function sum(a, b) {
    const arrA = a.split('').reverse();
    const arrB = b.split('').reverse();
    let longer = [];
    let theOther = [];
    if (arrA.length > arrB.length) {
        longer = [...arrA];
        theOther = [...arrB];
    } else {
        longer = [...arrB];
        theOther = [...arrA];
    }

    const result = [];
    for (let i = longer.length - 1; i >= 0; i--) {
        if (!theOther[i]) {
            result.push(Number(longer[i]));
            continue;
        }
        result.push(Number(longer[i]) + Number(theOther[i]));
    }

    return result.join('');

}