var newWin;

function centeredWindow(url, winname, w, h, scrollbars) {

    windowleft = (screen.width - w) / 2;
    windowtop = (screen.height - h) / 2;
    settings = '"toolbar=yes, directories=yes, menubar=yes, scrollbars=' + scrollbars + ', resizable=yes, status=yes, width=' + w + ', height=' + h + ', left=' + windowleft + ', top=' + windowtop + ' "';
    closewin();
    newWin = window.open(url, winname, settings);
    newWin.focus();
}

function closewin() {

    if (newWin && !newWin.closed) {
        newWin.close();
    }

}