function writeCookie(name, value, days){
    var expire = "";
    if(days != null){
        expire = new Date((new Date()).getTime() + days * 3600000);
        expire = "; expires=" + expire.toGMTString();
    }
    document.cookie = name + "=" + escape(value) + expire;
}
writeCookie('screenWidth', screen.width, 365);