code: function indexKeys(object) {
          var length = object ? object.length : undefined;
          if (isLength(length) && (isArray(object) || isString(object) || isArguments(object))) {
            return baseTimes(length, String);
          }
          return null;
        }
docs: Creates an array of index keys for `object` values of arrays,
  `arguments` objects, and strings, otherwise `null` is returned.
  
  @private
  @param {Object} object The object to query.
  @returns {Array|null} Returns index keys, else `null`.