jQuery UI Sortable, how to determine current location and new location in update event?

$('#sortable').sortable({
    start: function(e, ui) {
        // creates a temporary attribute on the element with the old index
        $(this).attr('data-previndex', ui.item.index());
    },
    update: function(e, ui) {
        // gets the new and old index then removes the temporary attribute
        var newIndex = ui.item.index();
        var oldIndex = $(this).attr('data-previndex');
        $(this).removeAttr('data-previndex');
    }
});