js中cookie练习

<script type="text/javascript">

        // 创建cookie  修改cookie 指定过期时间
        document.cookie = "username=Tom;expires=Thu, 18 Dec 2013 12:00:00 GMT;path=/";

        //读取cookie的值
        var readcookievalue = document.cookie;

        //删除cookie   不必写cookie的值，指定过去的时间即可
        document.cookie = "username=;expires=Thu, 18 Dec 2013 12:00:00 GMT;";

        //函数设置cookie的值
        function setcookie(cname, cvalue, exdays) {
            var d = new Date();
            d.setTime(d.getTime() + exdays * 24 * 60 * 60 * 1000);
            var expires = "expires=" + d.toGMTString();
            document.cookie = cname + "=" + cvalue + ";" + expires;

        }

        //函数获取cookie
        function getcookie(cname) {

            var name = cname + "=";
            var ca = document.cookie.split(";");

            for (var i = 0; i < ca.length; i++) {

                var c = ca[i].trim();
                if (c.indexOf(name) == 0)
                    return c.substring(name.length, c.length);

            }
            return "";

        }

        //检查cookie操作
        function checkcookie() {
            var user = getcookie("username");
            if (user != "") {

                alert("welcome again:" + user);
            } else {
                user = prompt("please enter your name:", "");
                if (user != "" && user != null) {

                    setcookie("username", user, 365);
                }

            }

        }

    </script>