1071-GreatestCommonDivisorofStrings.js

var gcdOfStrings = function (str1, str2) {
  if (str1 + str2 !== str2 + str1) return '';
  const gcd = (a, b) => (0 === b ? a : gcd(b, a % b));
  return str1.substring(0, gcd(str1.length, str2.length));
};