docs: function combine(arr1, arr2) {
          if (arr2 == null) {
              return arr1;
          }
  
          var i = -1, len = arr2.length;
          while (++i < len) {
              if (indexOf(arr1, arr2[i]) === -1) {
                  arr1.push(arr2[i]);
              }
          }
  
          return arr1;
      }