添加图片注释，不超过 140 字（可选）

方式一

const add = (num1, num2) => {
    // 获取最大长度
    const len = Math.max(num1.length, num2.length);
    // 补0
    num1 = num1.padStart(len, 0);
    num2 = num2.padStart(len, 0);
  
    let flag = 0,esult = ``,temp = 0;
    for(let i=len-1; i>=0;  i--){
      temp = flag + parseInt(num1[i]) + parseInt(num2[i])
      result = (temp%10) + result 
      flag = parseInt(temp/10)
    }
    // 判断是否进位
    return result = (flag === 1 ? '1' : '') + result;
} 

const n1 = "9007199254740990"
const n2 = "1229007199254740993443"

add(n1, n2);
 // "1229016206453995734433"

方式二

let n1 = BigInt('111111111111111111111111111111111111111')
let n2 = 222222222222222222222222222222222222222n

console.log(n1 + n2, n1 * n2, n1 - n2);

方式三

var addStrings = function(num1, num2) {
  if (+num1 === 0) return num2
  if (+num2 === 0) return num1
  const arr1 = num1.split('')
  const arr2 = num2.split('')
  let res = ''
  let flag = 0
  while (arr1.length || arr2.length || flag) {
    const a = arr1.length ? +arr1.pop() : 0
    const b = arr2.length ? +arr2.pop() : 0
    const s = a + b + flag
    const t = s % 10
    flag = s >= 10 ? 1 : 0
    res = t + res
  }
  return res
};

方式四

function addStrings(num1: string, num2: string): string {
  let i = num1.length - 1, j = num2.length - 1, add = 0;
  const ans: number[] = [];

  while(i >= 0 || j >= 0 || add!= 0) {
    const x = i >= 0 ? Number.parseInt(num1.charAt(i), 10) : 0;
    const y = j >= 0 ? Number.parseInt(num2.charAt(j), 10) : 0;
    const r = x + y + add;
    ans.unshift(r % 10);
    add = Math.floor(r / 10);
    i -= 1;
    j -= 1;
  }

  return ans.join('');
};

方式五

var addStrings = function (num1, num2) {
  let aLen = num1.length - 1;
  let bLen = num2.length - 1;
  let remainder = 0;
  let result = [];
  while (aLen >= 0 || bLen >= 0 || remainder !== 0) { // 注意最后一个进位
    const a = aLen >= 0 ? num1.charAt(aLen) - '0' : 0;
    const b = bLen >= 0 ? num2.charAt(bLen) - '0' : 0;
    const tempResult = a + b + remainder;
    result.push((tempResult) % 10);
    remainder = Math.floor(tempResult / 10);
    aLen -= 1;
    bLen -= 1;
  }
  return result.reverse().join('');
};