Optimizing Dependency Injection in NestJS

NestJS follows a modular and scalable architecture, relying heavily on Dependency Injection (DI) to manage services efficiently. In this article, we'll explore best practices for optimizing DI to improve performance, maintainability, and testability.

Understanding Dependency Injection in NestJS

Dependency Injection in NestJS follows a provider-based approach where dependencies are injected via the constructor.

Example:

@Injectable()
export class UserService {
  constructor(private readonly userRepository: UserRepository) {}

  getUsers() {
    return this.userRepository.findAll();
  }
}

Here, UserService depends on UserRepository, which NestJS automatically injects.

Optimizing Dependency Injection

1. Use @Injectable({ scope: Scope.REQUEST }) When Necessary

By default, providers in NestJS are singleton, meaning they are instantiated once per application lifecycle. However, for request-scoped dependencies (like database transactions), use request-scoped providers.

@Injectable({ scope: Scope.REQUEST })
export class RequestScopedService {
  constructor(@Inject(REQUEST) private request: Request) {}

  getRequestData() {
    return this.request.user;
  }
}

🔹 Use Cases: User session management, per-request logging, or multi-tenant handling.

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2. Avoid Circular Dependencies Using Interfaces or Forward References

Circular dependencies occur when two or more providers depend on each other. Example:

@Injectable()
export class AService {
  constructor(private readonly bService: BService) {}
}

@Injectable()
export class BService {
  constructor(private readonly aService: AService) {}
}

Solution 1: Use forwardRef
Modify one of the dependencies using forwardRef:

@Injectable()
export class AService {
  constructor(
    @Inject(forwardRef(() => BService)) private readonly bService: BService,
  ) {}
}

Solution 2: Use an Interface Token
Create an abstraction using an interface and manually inject it.

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3. Implement Provider Factories for Dynamic Dependencies

If a service requires dynamic configuration, use factory providers:

@Module({
  providers: [
    {
      provide: 'CONFIG_OPTIONS',
      useFactory: async () => {
        const config = await loadConfig();
        return config.database;
      },
    },
  ],
})
export class AppModule {}

This is useful for database connections, third-party integrations, or environment-based configurations.

Conclusion

Optimizing DI in NestJS ensures better performance, maintainability, and testability. Use request-scoped providers, avoid circular dependencies, and leverage factory providers for dynamic configurations.