docs: function union(arr1, arr2) {
      if (arr1.length == 0) {
          return arr2;
      }
  
      var index;
      for (var i = 0; i < arr2.length; i++) {
          if ((index = arr1.indexOf(arr2[i])) !== -1) {
              arr1.splice(index, 1);
          }
      }
      return arr1.concat(arr2);
  }