Largest prime palindrome under a max

findLargestPrimePalindrome.js

function findLargestPrimePalindrome(max) {
    if (max === 0) {
        return null;
    }

    if (isPalindrome(max) && isPrime(max)) {
        return max;
    }

    return findLargestPrimePalindrome(max - 1);
}