NavigateEvent: свойство formData

navigation.addEventListener("navigate", (event) => {
  // Some navigations, e.g. cross-origin navigations, we
  // cannot intercept. Let the browser handle those normally.
  if (!event.canIntercept) {
    return;
  }

  // Don't intercept fragment navigations or downloads.
  if (event.hashChange || event.downloadRequest !== null) {
    return;
  }

  event.intercept({
    handler() {
      if (event.formData) {
        processFormDataAndUpdateUI(event.formData, event.signal);
      } else {
        doSinglePageAppNav(event.destination, event.signal);
      }
    },
  });
});