大数相加解决

因为有js有数字大小限制，所以只能转成字符串一位一位的相加

function add(a, b) {
  const arrA = a.split('').reverse();
  const arrB = b.split('').reverse();
  const result = [];

  const type = (a) => {
    if (a && !isNaN(Number(a))) {
      return Number(a);
    }
    return 0;
  }

  let tem = 0;
  for (let index = 0; index < Math.max(arrA.length, arrB.length); index++) {
    const mm = type(arrA[index]);
    const nn = type(arrB[index]);
    const next = nn + mm + tem;
    tem = next > 9 ? 1 : 0;
    result.unshift(next % 10);
  }
  tem > 0 && result.unshift(1);
  return result.join('');
}

console.log(add('9991111111', '9999999999999999999999'));