Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
if (mw.config.get('wgNamespaceNumber') == 10) { // Only if we're in Template: space
 
  // Grab main page tab
  var sandtabMainTab = $('#left-navigation li:not(.selected)').slice(0,1).css('opacity','0.9');

  // Clone main page tab
  var sandtabTab = sandtabMainTab.clone(true).attr('id', sandtabMainTab.attr('id')+'-').css('opacity','0.9').removeClass('new');
 
  // Construct URL
  var sandtabURL = 'http://' + wgPageContentLanguage + '.' + wgNoticeProject + '.org/w/index.php?title=' + wgPageName + '/sandbox';
 
  // Set sandbox tab URL, text, and title 
  sandtabTab.find('a').attr('href', sandtabURL).empty().text('Sandbox').attr('title', "Navigate to this page's sandbox");

  // Get sandbox existence status via Ajax
  $.ajax({
     url:"http://en.wikipedia.org/w/api.php?action=query&titles=" + encodeURIComponent(wgPageName + '/sandbox') + "&format=xml", 
     dataType: "xml", 
     type: "GET",
     success:parseIt
  });
 
  sandtabTab.insertBefore($('#right-navigation li').slice(0,1));

}

// Ajax function on data return: If 'missing' field is defined, page is missing, so add 'new' class to tab, which turns it red
function parseIt (xml){
   var missing = $(xml).find('page').attr('missing');
   if (typeof missing != "undefined") sandtabTab.addClass('new');
}