Injectable & Providers — Series 2

Preview — 3 of 10 questions

HeroList needs HeroService. Why ask for it rather than construct it?

javascript

export class HeroList {
  private heroes = inject(HeroService);   // instead of new HeroService(new HttpClient(...))
}

ABecause the component would otherwise have to know how to build the service and everything the service needs — and every test would have to build it too. Asking for it leaves construction to the injector, which is also what makes substitution possible

BBecause new is forbidden in Angular classes

CBecause new creates a copy of the service per change detection cycle

DBecause services must be created before the component exists

What does this achieve?

javascript

@Injectable({ providedIn: 'root' })
export class HeroService {}

AA new instance per component that injects it

BRegistration only for the root component's subtree

COne instance for the whole application, created lazily on first injection — and if nothing injects it, the class can be dropped from the bundle entirely

DAn instance created eagerly at bootstrap, always

Which of these classes needs the decorator?

javascript

class Logger { log(m: string) { console.log(m); } }

class HeroService { constructor(private http: HttpClient) {} }

AHeroService — a class Angular has to construct with dependencies needs the decorator, because that is what emits the constructor's type metadata. A dependency-free Logger provided by useClass can work without it, but decorating both is the convention

BNeither; the decorator is optional everywhere

CBoth, always, or the compiler errors

DLogger only, since it has no constructor parameters

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