// The client event StartEditing takes two parameters sender and e
// sender is the object which is raising the event
// e is the TabItemCancelEventArgs
function WebTab1_StartEditing(sender, e) {
var tab = sender;
//Gets the index of the tab raising the event
var tabIndex = e.get_tabIndex();
//Assuming you have a label called labelOutput on the form
var label = $get('labelOutput');
if (tabIndex == 3) {
//Cancels the StartEditing event
e.set_cancel(true);
label.innerHTML = "Editing not allowed for tab with index " + tabIndex +"!";
}
else
label.innerHTML = "Editing started...";
}