code:

function PreloadDestinationThing() : ReactElement {
  const router = useRouter();
  const [href, setHref] = useState<string>(null);
  useEffect(() => {
    if (router.isReady && href != null) {
      let dest;
      if (Array.isArray(router.query.dest)) {
        dest = router.query.dest[0] ?? '/';
      } else {
        dest = router.query.dest ?? '/';
      }
      router.prefetch(dest);
      setHref(dest);
    }
  }, [router, router.isReady, href]);

...