function bigIntAdd(a, b) {
// 取两数中最大长度
const length = Math.max(a.length, b.length)
// 不足补 0
a = a.padStart(length, '0')
b = b.padStart(length, '0')
let sum = ''
let t = 0
let f = 0 // 进位
for (let i = length - 1; i >=0; i--) {
t = Number(a[i]) + Number(b[i]) + f
f = Math.floor(t / 10)
sum = t % 10 + sum;
}
if (f == 1) {
sum = '1' + sum
}
return sum;
}
let a = "9007199254740991";
let b = "1234567899999999999";
const result = bigIntAdd(a, b);
console.log(result)