编辑代码

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)