Agent Skills: NestJS Best Practices

NestJS Best Practices

Overview

Grounded in the Official NestJS Documentation, this skill enforces modular architecture, dependency injection scoping, exception filters, DTO validation with class-validator, and Drizzle ORM integration patterns.

When to Use

• Designing/refactoring NestJS modules or dependency injection
• Creating exception filters, validating DTOs, or integrating Drizzle ORM
• Reviewing code for anti-patterns or onboarding to a NestJS codebase

Instructions

1. Modular Architecture

Follow strict module encapsulation. Each domain feature should be its own @Module():

• Export only what other modules need — keep internal providers private
• Use forwardRef() only as a last resort for circular dependencies; prefer restructuring
• Group related controllers, services, and repositories within the same module
• Use a SharedModule for cross-cutting concerns (logging, configuration, caching)

See references/arch-module-boundaries.md for enforcement rules.

2. Dependency Injection

Choose the correct provider scope based on use case:

| Scope | Lifecycle | Use Case | |-------------|------------------------------|---------------------------------------------| | DEFAULT | Singleton (shared) | Stateless services, repositories | | REQUEST | Per-request instance | Request-scoped data (tenant, user context) | | TRANSIENT | New instance per injection | Stateful utilities, per-consumer caches |

• Default to DEFAULT scope — only use REQUEST or TRANSIENT when justified
• Use constructor injection exclusively — avoid property injection
• Register custom providers with useClass, useValue, useFactory, or useExisting

See references/di-provider-scoping.md for enforcement rules.

3. Request Lifecycle

Understand and respect the NestJS request processing pipeline:

Middleware → Guards → Interceptors (before) → Pipes → Route Handler → Interceptors (after) → Exception Filters

• Middleware: Cross-cutting concerns (logging, CORS, body parsing)
• Guards: Authorization and authentication checks (return true/false)
• Interceptors: Transform response data, add caching, measure timing
• Pipes: Validate and transform input parameters
• Exception Filters: Catch and format error responses

4. Error Handling

Standardize error responses across the application:

• Extend HttpException for HTTP-specific errors
• Create domain-specific exception classes (e.g., OrderNotFoundException)
• Implement a global ExceptionFilter for consistent error formatting
• Use the Result pattern for expected business logic failures
• Never silently swallow exceptions

See references/error-exception-filters.md for enforcement rules.

5. Validation

Enforce input validation at the API boundary:

• Enable ValidationPipe globally with transform: true and whitelist: true
• Decorate all DTO properties with class-validator decorators
• Use class-transformer for type coercion (@Type(), @Transform())
• Create separate DTOs for Create, Update, and Response operations
• Never trust raw user input — validate everything

See references/api-validation-dto.md for enforcement rules.

6. Database Patterns (Drizzle ORM)

Integrate Drizzle ORM following NestJS provider conventions:

• Wrap the Drizzle client in an injectable provider
• Use the Repository pattern for data access encapsulation
• Define schemas in dedicated schema files per domain module
• Use transactions for multi-step operations
• Keep database logic out of controllers

See references/db-drizzle-patterns.md for enforcement rules.

Best Practices

| Area | Do | Don't | |--------------------|------------------------------------------|------------------------------------------| | Modules | One module per domain feature | Dump everything in AppModule | | DI Scoping | Default to singleton scope | Use REQUEST scope without justification| | Error Handling | Custom exception filters + domain errors | Bare try/catch with console.log | | Validation | Global ValidationPipe + DTO decorators | Manual if checks in controllers | | Database | Repository pattern with injected client | Direct DB queries in controllers | | Testing | Unit test services, e2e test controllers | Skip tests or test implementation details| | Configuration | @nestjs/config with typed schemas | Hardcode values or use process.env |

Examples

Example: New Domain Module with Validation

When building a "Product" feature, follow this workflow:

1. Create the module with proper encapsulation:

// product/product.module.ts
@Module({
  imports: [DatabaseModule],
  controllers: [ProductController],
  providers: [ProductService, ProductRepository],
  exports: [ProductService], // Only export what others need
})
export class ProductModule {}

2. Create validated DTOs:

// product/dto/create-product.dto.ts
import { IsString, IsNumber, IsPositive, MaxLength } from 'class-validator';

export class CreateProductDto {
  @IsString() @MaxLength(255) readonly name: string;
  @IsNumber() @IsPositive() readonly price: number;
}

3. Service with error handling:

@Injectable()
export class ProductService {
  constructor(private readonly productRepository: ProductRepository) {}

  async findById(id: string): Promise<Product> {
    const product = await this.productRepository.findById(id);
    if (!product) throw new ProductNotFoundException(id);
    return product;
  }
}

4. Verify module registration:

# Check module is imported in AppModule
grep -r "ProductModule" src/app.module.ts

# Run e2e to confirm exports work
npx jest --testPathPattern="product"

Constraints and Warnings

1. Do not mix scopes without justification — REQUEST-scoped providers cascade to all dependents
2. Never access database directly from controllers — always go through service and repository layers
3. Avoid forwardRef() — restructure modules to eliminate circular dependencies
4. Do not skip ValidationPipe — always validate at the API boundary with DTOs
5. Never hardcode secrets — use @nestjs/config with environment variables
6. Keep modules focused — one domain feature per module, avoid "god modules"

References

• references/architecture.md — Deep-dive into NestJS architectural patterns
• references/ — Individual enforcement rules with correct/incorrect examples
• assets/templates/ — Starter templates for common NestJS components