Copy the textbox value into another

-- Script tag ...
<script type="text/javascript">
$(document).ready(function () {
$('input[id$=txtUserName]').keyup(function () {
var txtClone = $(this).val();
$('input[id$=txtUpdateBox]').val(txtClone)
});
});
</script>

------ aspx control.
<asp:TextBox ID="txtUserName" runat="server" />
<asp:TextBox ID="txtUpdateBox" runat="server" />