Check if a string consists of a repeated character sequence

const consistsRepeatedSubstring = (str) => `${str}${str}`.indexOf(str, 1) !== str.length;

const consistsRepeatedSubstring = (str: string): boolean => `${str}${str}`.indexOf(str, 1) !== str.length;

consistsRepeatedSubstring('aa'); // true
consistsRepeatedSubstring('aaa'); // true
consistsRepeatedSubstring('ababab'); // true
consistsRepeatedSubstring('abc'); // false