How to get session value using javascript

I have a class to deal with session variables. Here is a sample attached:

namespace General
{

    public class Session
    {
        public Session()
        {
        }


        public static string UserID
        {
            get { return HttpContext.Current.Session["UserID"] as string ?? String.Empty; }
            set { HttpContext.Current.Session["UserID"] = value;  }
        }

        public static string departFlightID
        {
            get { return HttpContext.Current.Session["departFlightID"] as string ?? String.Empty; }
            set { HttpContext.Current.Session["departFlightID"] = value; }
        }

        public static string returnFlightID
        {
            get { return HttpContext.Current.Session["returnFlightID"] as string ?? String.Empty; }
            set { HttpContext.Current.Session["returnFlightID"] = value; }
        }
}

}

Now at some point I store the flightID in:

General.Session.departFlightID = flightID;

And at another point I want to retrieve this value using Javascript. I have seen examples here but they won't work (there's no error but they will return EMPTY).

Most recent tries:

 var session = '<%= General.Session.departFlightID %>';
 var session = '<%= HttpContext.Current.Session["departFlightID"] %>';

It does work if I set value on page load, but I am running a webmethod where I create a html and then send that html back to be populated inside a div to make a ticket there. I need to get the session value but it does not update.

To make it more clear here is the Javascript code:

<script type="text/javascript">


function addTicket(flightID, flightType) {
    PageMethods.addTicket(flightID, flightType, OnGetMessageSuccess);
}

function OnGetMessageSuccess(result, userContext, methodName) {

    var pageUrl = document.URL;
    if (pageUrl.indexOf("&ret_date=&") !== -1) {

        var session = '<%= HttpContext.Current.Session["departFlightID"] %>';

        alert(session);
        result = result + "<a onclick=\"searchflight.abortAllAjax()\" data-index=\"6\" class=\"bookNow\" title=\"Book Now!\" href=\"#\">Book Now!</a>";
        result = result + "</div>    </div>  </div>";
        document.getElementById("detail").innerHTML = result;
    }

}

And here is the webmethod:

[System.Web.Services.WebMethod]
    public static string addTicket(string flightID, string flightType)
    {
        //GET FLIGHT DETAILS  FROM DATATABLE OR SESSION DATA TABLE
        string tmpHtml = string.Empty;
        tmphtml="sample";
        string flightID="123123213";
        General.Session.departFlightID=flightID;

    return tmphtml;
    }