AJAX :: How To Read Textbox Which Is Inside 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>

[Code] ....

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....