how can stop navigation on the first row and on the last row of row selection from up and down keys?

if (e.keyCode == 40){ //arrow down

         table.$('tr.selected').removeClass('selected');
        tr.next().addClass('selected');
        tr = table.$('tr.selected');

        }
        if (e.keyCode == 38){ //arrow up

         table.$('tr.selected').removeClass('selected');
           tr.prev().addClass('selected');
           tr = table.$('tr.selected');

        }
        data = table.row('tr.selected').data();