Services

Services are injectable classes (`@Injectable`) holding reusable logic: HTTP clients, facades over stores, coordination between features. They keep components thin and testable.

					@Injectable({ providedIn: 'root' })
export class UserApi {
  constructor(private http: HttpClient) {}
  getUser(id: string) { return this.http.get<User>(`/api/users/${id}`); }
}
				

`providedIn: 'root'` tree-shakes singletons. Feature-scoped providers create per-lazy-route instances when needed.

On interviews: service versus component state, testing with mocks, and avoiding god-services that know every feature.

Common pitfalls: storing UI view state in root singletons, HttpClient calls without error mapping, and circular service dependencies.

The trade-off is convenient root singletons versus hidden global state that outlives feature boundaries.

Checklist:

• One responsibility per service.
• Root provide for app-wide singletons.
• Feature providers for isolated state.
• Return Observables or Promises consistently.