component.ts

@Component({})
export class AppComponent {
  readonly noneMessage = "nothing";

  readonly selectSeat$ = new Subject<number>();

  readonly selectedMessage$ = this.selectSeat$.pipe(
    scan(registerSeats, new Set<number>()),
    startWith(new Set<number>()),
    map(set => (set.size ? Array.from(set).join(", ") : this.noneMessage)),
    share()
  );

  readonly cost$ = this.selectSeat$.pipe(
    reduce(registerSeats, new Set()),
    map(selected => selected.size * 3)
  );
}