Scopes & Advanced DI — Series 3

Preview — 3 of 10 questions

What should you expect from these hooks?

javascript

@Injectable({ scope: Scope.REQUEST })
export class TenantContext implements OnModuleInit, OnModuleDestroy {
  onModuleInit() { /* never runs? */ }
  onModuleDestroy() { /* never runs? */ }
}

AThey fire once per request, replacing the module-level semantics

BonModuleInit fires per request while onModuleDestroy fires once at shutdown

CThey fire once at bootstrap against a prototype instance

DModule-lifecycle hooks are tied to the module's lifecycle, and a request-scoped provider has no instance at that time — so they are not invoked for it; per-request setup belongs in the constructor or in an explicit initialisation the consumer calls

What is the outcome?

javascript

@Injectable({ scope: Scope.REQUEST })
export class RequestUser { constructor(@Inject(REQUEST) readonly req: Request) {} }

@Injectable()                                  // default scope
export class AuditService { constructor(private readonly user: RequestUser) {} }

AStartup fails: a default-scoped provider may not depend on a request-scoped one

BAuditService is promoted to request scope automatically — the declared scope is a floor, not a ceiling, so it is instantiated per request along with everything that depends on it

CAuditService stays a singleton and receives the first request's RequestUser

DAuditService receives a proxy that resolves the current request lazily

What does injecting @Inject('LOGGER') produce?

javascript

@Injectable({ scope: Scope.TRANSIENT })
export class Logger {}

@Module({
  providers: [Logger, { provide: 'LOGGER', useExisting: Logger }],
})
export class AppModule {}

AThe alias resolves to the target token, so the target's scope governs: 'LOGGER' behaves like Logger and yields a transient instance per consumer, because useExisting is an indirection rather than a second registration

BA default-scoped singleton, since the alias declares no scope of its own

CA resolution error, because a scoped provider cannot be aliased

DA frozen snapshot of the first Logger instance created

Answer all 10 questions (7 more), see explanations for every answer, and track your score.