Unable to pull data from ViewBag in MVC

I am trying to extract data from a ViewBag and paste it into a hidden form field via JS before submitting.

This does not work.

During debugging, I found that part of the JS expression is ignored, and as a result, "var" evaluates to an empty string.

Here is the part of JS that should perform the assignment:

function setid() {
   var id = '@(ViewBag.id)';
   $('#idField').val(id);
};

in debug I get the following expression, which of course results in a null identifier:

function setid() {
    var id = '';
    $('#idField').val(id);
  };

(I'm not sure this is the only obstacle)

This JS function is called by the argument 'OnBegin' 'Ajax.BeginForm' on the same page.

This is a hidden html hidden field that is in the form I'm trying to assign an id value to:

input type  ="hidden" name ="id" id ="idField" value ="" 

and the next is a controller that returns Partial, which is displayed as a jquery dialog:

public ActionResult openDialog()
    {
        ViewBag.id = TempData["id"];
        return PartialView("EndLoadDetailsDialog");
    }

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