Question

I need to do a postback and save the data in the screen to session when the screen is closed, for this i am using the onBeforeUnload event, and placed a hidden button in the screen. In the on before unload event I am calling the click event to call the button server event. But the event is not firing. Is anything I am missing here.

<asp:Button Style="display: none" runat="server" ID="btnHidUpdate" OnClick="btnHidUpdate_Click" />
<script type="text/javascript">
  $(document).ready(function () {
    window.onbeforeunload = updateSessionBeforeUnload; 
  });
</script>

In .js file:

function updateSessionBeforeUnload() {
  var hidUpdate = $('[id$="btnHidUpdate"]')[0];
  hidUpdate.click();
}

In .cs code behind:

protected void btnHidUpdate_Click(object sender, EventArgs e)
{
  UpdateSession();
}

Answer 1

The problem is that the page loads the next page before it can execute the button click.

Use the Jquery Unload event e.g.

$(function(){
    $(window).unload(function(){
       // put synchronous code here to persist what you need.
    });
});

You can use an Ajax event like Yuriy says however you must set async to false like this:

      $.ajax({
           type: "POST",
           url: "Url/To/Persistance",
           dataType: "json",
           data : { "data" : "to be persisted"},
           async: false
      });

EDIT
I would avoid the click event all together and do something like this:

$(function(){
    $(window).unload(function(event){
       var hidUpdate = $('[id$="btnHidUpdate"]')[0];
       __doPostBack(hidUpdate.attr('id'),'');
    });
});

However if you must click the button try this

$(function(){
    $(window).unload(function(event){
       var hidUpdate = $('[id$="btnHidUpdate"]')[0];
       hidUpdate.click()
       alert(
         "Default: " + event.isDefaultPrevented() + "\n" + 
         "ImedPropStopped: " + event.isImmediatePropagationStopped() + "\n" + 
         "PropStopped: " + event.isPropagationStopped()
       );
    });
});

And tell us what the alert says ?