How to determine postback with javascript and asp.net mvc?

I am currently using ASP.NET MVC2 and I have a ActionController that allows me to save only if the form content is valid, something like

public ActionResult Edit(Guid id)
{
    //....
    return View();
}

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(Guid id, DTOUserEdit dto)
{
    if ( ModelState.IsValid) Save(user);
    return ModelState.IsValid ? RedirectToAction("Index") : (ActionResult)View(dto);
}

This occasion that, if I'm completing incorrectly the editing form it shows a开发者_Go百科gain repeatedly.

My question is if I can trust in the following piece of javascript code to determine whether a call is a PostBack, in other words if it helps me to know it's not the first time that I'm visiting that page. I read somewhere that I should not trust in document.referrer because some proxies delete it, then what code should I use?

function isPostBack() {
    if (document.referrer != null && document.referrer == window.location.href)
        return true;
    return false;
}

This code could be easily hacked by malicious user or Javascript could be disabled at all.

I don't know exactly your requirements, but I would probably prefer server side check during view rendering. So instead of

<script type="text/javascript">
    if (isPostBack())
    {
        alert("Check your data!");
    }
</script>

I would use

<% 
    if (Request.HttpMethod == "POST") 
    {
%>
    <script type="text/javascript">
        alert("Check your data!");
    </script>
<% 
    }
%>

You could expose a public property on your page :

public bool IsPostBack { get; set; }

Set this property to true / false by checking the !IsPostBack in your Codebehind and assign the value to a hidden variable on the page.

On the client side read the value fromt he hidden variable.

Hope this helps