Accessing ASP HiddenField in javascript
<p>I have been searching through here and google for a few days now, trying to figure out why I cannot get the value of a hiddenfield variable in javascript.  When accessed, the value is returned as undefined.<br>
I have an ASP HiddenField inside an UpdatePanel which is part of a custom user control in a .aspx web page (standard issue).  </p>

<p>In my user control, I need to get the .Value of the HiddenField (hdnServer) in javascript after setting it in C#.  But for some reason the following is not getting the correct value.  </p>

<p>The MessageBox in the C# code returns the correct value (the code here has test values), but when accessed in javascript is undefined.  </p>

<p>userControl.ascx:</p>

<pre><code>//this function is called when the timer created in document.ready() elapses
//returns the correct hdnServer value in the check. 
    var checkHdn = function () {
        var temp = document.getElementById("&lt;%=hdnServer.ClientID%&gt;").value;
        temp = temp.toString();
        if (temp != "") {
            $('#LoadingViewer').hide();
            clearInterval(checkSrv);

            //enable start button
            $('#startBtn').attr("Enabled", "true");
        }
    };

  function RdpConnect() {

                //serverName = undefined here.  should be ip address when set in c# 
                var serverName = document.getElementById("&lt;%= hdnServer.ClientID %&gt;").value;
                alert(serverName);
                if (serverName != "") {
                    MsRdpClient.Server = serverName;
                }
            };
</code></pre>

<p>userControl.ascx.cs code-behind:</p>

<pre><code>public partial class userControl : System.Web.UI.UserControl
    {
        System.Timers.Timer timer; 

        protected void Page_Load(object sender, EventArgs e)
        {
            timer = new System.Timers.Timer(5000);
            timer.Start();
        }

        protected void testOnTick(object sender, System.Timers.ElapsedEventArgs e)
        {
                hdnServer.Value = "test value";
                startBtn.Enabled = true;
                timer.Enabled = false;
        }
    }
</code></pre>

<p>Here is the asp for HiddenField just in case: userControl.ascx:</p>

<pre><code>&lt;asp:ScriptManager ID="ScriptManager1" runat="server" /&gt;
&lt;asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Always"&gt;
    &lt;Triggers&gt;
         &lt;!--trigger not used  --&gt;
       &lt;!-- &lt;asp:AsyncPostBackTrigger ControlID="Timer1" EventName="Tick" /&gt;--&gt;
    &lt;/Triggers&gt;
    &lt;ContentTemplate&gt;
        &lt;asp:HiddenField ID="hdnServer" runat="server" /&gt;
        &lt;asp:Label ID="Label1" Text="Loading, please wait." CssClass="loading" runat="server"
            Font-Size="XX-Large" /&gt;
    &lt;/ContentTemplate&gt;
&lt;/asp:UpdatePanel&gt;
</code></pre>

<p>Thank you for any advice in advance!</p>

<p>EDIT:  messagebox removed..
Here is rendered html:  <a href="http://pastie.org/3122247" rel="nofollow">http://pastie.org/3122247</a> </p>
