There is a small issue in the solution provided. if the input is "0009", then the output should be 9 (which is largest palindromic number) but this solution returns "0", as the first half is full zeroes. May be the below code will help.

class Solution {

largestPalindromic(num) {

const freq = {};

let firstHalf = '',

  middle = '';



for (let i = 0; i < num.length; i++) {

  freq[num[i]] = (freq[num[i]] || 0) + 1;

}

for (let i = 9; i >= 0; i--) {

  if (freq[i] && freq[i] % 2 !== 0 && middle === '') {

    middle = i.toString();

  }

  firstHalf += i.toString().repeat(Math.floor(freq[i] / 2));

}



if (firstHalf === '' ||firstHalf.match(/^0+$/) ) {

  return middle === '' ? 0 : middle;

} 

/* else if (firstHalf.match(/^0+$/)) {

  return 0;

} */



return firstHalf + middle + [...firstHalf].reverse().join('');

}

}