let a = "9007199254740991";
let b = "1234567899999999999";
function add(a, b) {
    let maxLength = Math.max(a.length, b.length);
    a = a.padStart(maxLength, 0);
    b = b.padStart(maxLength, 0);
    let t = 0;
    let f = 0;
    let sum = "";
    for (let i = maxLength - 1; i >= 0; i--) {
        t = parseInt(a[i]) + parseInt(b[i]) + f;
        f = Math.floor(t / 10);
        sum = t % 10 + sum;
    }
    return f == 1 ? "1" + sum : sum;
}
console.log(add(a, b));