how to add value to session in javascript code example

Example 1: set a value in session using javascript

<script type="text/javascript">
    function GetUserName()
    {

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

Example 2: set a value in session using javascript

<script type="text/javascript">
function SetUserName()
{
    var userName = "Shahid Bhat";
    '<%Session["UserName"] = "' + userName + '"; %>';
     alert('<%=Session["UserName"] %>');
}
</script>