编辑代码

const add = (a,b) => {
    let maxLength = Math.max(a.length,b.length)
    a = a.padStart(maxLength, 0);
    b = b.padStart(maxLength, 0);

    let f = 0;// 进位
    let s = 0;
    let sum = ""
    for(let i = maxLength - 1; i >= 0; i--){
        s = Number(a[i]) + Number(b[i]) + f;
        f = Math.floor(s/10);
        sum = s%10 + sum;
    }
    if(f == 1){
      sum = "1" + sum;
    }
   return sum;
}

let a = "9007199254740991";
let b = "1234567899999999999";

console.log(add(a ,b))