feat: add nestjs development patterns

skills/nestjs-patterns/SKILL.md

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+---
+name: nestjs-patterns
+description: NestJS architecture patterns for modules, controllers, providers, DTO validation, guards, interceptors, config, and production-grade TypeScript backends.
+origin: ECC
+---
+
+# 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 {
+  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(),
+      });
+    }
+
+    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 partially.
+- Prefer async provider initialization for DB/cache clients with explicit health checks.
+- Keep background jobs and event consumers in their own modules, not inside HTTP controllers.
+- Make rate limiting, auth, and audit logging explicit for public endpoints.