两个超大的数字相加

function largeCount(f, t) {
    f += "";
    t += "";
    let fl = f.length,
        tl = t.length,
        arr = [],
        i = 0,
        spacing = fl - tl,
        len = 0,
        lastMan = [],
        fr = f.split("").reverse(),
        tr = t.split("").reverse(),
        current = 0,
        curr = 0,
        isT, maxLen;
    if (spacing > 0) {
        len = tl;
        lastMan = fr;
        maxLen = fl;
    } else {
        len = fl;
        lastMan = tr;
        maxLen = tl;
    }
    function course(current, i) {
        if (current > 9) {
            arr[i] = current % 10;
            arr[i + 1] = 1;
            return true;
        } else {
            arr[i] = current;
            return false;
        }
    }
    for (; i < len;) {
        curr = arr[i] ? arr[i] : 0;
        current = fr[i] * 1 + tr[i] * 1 + curr * 1;
        course(current, i++);
        if (i === len) {
            if (arr[i]) {
                do {
                    curr = lastMan[i];
                    if (curr) {
                        current = lastMan[i] * 1 + arr[i] * 1;
                        isT = course(current, i++);
                    } else {
                        isT = false;
                    }
                } while (isT && i < maxLen);
            }
            arr = [...arr, ...lastMan.slice(i)];
        }
    }
    return arr.reverse().join("");
}