code: function(items) {
              if(!items)
                  return;
  
              var existingIds = [];
              var lastId;
              for(var i = 0; i < items.length; i++) {
                  if(items[i].id) {
                      existingIds.push(items[i].id);
                      if(!lastId)
                          lastId = items[i].id;
                      else
                          lastId = Math.max(lastId, items[i].id);
                  }
              }
              for(var j = 0; j < items.length; j++) {
                  if(!items[j].id) {
                      lastId++;
                      items[j].id = lastId;
                  }
              }
              return items;
          }
docs: You can use the name convention functions started with underscore are
  private. Really calls to functions starting with underscore are
  filtered, for example:
  
  @example
  $('#element').jqueryPlugin('_pseudoPrivateMethod');  // Will not work
   Method that create ids for items that does not have one