docs: function normalizeList(items) {
      if (!items) {
          return [];
      }
      var list;
      if (Array.isArray(items)) {
          list = [].concat(items);
      } else if (typeof items === 'string') {
          list = items.split(/[\s,]+/g).filter(Boolean);
      } else {
          throw new Error('Invalid items: ' + items);
      }
      return list;
  }