[Javascript] How to do big integers sum

/**
 * Big integer sum
 * Using strings to represent big integers
 * @param {string} a
 * @param {string} b
 * @returns {string}
 */
function bigIntSum(a, b) {
  const maxLength = Math.max(a.length, b.length);
  const aStr = a.padStart(maxLength, "0");
  const bStr = b.padStart(maxLength, "0");
  let carry = 0;
  let result = "";
  for (let i = maxLength - 1; i >= 0; i--) {
    const sum = Number(aStr[i]) + Number(bStr[i]) + carry;
    const digit = sum % 10;
    result = `${digit}${result}`;
    carry = Math.floor(sum / 10);
  }

  if (carry === 1) {
    result = `${carry}${result}`;
  }

  return result;
}

const res = sum("1354523252442512423", "999999239");
console.log(res);