code: function( index ) {
          var i = index + 1;
          while ( !groups[ i ] && i < groups.length ) {
              i++;
          }
          if ( i < groups.length ) {
              return i;
          }
      }
docs: Get the span element from groups[] that is immediately after given index.
  
  This can be used to find the reference node for an insertBefore() call.
  If no element exists at a larger index, returns undefined. (In this case,
  you'd use appendChild() instead.)
  
  Note that index needn't itself exist in groups[].