vanilla js でクッキーの値を取得 参考:https://developer.mozilla.org/ja/docs/Web/API/Document/cookie
document.cookie = "test1=Hello";
document.cookie = "test2=World";
var cookieValue = document.cookie.replace(/(?:(?:^|.*;\s*)test2\s*\=\s*([^;]*).*$)|^.*$/, "$1");
function alertCookieValue() {
alert(cookieValue);
}