How to read Textbox which is inside Ajax tab control into javascript function

How to read the Textbox which is inside Ajax tab container. This is my design

<ajax:TabContainer ID="TabContainer1" runat="server" >
        <ajax:TabPanel ID="tbpnl1" runat="server">
            <HeaderTemplate>
                New User
            </HeaderTemplate>
            <ContentTemplate>
                <asp:Panel ID="UserReg" runat="server">
                    <table align="center">
                        <tr>
                            <td>
                            </td>
                            <td align="right">
                            </td>
                            <td align="center">
                                <b>Registration Form</b>
                            </td>
                        </tr>
                        <tr>
                            <td>
                            </td>
                            <td align="right">
                                UserName:
                            </td>
                            <td>
                                <asp:TextBox ID="txtuser" onblur="alert();"

  runat="server"></asp:TextBox>                           </td>
                        </tr>
                    </table>
                </asp:Panel>
            </ContentTemplate>
        </ajax:TabPanel>

Here is my sample script that I need to call onblur event of text box

<script type="text/javascript">
    function alert() {
        var value = document.getElementById("TabContainer1_tbpnl1_txtuser").value;
        alert(value);
    }
</script>

But didn't work for me can any one help