patterns-dependency-injection.md

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title: Use NestJS Dependency Injection impact: HIGH impactDescription: Enables testability, loose coupling, and maintainability tags: patterns, dependency-injection, nestjs, testing

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Use NestJS Dependency Injection

Impact: HIGH

All services, repositories, and providers are managed by NestJS DI container. Never instantiate classes manually.

Incorrect (manual instantiation):

@Injectable()
export class ItemService {
  private repo = new ItemRepository(new PrismaService()); // Wrong!
  private cache = new CacheService(new RedisService());    // Wrong!
}

Correct (constructor injection):

@Injectable()
export class ItemService {
  constructor(
    private readonly itemRepo: ItemRepository,
    private readonly cache: CacheService,
  ) {}
}

// Module registers providers
@Module({
  imports: [PrismaModule, RedisModule],
  controllers: [ItemController],
  providers: [ItemService, ItemRepository],
  exports: [ItemService],
})
export class ItemModule {}

Testing Benefits

DI makes mocking trivial:

const module = await Test.createTestingModule({
  providers: [
    ItemService,
    { provide: ItemRepository, useValue: mockRepository },
    { provide: CacheService, useValue: mockCache },
  ],
}).compile();

Rules

• All classes that need dependencies use @Injectable() decorator
• Dependencies injected via constructor
• Module providers array declares what's available
• Module exports array declares what other modules can use
• Use interfaces for repositories when you want to swap implementations
• Never use new to create injected services