how to use session using javascript

<script type="text/javascript">
function SetUserName()
{
    var userName = "Shahid Bhat";
    '<%Session["UserName"] = "' + userName + '"; %>';
     alert('<%=Session["UserName"] %>');
}
</script>BY LOVE
sessionStorage.setItem("SessionName", "SessionValue");<script type="text/javascript">
    function GetUserName()
    {

        var username = '<%= Session["UserName"] %>';
        alert(username );
    }
</script>