两个超大的数字相乘

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("");
}

function multiply(f, t) {
    let current, currentLevel, i, j, mult, a1, a2, zero;
    let fl = f.length;
    let tl = t.length;
    let result = [];
    let fArr = [];
    let tArr = [];
    let maxLen = fl > tl ? fl : tl;
    f = reverse(f);
    t = reverse(t);
    function reverse(s) {
        return s.split('').reverse().join('');
    }
    function addZero(n) {
        let t = '';
        for (j = 0; j < n; j++) {
            t += '0';
        }
        return t;
    }
    for (i = 0; i < maxLen; i++) {
        zero = addZero(i);
        a1 = f[i];
        a2 = t[i];
        if (a1) {
            fArr[i] = [a1, zero];
        }
        if (a2) {
            tArr[i] = [a2, zero];
        }
    }
    for (i = 0; i < fl; i++) {
        current = fArr[i];
        for (j = 0; j < tl; j++) {
            currentLevel = tArr[j];
            result.push((current[0] * 1) * (currentLevel[0] * 1) + current[1] + currentLevel[1]);
        }
    }
    maxLen = result.length;
    mult = result[0];
    for (i = 1; i < maxLen; i++) {
        mult = largeCount(mult, result[i]);
    }
    return mult;
}