const isPalindrome = num => parseInt(num.toString().split('').reverse().join(''), 10) === num;

// The problem is that I wasn't returning the result of the nextPalindrome recursive call
// I think this works now
const nextPalindrome = (num) => {
  if(isPalindrome(num)) {
    return num;
  } else {
    return nextPalindrome(num + 1);
  }
};

console.log(nextPalindrome(91));