Add a condition with .length
see below,
if ($(".row_selected").length > 1) { //more than one row selected
alert('Please select one row');
} else if ($(".row_selected").length) { //one row selected
alert('Row(s) are selected.')
} else { // none selected
alert('No rows are selected.')
}
Seems like the row_selected
is applied to the row only on selection so you don't need :visible
check.