function indexKeys(object) {
	  var length = object ? object.length : undefined;
	  if (isLength(length) &&
	      (isArray(object) || isString(object) || isArguments(object))) {
	    return baseTimes(length, String);
	  }
	  return null;
	}
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`.