generateDraftTitle = title => {

  let c = 0,
    draftTitle;
  do {
    draftTitle = 'Draft ' + (c ? (c + 1) + ' ' : '') + 'of \'' + title + '\'';
    c++;
  } while ($tw.wiki.tiddlerExists(draftTitle));
  return draftTitle;

}
Modified TW-Code from Navigator widget
https://github.com/Jermolene/TiddlyWiki5/blob/master/core/modules/widgets/navigator.js