js获取cookie

function get_Cookie(cookieName) {
    var cookieString = document.cookie;
    var start = cookieString.indexOf(cookieName + '=');
    if (start == -1)
        return null;
    start += cookieName.length + 1;
    var end = cookieString.indexOf(';', start);
    if (end == -1) return unescape(cookieString.substring(start));
    return unescape(cookieString.substring(start, end));
}