Control ID Changed and GetElemntByID Return NUll

I have this code

<asp:UpdatePanel ID="UpdatePanel3" runat="server">
<ContentTemplate>
<asp:TextBox ID="TxURL" runat="server"></asp:TextBox>
<asp:Button ID="Test" runat="server" Text="Button" OnClick="Test_Onclick" />
</ContentTemplate>
</asp:UpdatePanel>

I try to run this script

var TextControl= document.getElementByID("TxURL")

or

var TextControl = $get("TxURL")

but return value is NULL.

so How, to get Item in Client with the orignal Name. not the generated Name

Thanks.

U may be using Master Pages or UserControl thats why the client id of the control are not the same which u give to the control on the design time

try to run this script

var TextControl= document.getElementByID("<%= TxURL.ClientID %>")

or

var TextControl = $get("<%= TxURL.ClientID %>")