docs: function isEmpty(val) {
      return (isUndefined(val) ||
          isNull(val) ||
          (isString(val) && val.length === 0) ||
          (isNumber(val) && isNaN(val)) ||
          (isPlainObject(val) && !array_1.keys(val).length) ||
          (isArray(val) && !val.length));
  }