function getCookie(name) {
    const cookies = document.cookie ? document.cookie.split(";") : [];

    for (const rawCookie of cookies) {
        const cookie = rawCookie.trim();

        if (cookie.startsWith(`${name}=`)) {
            return decodeURIComponent(cookie.slice(name.length + 1));
        }
    }

    return null;
}

11-2. ヘッダーへ付ける