function getCookie(cname) {
    let name = cname + "=";
    let ca = document.cookie.split(";");
    for (let i = 0; i < ca.length; i++) {
        let c = ca[i].trim();
        if (c.indexOf(name) == 0) {
            return c.substring(name.length, c.length);
        }
    }
}

调用：

getCookie("user_device")