// The client event MouseOut takes two parameters sender and e
        // sender  is the object which is raising the event
        // e is the TabMouseEventArgs
        function WebTab1_MouseOut(sender, e) {
            var tab =sender;
            //Gets the index of the tab involved in the MouseOut event
            var tabIndex = e.get_tabIndex();
            //Gets index of scroll button involved in mouse event. 
            //Possible values: 0 - right or top scroll button, 1 - left or bottom scroll button,
            // -1 - it is event over tab item or add-new-tab button
            var scrollButtonIndex = e.get_scrollButtonIndex();
//Assuming you have a label called labelOutput on the form
            var label = $get('labelOutput');
            if (e.isAddNewTabButton()) {
                label.innerHTML = "Mouse out of add new tab button";
            }
            else if (e.isCloseButton()) {
                label.innerHTML = "Mouse out of close button with tab index" + tabIndex;
            }
            else
                if (tabIndex != -1) {
                label.innerHTML = "Mouse out of the tab with index '" + tabIndex + "'";
            }
            else {
                if (scrollButtonIndex == 0) {
                    label.innerHTML = "Mouse out of the right scroll button";
                }
                else
                    label.innerHTML = "Mouse out of the left scroll button";
            }    
        }