api-framework-nestjs

NestJS Patterns

Quick Guide: NestJS is an opinionated, modular Node.js framework built on TypeScript. Use modules to organize features, controllers for HTTP routing, services for business logic with dependency injection, DTOs with class-validator for validation, guards for auth, and exception filters for error handling. Key gotchas: always register services in module providers, always enable ValidationPipe globally with whitelist: true, never put business logic in controllers, never instantiate services with new. NestJS 11 is the current stable version (opt-in SWC compiler, Express v5, reversed termination hooks).

<critical_requirements>

CRITICAL: Before Using This Skill

All code must follow project conventions in CLAUDE.md (kebab-case, named exports, import ordering, import type, named constants)

(You MUST use @Injectable() on every service and register it in the module providers array)

(You MUST enable ValidationPipe globally with whitelist: true and forbidNonWhitelisted: true)

(You MUST use DTOs with class-validator decorators for ALL request body validation — never validate manually in controllers)

(You MUST throw NestJS built-in HTTP exceptions (NotFoundException, BadRequestException, etc.) — never send raw status codes)

(You MUST use constructor injection for dependencies — never instantiate services manually with new)

</critical_requirements>

Auto-detection: NestJS, @nestjs/common, @nestjs/core, @Module, @Controller, @Injectable, @Get, @Post, @Body, @Param, @Query, @UseGuards, @UseInterceptors, @UsePipes, @UseFilters, CanActivate, NestInterceptor, PipeTransform, ExceptionFilter, ValidationPipe, class-validator, class-transformer

When to use:

• Building structured backend APIs with TypeScript and dependency injection
• Applications requiring modular architecture with clear separation of concerns
• REST APIs with declarative validation, authentication, and role-based access
• Projects needing the guard/interceptor/pipe/filter request lifecycle

When NOT to use:

• Simple scripts or serverless functions that don't need a framework
• Projects where Express/Fastify alone is sufficient (no DI, no modules needed)
• Frontend code

Detailed Resources:

• [examples/core.md](examples/core.md) — Feature modules, CRUD, DTOs, dynamic modules, exception filters, custom providers
• [examples/database.md](examples/database.md) — NestJS DI patterns for database integration, transactions
• [examples/auth.md](examples/auth.md) — Passport.js integration, JWT strategy, auth guards, RBAC
• [examples/testing.md](examples/testing.md) — Unit testing with Test.createTestingModule, e2e with supertest
• [examples/advanced.md](examples/advanced.md) — Interceptors, custom pipes, custom decorators, config, CQRS, Swagger
• [reference.md](reference.md) — CLI commands, project structure, decorator tables, decision frameworks

<philosophy>

Philosophy

NestJS enforces a modular, decorator-driven architecture inspired by Angular. Every feature is organized into modules containing controllers (HTTP layer), services (business logic), and supporting infrastructure (guards, pipes, interceptors, filters).

Core principles:

1. Modularity — Group related controllers, services, and providers into feature modules. Modules are the primary organizational unit.
2. Dependency injection — Never instantiate services manually. Declare them as @Injectable() and let NestJS resolve the dependency graph via constructor injection.
3. Decorator-driven — Decorators (@Controller, @Get, @Body, @UseGuards) attach metadata that NestJS uses to build routing, validation, and middleware pipelines.
4. Separation of concerns — Controllers handle HTTP request/response. Services handle business logic. Guards handle authorization. Pipes handle validation/transformation. Filters handle exceptions.
5. Convention over configuration — Follow NestJS conventions (one module per feature, one controller per resource, DTOs for validation) to get batteries-included functionality.

</philosophy>

<patterns>

Key Patterns

Module System

Every NestJS app has a root AppModule that imports feature modules. Each feature module groups its controller, service, and providers. Export services that other modules need.

// Feature module — one per resource
@Module({
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService], // Available to other modules
})
export class UsersModule {}

Why good: Encapsulation per feature, explicit dependency graph via imports/exports, testable in isolation

See [examples/core.md](examples/core.md) for complete CRUD module, dynamic modules, and custom providers.

Controllers — Thin Routing Layer

Controllers should only extract request data and delegate to services. No business logic.

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

  @Get(":id")
  findOne(@Param("id", ParseIntPipe) id: number) {
    return this.usersService.findOne(id);
  }

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

Why good: ParseIntPipe validates and converts param, @HttpCode for explicit status, thin delegation to service

Anti-pattern: Business logic, manual validation, or database access in controllers — always delegate to services.

DTOs with class-validator

Use DTOs with class-validator decorators for all request validation. Enable ValidationPipe globally.

const MIN_PASSWORD_LENGTH = 8;

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

  @IsString()
  @MinLength(MIN_PASSWORD_LENGTH)
  password: string;
}

// Update DTO — reuses validation rules
export class UpdateUserDto extends PartialType(CreateUserDto) {}

// main.ts — Enable globally
app.useGlobalPipes(
  new ValidationPipe({
    whitelist: true, // Strip unknown properties
    forbidNonWhitelisted: true, // Reject unknown properties
    transform: true, // Auto-transform to DTO instances
  }),
);

Why good: Declarative validation, whitelist prevents mass-assignment, PartialType avoids duplicating rules

See [examples/core.md](examples/core.md) for nested DTOs, query DTOs with pagination, and validation groups.

Services and Dependency Injection

Services contain business logic. Decorate with @Injectable() and inject via constructor.

@Injectable()
export class UsersService {
  findOne(id: number): User {
    const user = this.users.find((u) => u.id === id);
    if (!user) {
      throw new NotFoundException(`User with id ${id} not found`);
    }
    return user;
  }
}

Why good: @Injectable() enables DI, throws NestJS HTTP exceptions, pure business logic with no HTTP concerns

Custom Providers

Use token-based injection for non-class providers (factory, value, class providers):

const DATABASE_CONNECTION = "DATABASE_CONNECTION";

const databaseProvider = {
  provide: DATABASE_CONNECTION,
  useFactory: async (configService: ConfigService) => {
    return createConnection(configService.get("database"));
  },
  inject: [ConfigService],
};

// Inject with @Inject token
constructor(@Inject(DATABASE_CONNECTION) private readonly db: Connection) {}

See [examples/core.md](examples/core.md) for complete provider examples.

Exception Handling

Throw NestJS built-in HTTP exceptions from services. Use exception filters for custom error response formatting.

// Service — throw built-in exceptions
throw new NotFoundException("Resource not found");
throw new ConflictException("Resource already exists");
throw new BadRequestException("Invalid input");
throw new UnauthorizedException("Authentication required");

Key point: NestJS