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

(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 — Feature modules, CRUD, DTOs, dynamic modules, exception filters, custom providers
• examples/database.md — NestJS DI patterns for database integration, transactions
• examples/auth.md — Passport.js integration, JWT strategy, auth guards, RBAC
• examples/testing.md — Unit testing with Test.createTestingModule, e2e with supertest
• examples/advanced.md — Interceptors, custom pipes, custom decorators, config, CQRS, Swagger
• 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 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 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 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 auto-converts these to proper HTTP responses with correct status codes. Never send raw status codes.

For custom error response shapes, use a global @Catch() exception filter. See examples/core.md.

---

Guards and Middleware

Guards decide whether a request proceeds (authorization). Middleware runs before routing (logging, CORS).

// Guard — implements CanActivate
@Injectable()
export class JwtAuthGuard implements CanActivate {
  async canActivate(context: ExecutionContext): Promise<boolean> {
    const request = context.switchToHttp().getRequest<Request>();
    // Validate token, attach user to request
    return true;
  }
}

// Apply to routes
@UseGuards(JwtAuthGuard, RolesGuard)
@Controller("admin")
export class AdminController {}

Why good: Guards are injectable (can use services), composable (run in order), use Reflector for metadata-driven access control

See examples/auth.md for JWT auth, Passport.js integration, RBAC, and @Public() decorator.

---

Interceptors

Interceptors wrap handler execution for cross-cutting concerns (response wrapping, logging, caching).

@Injectable()
export class TransformResponseInterceptor<T> implements NestInterceptor {
  intercept(
    context: ExecutionContext,
    next: CallHandler,
  ): Observable<ApiResponse<T>> {
    return next.handle().pipe(
      map((data) => ({
        success: true,
        data,
        timestamp: new Date().toISOString(),
      })),
    );
  }
}

See examples/advanced.md for logging, caching, and custom pipe patterns.

</patterns>

---

<decision_framework>

Decision Framework

Request Lifecycle

Incoming Request
  → Middleware (logging, CORS, body parsing)
    → Guards (authentication, authorization)
      → Interceptors (pre-handler: transform request, start timing)
        → Pipes (validation, transformation)
          → Route Handler (controller method)
        → Interceptors (post-handler: transform response, log timing)
  → Exception Filters (catch and format errors)
→ Response

Which Layer to Use

Need to process raw request before routing?
├─ YES → Middleware (logging, CORS, rate limiting)
└─ NO → Does it decide allow/deny for a route?
    ├─ YES → Guard (auth, roles, permissions)
    └─ NO → Does it transform/validate input data?
        ├─ YES → Pipe (validation, type coercion)
        └─ NO → Does it wrap handler execution?
            ├─ YES → Interceptor (timing, caching, response mapping)
            └─ NO → Does it handle errors?
                ├─ YES → Exception Filter
                └─ NO → Put it in the service layer

Module Organization

Is this a cross-cutting concern (auth, config, logging)?
├─ YES → Global module or shared module
└─ NO → Is it a business feature (users, orders, products)?
    ├─ YES → Feature module (users.module.ts)
    └─ NO → Is it infrastructure (database, cache, queue)?
        ├─ YES → Infrastructure module
        └─ NO → Part of the closest feature module

</decision_framework>

---

<red_flags>

RED FLAGS

High Priority Issues:

• Putting business logic in controllers instead of services
• Missing @Injectable() on services (DI fails silently at runtime)
• Not enabling ValidationPipe globally (DTOs are not validated)
• Using any for request body instead of typed DTOs
• Instantiating services with new instead of constructor injection
• Throwing raw Error instead of NestJS HTTP exceptions (produces 500 instead of proper status)

Medium Priority Issues:

• Not exporting services from modules (other modules can't import them)
• Importing the entire module when you only need one service
• Missing whitelist: true on ValidationPipe (mass-assignment vulnerability)
• Using @Res() decorator outside streaming scenarios (opts out of NestJS response handling)
• Not using PartialType / PickType / OmitType for update DTOs (duplicated validation)

Common Mistakes:

• Circular module dependencies — restructure with forwardRef() or extract shared logic
• Forgetting to register providers in the module — service injection fails at runtime
• Using synchronous guards for async operations — return Promise<boolean> or Observable<boolean>
• Not handling all exception types in custom filters — always have a catch-all for unknown errors

Gotchas and Edge Cases:

• @UseGuards(AuthGuard) takes a class reference, not an instance — NestJS instantiates via DI
• ValidationPipe with transform: true converts query params to their declared types automatically
• Guards execute AFTER middleware but BEFORE interceptors and pipes
• @Catch() with no arguments catches ALL exceptions, not just HttpException
• IntrinsicException (NestJS 11) throws without framework auto-logging — useful for expected flow control
• NestJS 11: Termination lifecycle hooks (OnModuleDestroy, OnApplicationShutdown) now execute in reverse order
• NestJS 11: Express v5 requires named wildcards (/*splat instead of /*)
• NestJS 11: SWC is a supported opt-in compiler via nest-cli.json ("builder": "swc") — 20x faster builds than tsc
• NestJS 11: ParseDatePipe is now built-in — no need for custom date parsing pipes
• Request-scoped providers (Scope.REQUEST) affect performance — use only when needed
• forwardRef() should be a last resort — circular deps usually signal a design issue

</red_flags>

---

<critical_reminders>

CRITICAL REMINDERS

(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)

Failure to follow these rules will produce unvalidated, untestable NestJS code with broken dependency injection.

</critical_reminders>