Key in only Numbers in Text Box using Java Script For ASP.Net

//JavaScript Function:

<script language="javascript" type="text/javascript">

function num()

{

if (event.keyCode <> 57) {

event.keycode = 0;

return false;

}

else

return true;

}

script>

//Function Call in text box:
<asp:TextBox ID="TExternalDocumentNo" onkeyPress="return num()" runat="server">asp:TextBox>