NestJS Controllers & Services Standards

Priority: P0 (FOUNDATIONAL)

Layer separation standards and dependency injection patterns for NestJS applications.

Controllers

• Role: Handler only. Delegate all logic to Services.
• Context: Use ExecutionContext helpers (switchToHttp()) for platform-agnostic code.
• Custom Decorators:
• Avoid: @Request() req -> req.user (Not type-safe).
• Pattern: Create typed decorators like @CurrentUser(), @DeviceIp().

typescript export const CurrentUser = createParamDecorator( (data: unknown, ctx: ExecutionContext) => ctx.switchToHttp().getRequest().user, );

DTOs & Validation

• Strictness:
• whitelist: true: Strip properties without decorators.
• Critical: forbidNonWhitelisted: true: Throw error if unknown properties exist.
• Transformation:
• transform: true: Auto-convert primitives (String '1' -> Number 1) and instantiate DTO classes.
• Documentation:
• Automation: Use the @nestjs/swagger CLI plugin (nest-cli.json) to auto-detect DTO properties without manual @ApiProperty() tags.

Interceptors (Response Mapping)

• Standardization: specific responses should be mapped in Interceptors, not Controllers.
• Use map() to wrap success responses (e.g. { data: T }).
• Refer to API Standards for PageDto and ApiResponse.
• Use catchError() to map low-level errors (DB constraints) to HttpException (e.g. ConflictException) before they hit the global filter.

Services & Business Logic

• Singleton: Default.
• Stateless: Do not store request-specific state in class properties unless identifying as Scope.REQUEST.

Pipes & Validation

• Global: Register ValidationPipe globally.
• Route Params: Fail fast. Always use ParseIntPipe, ParseUUIDPipe on all ID parameters.

@Get(':id')
findOne(@Param('id', ParseIntPipe) id: number) { ... }

Lifecycle Events

• Init: Use OnModuleInit for connection setup.
• Destroy: Use OnApplicationShutdown for cleanup. (Requires enableShutdownHooks()).