<script>
 
    var dhxWins = new dhtmlXWindows();
    ...
    // making window modal
    dhxWins.window(id).setModal(true);
    // making window modeless
    dhxWins.window(id).setModal(false);
    // is window modal
    var isModal = dhxWins.window(id).isModal(); // true/false</script>