How to tell if a page unload in ASP is a PostBack

This seems like a common question but search is not returning anything.

I have the following code that executes before the page unloads.The problem is if the unload is a postback i do not want to fire my warning to the user but i can’t figure out how to differentiate between a postback and a user navigating to another page for example.

// This is executed before the page actually unloads
        $(window).bind("beforeunload", function () {

            if (prompt) {

                //prompt
                return true;
            }
            else {

                //reset our prompt variable
                prompt = true;
            }
        })

Running script in the code behind i.e. if Page.IsPostBack then set prompt is not an option.

Any ideas?

EDIT:

Here is the solution I ended up with:

 function DoNotPrompt() {
              prompt = false;
        }

I then added this to all the controls where the user could do something that result in a post back.

OnClientClick="DoNotPrompt()

Then checked this flag and only returned a string in “beforeunload” if the user was really moving away from the page i.e. not a postback.

I also had to use this code:
var magicInput = document.getElementById(‘__EVENTTARGET’);

    if (magicInput && magicInput.value) {
        // the page is being posted back by an ASP control 
        prompt = false;
    }

The reason being i had a custom user control that was a list box and I could not add the above method. So used this to catch that event and set the flag to false.

Not the most elegent solution.

Thanks,
Michael

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Answers:

Method 1

Method 2

Method 3

Answers:

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Method 1

You can capture the submit and reset the onbeforeunload as:

jQuery(function($) {
    var form = $('form'), oldSubmit = form[0].onsubmit;
    form[0].onsubmit = null;

    $('form').submit(function() {       
        // reset the onbeforeunload
        window.onbeforeunload = null;

        // run what actually was on
        if(oldSubmit)           
            oldSubmit.call(this);           
    });
});

This is a tested code from my pages 🙂

Method 2

This may not cover all of the postback situations, but you can tell if the page was posted back by an ASP control by interrogating the __EVENTTARGET hidden input.

This input is set by ASP when the page is posted back by an ASP control.

var magicInput = document.getElementById('__EVENTTARGET');

if (magicInput && magicInput.value) {
   // the page is being posted back by an ASP control
}

Method 3

JavaScript runs on the client; as far as the client is concerned, a page does not maintain state from one view to the next. Postbacks are entirely an ASP.NET concept.

You can get around this by running some code on the server-side which defines a JavaScript variable based on whether or not Page.IsPostBack is true.

Example:

<%@ Page Language="C#" AutoEventWireup="true"  %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title>Page.IsPostBack -> client</title>

    <script type="text/javascript">
        var isPostback = <%= Page.IsPostBack %>;
        console.log("IsPostBack: " + isPostback);
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:Button runat="server" ID="btnTest" Text="Click me..." />
    </div>
    </form>
</body>
</html>

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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