How do I popup runtime message using confirmation?

I used OnClientClick="javascript(myConfirm)" of update button to ask users for confirmation.  The message includes user's selection of gridview (checkbox).  The issue is that client function is ahead of server side's updating the user's selections on different gridview's page (the myConfirm needs to force one more PostBack to pick user's selection on new gridview's page).  How can I do it?
function myConfirm() {var selectedKeys=document.getElementByID("LabelSelectedKeys");  confirm("You selected"+selectedKeys.Text);}
LabelSelectedKeys are viewstate tracked user's selected entries cross gridview's pages.  I'll give exampe: user select 1,2 on page1 and then move to page 9 to select 90, 20.   User click the update but lat two selection is not showing in the confirm (though the final update command is fine as the button's postback take effects).   Is there anyway to solve this (like force postback on myConf ...