aware settings and database integrations
- Testing NestJS units or HTTP endpoints

## Project Structure

```text
src/
├── app.module.ts
├── main.ts
├── common/
│   ├── filters/
│   ├── guards/
│   ├── interceptors/
│   └── pipes/
├── config/
│   ├── configuration.ts
│   └── validation.ts
├── modules/
│   ├── auth/
│   │   ├── auth.controller.ts
│   │   ├── auth.module.ts
│   │   ├── auth.service.ts
│   │   ├── dto/
│   │   ├── guards/
│   │   └── strategies/
│   └── users/
│       ├── dto/
│       ├── entities/
│       ├── users.controller.ts
│       ├── users.module.ts
│       └── users.service.ts
└── prisma/ or database/
```

- Keep domain code inside feature modules.
- Put cross-cutting filters, decorators, guards, and interceptors in `common/`.
- Keep DTOs close to the module that owns them.

## Bootstrap and Global Validation

```ts
async function bootstrap() {
  const app = await NestFactory.create(AppModule, { bufferLogs: true });

  app.useGlobalPipes(
    new ValidationPipe({
      whitelist: true,
      forbidNonWhitelisted: true,
      transform: true,
      transformOptions: { enableImplicitConversion: true },
    }),
  );

  app.useGlobalInterceptors(new ClassSerializerInterceptor(app.get(Reflector)));
  app.useGlobalFilters(new HttpExceptionFilter());

  await app.listen(process.env.PORT ?? 3000);
}
bootstrap();
```

- Always enable `whitelist` and `forbidNonWhitelisted` on public APIs.
- Prefer one global validation pipe instead of repeating validation config per route.

## Modules, Controllers, and Providers

```ts
@Module({
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService],
})
export class UsersModule {}

@Controller('users')
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

  @Get(':id')
  getById(@Param('id', ParseUUIDPipe) id: string) {
    return this.usersService.getById(id);
  }

  @Post()
  create(@Body() dto: CreateUserDto) {
    return this.usersService.create(dto);
  }
}

@Injectable()
export class UsersService {
  constructor(private readonly usersRepo: UsersRepository) {}

  async create(dto: CreateUserDto) {
    return this.usersRepo.create(dto);
  }
}
```

- Controllers should stay thin: parse HTTP input, call a provider, return response DTOs.
- Put business logic in injectable services, not controllers.
- Export only the providers other modules genuinely need.

## DTOs and Validation

```ts
export class CreateUserDto {
  @IsEmail()
  email!: string;

  @IsString()
  @Length(2, 80)
  name!: string;

  @IsOptional()
  @IsEnum(UserRole)
  role?: UserRole;
}
```

- Validate every request DTO with `class-validator`.
- Use dedicated response DTOs or serializers instead of returning ORM entities directly.
- Avoid leaking internal fields such as password hashes, tokens, or audit columns.

## Auth, Guards, and Request Context

```ts
@UseGuards(JwtAuthGuard, RolesGuard)
@Roles('admin')
@Get('admin/report')
getAdminReport(@Req() req: AuthenticatedRequest) {
  return this.reportService.getForUser(req.user.id);
}
```

- Keep auth strategies and guards module-local unless they are truly shared.
- Encode coarse access rules in guards, then do resource-specific authorization in services.
- Prefer explicit request types for authenticated request objects.

## Exception Filters and Error Shape

```ts
@Catch()
export class HttpExceptionFilter implements ExceptionFilter {
  private readonly logger = new Logger(HttpExceptionFilter.name);

  catch(exception: unknown, host: ArgumentsHost) {
    const response = host.switchToHttp().getResponse<Response>();
    const request = host.switchToHttp().getRequest<Request>();

    if (exception instanceof HttpException) {
      return response.status(exception.getStatus()).json({
        path: request.url,
        error: exception.getResponse(),
      });
    }

    this.logger.error(
      `Unhandled exception at ${request.url}: ${exception instanceof Error ? exception.message : exception}`,
      exception instanceof Error ? exception.stack : undefined,
    );

    return response.status(500).json({
      path: request.url,
      error: 'Internal server error',
    });
  }
}
```

- Keep one consistent error envelope across the API.
- Throw framework exceptions for expected client errors; log and wrap unexpected failures centrally.

## Config and Environment Validation

```ts
ConfigModule.forRoot({
  isGlobal: true,
  load: [configuration],
  validate: validateEnv,
});
```

- Validate env at boot, not lazily at first request.
- Keep config access behind typed helpers or config services.
- Split dev/staging/prod concerns in config factories instead of branching throughout feature code.

## Persistence and Transactions

- Keep repository / ORM code behind providers that speak domain language.
- For Prisma or TypeORM, isolate transactional workflows in services that own the unit of work.
- Do not let controllers coordinate multi-step writes directly.

## Testing

```ts
describe('UsersController', () => {
  let app: INestApplication;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [UsersModule],
    }).compile();

    app = moduleRef.createNestApplication();
    app.useGlobalPipes(new ValidationPipe({ whitelist: true, transform: true }));
    await app.init();
  });
});
```

- Unit test providers in isolation with mocked dependencies.
- Add request-level tests for guards, validation pipes, and exception filters.
- Reuse the same global pipes/filters in tests that you use in production.

## Production Defaults

- Enable structured logging and request correlation ids.
- Terminate on invalid env/config instead of booting