s.js

/**
 * @param {string} n
 * @return {string}
 */
var nearestPalindromic = function(n) {
    const nN = BigInt(n);
    if (n == 11n) {
        return "9";
    }
    const palindrome = getUniquePalindrome(n);
    if (palindrome <= 0) {
        return palindrome.toString();
    }
    const nPalindrome = BigInt(palindrome);
    const res = [ palindrome ];
    const cnt = (n + '').length;
    for (let i = 0; i < cnt; i++) {
        res.push(getPalindrome(nN + BigInt(Math.pow(10, i))));
        res.push(getPalindrome(nN - BigInt(Math.pow(10, i))));
    }
    // console.log(res);
    return res.reduce((acc, cur) =>
        cur != n &&
        (
            Math.abs(Number(cur - nN)) < Math.abs(Number(acc - nN)) ||
            (cur < acc && Math.abs(Number(cur - nN)) == Math.abs(Number(acc - nN)))
        ) ? cur : acc, 0n).toString();
};

const getPalindrome = n => {
    const digits = ('' + n).split('');
    for(let i = 0, len = digits.length, mid = (len / 2); i < mid; i++) {
        digits[len-i-1] = digits[i];
    }
    return BigInt(digits.join(''));
};

const getUniquePalindrome = n => {
    let result = getPalindrome(n);

    if (result != n) {
        return result;
    }
    // console.log(result, n, result[Math.ceil(result.length / 2) - 1]);
    result = ('' + result).split('');
    
    if (result[Math.ceil(result.length / 2) - 1] > 1) {
        result[Math.ceil(result.length / 2) - 1] = Number(result[Math.ceil(result.length / 2) - 1]) - 1;
    } else {
        
        result[Math.ceil(result.length / 2) - 1] = Number(result[Math.ceil(result.length / 2) - 1]) + 1;
    }
    console.log(result);
    return getPalindrome(result.join(''));
};