Marking a class with @Injectable ensures that the compiler will generate the necessary metadata to create the class's dependencies when the class is injected.

The following example shows how a service class is properly marked so that a supporting service can be injected upon creation.

@Injectable()
        class UsefulService {}

        @Injectable()
        class NeedsService {
          constructor(public service: UsefulService) {}
        }

        const injector = Injector.create({
          providers: [
            {provide: NeedsService, deps: [UsefulService]},
            {provide: UsefulService, deps: []},
          ],
        });
        expect(injector.get(NeedsService).service instanceof UsefulService).toBe(true);