validation.ts

const isValid$ = (
  merge(
    value$.pipe(
      distinctUntilChanged(),
      map(() => undefined),
    ),
    value$.pipe(
      debounceTime(500),
      withLatestFrom(isComplete$),
      filter(([_, isComplete]) => isComplete),
      switchMap(([value]) => (
        validateValue(value).pipe(
          takeUntil(value$),
        )
      )),
    ),
  );
);