In controller action:

if(success)
{
    TempData["Alert"] = "Alert message";    
}

The view which SomeAction action returns (or in layout view):




@{
    var message = TempData["Alert"] ?? string.Empty;
 }

<script type="text/javascript">
    var message = '@message';
    if(message)
        alert(message);
</script>






Go to controller: 
 
[HttpPost]
        public ActionResult Index(ModelName objmodel)
        {
            try
            {
      TempData["AlertMessage"] = "Success Alert Message";
                    return RedirectToAction("Index");
     }
            catch(Exception ex)
            {
                TempData["AlertMessage"] = "Exception Message.";
                return RedirectToAction("Index");
            }

        }

In  _Layout.cshtml: 


<head>
@{
    var message = TempData["AlertMessage"] ?? string.Empty;
}
<script type="text/javascript">
    var message = "@message";
    if (message)
        alert(message);
</script>

</head>