ViewBag and ViewData

I'm starting a career in development, and in the project here, I found a ViewData being used, and through my searches I can not update it in a ActionResult in the controller. I ended up testing with a ViewBag too, where I was also unhappy. Remembering that the value of these two will be allocated in page rendering, correct? (Correct me, please)

How could I do to retrieve this values, via ajax , to another question, since my .js functions are in another file, not located within Index.cshtml

Edit 1: In my current scenario, the following is happening:

public IActionResult Index()
{
   // Crio minha ViewData
   ViewData["NomeDaViewData"] = null;
   UpdateViewData(); // aqui onde vou atualizá-la

   return View();
}

private void UpdateViewData()
{
   var dado = _myReposiory.BuscaValor();
   ViewData["NomeDaViewData"] = dado;
}

Soon after in my Index game the value of ViewData in an input hidden, where I can work in my .js file quietly. But I see I can better this way, just do not know how.