How to access Session variables and set them in javascript?

Javascript can not directly set session values. For setting session values from javascript I do ajax call as follows.

Check Online

At ASPx file or html,

 <script type="text/javascript">
 $(function(){
   //Getting values from session and saving in javascript variable.
   // But this will be executed only at document.ready.
   var firstName = '<%= Session["FirstName"] ?? "" %>';
   var lastName = '<%= Session["LastName"] ?? "" %>';

   $("#FirstName").val(firstName);
   $("#LastName").val(lastName);

   $('Button').click(function(){
     //Posting values to save in session
     $.post(document.URL+'?mode=ajax', 
     {'FirstName':$("#FirstName").val(),
     'LastName':$("#LastName").val()
     } );
   });

 });

At Server side,

protected void Page_Load(object sender, EventArgs e)
 {
      if(Request.QueryString["mode"] != null && Request.QueryString["mode"] == "ajax")
      {
        //Saving the variables in session. Variables are posted by ajax.
        Session["FirstName"] = Request.Form["FirstName"] ?? "";
        Session["LastName"] = Request.Form["LastName"] ?? "";
      }
 }

For getting session values, as said by Shekhar and Rajeev

var firstName = '<%= Session["FirstName"] ?? "" %>';

Hope this helps.

Accessing & Assigning the Session Variable using Javascript:

Assigning the ASP.NET Session Variable using Javascript:

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

Accessing ASP.NET Session variable using Javascript:

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

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

You can't access Session directly in JavaScript.

You can make a hidden field and pass it to your page and then use JavaScript to retrieve the object via document.getElementById