// The client event EndEditing takes two parameters sender and e
// sender is the object which is raising the event
// e is the TabItemEndEditingEventArgs
function WebTab1_EndEditing(sender, e) {
var tab = sender;
//Gets the index of the tab raising the event
var tabIndex = e.get_tabIndex();
//Gets the old text
var oldText = tab.get_tabs()[tabIndex].get_text();
//Gets the new text
var newText = e.get_text();
//Assuming you have a label called labelOutput on the form
var label = $get('labelOutput');
if (!isNaN(newText)) {
//Cancels the EndEditing event
e.set_cancel(true);
label.innerHTML = "Numbers not allowed!";
}
else
label.innerHTML = "Changed the text from '" + oldText + "' to '" + newText + "' at index " + tabIndex + "!";
}