code: function union() {
      var index = -1,
          result = [],
          flattened = concat.apply(result, arguments),
          length = flattened.length;
  
      while (++index < length) {
        if (indexOf(result, flattened[index]) < 0) {
          result.push(flattened[index]);
        }
      }
      return result;
    }
docs: Computes the union of the passed-in arrays.
  
  @static
  @memberOf _
  @category Arrays
  @param {Array} [array1, array2, ...] Arrays to process.
  @returns {Array} Returns a new array of unique values, in order, that are
  present in one or more of the arrays.
  @example
  
  _.union([1, 2, 3], [101, 2, 1, 10], [2, 1]);
  // => [1, 2, 3, 101, 10]