NestJS providers

Providers are injectable classes (or values/factories) registered in a module's `providers` array. `@Injectable()` marks a class for DI. Services, repositories, and adapters are typical providers.

					@Injectable()
export class UsersService {
  constructor(private readonly repo: UsersRepository) {}

  async create(dto: CreateUserDto) {
    return this.repo.save(dto);
  }
}
				

Custom providers use `useClass`, `useValue`, `useFactory`, and injection tokens for interfaces. Scope defaults to singleton per module; `Scope.REQUEST` creates one instance per HTTP request — use sparingly due to cost.

On interviews: explain provider lifetime, when factories beat classes, and how repositories hide persistence from services.

Common pitfalls: request-scoped providers everywhere, registering the same provider in multiple modules without exporting, and services that know HTTP details.

Checklist:

• Mark domain services `@Injectable()`.
• Hide persistence behind repository providers.
• Prefer singleton scope unless request context is required.
• Use tokens for interface-to-implementation binding.