How to get the button id using javascript?

Hi,

Usercontrol have a button with id=hdnBtn as below,

<pre lang="asp">
&lt;%@ Control Language="C#" AutoEventWireup="true" CodeBehind="Header.ascx.cs" Inherits="M2iSTS2.UserControl.Header" %&gt;

&lt;link href="/Styles/styles.css" rel="stylesheet" type="text/css" /&gt;
&lt;script language="javascript" type="text/javascript" src="../Script/Map.js"&gt;&lt;/script&gt;

&lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;

     &lt;input type="button" value="Button1" id="hdnBtn" runat="server" onserverclick="assignValue"/&gt;
 &lt;div id="header" class="header"&gt;
    &lt;h1 class="h1"&gt;TrackU: Student and Vehicle Tracking System&lt;/h1&gt;
        &lt;p class="p"&gt;&lt;a class="a" href="/Account/login.aspx"  önclick='setSession()'&gt;Logout&lt;/a&gt;&lt;/p&gt;
  &lt;/div&gt;
  &lt;div&gt;
    &lt;ul class="ul"&gt;
        &lt;li class="li"&gt;&lt;a class="lia" href="/Admin/Fleet.aspx"&gt;Fleet&lt;/a&gt;&lt;/li&gt;
        &lt;li class="li"&gt;&lt;a class="lia" href="/Admin/StudentInfo.aspx"&gt;Student Info&lt;/a&gt;&lt;/li&gt;
        &lt;li class="li"&gt;&lt;a class="lia" href="/Admin/RouteSettings.aspx"&gt;Route Settings&lt;/a&gt;&lt;/li&gt;
        &lt;li class="li"&gt;&lt;a class="lia" href="/Admin/Reports.aspx"&gt;Reports&lt;/a&gt;&lt;/li&gt;
    &lt;/ul&gt;
  &lt;/div&gt;


</pre>

javscript function as below,

<pre lang="javscript">
function setSession() {
    alert(document.getElementById('hdnBtn'));

}
</pre>

but the <pre lang="cs">alert(document.getElementById('hdnBtn'));</pre>

giving the 'null'.why? how to get the id of button in usercontrol?

Thanks