Need Help on allowing only number on textbox in ASP.Net

Hi,

am using VS2010 and i am trying to block users typing alphabets and special charecters( allowed only numbers). ?I have written javascript and calling onkeydown and onkeyup events of my textbox. seems havascript doesn't firing up. am 

getting Uncaught ReferenceError javascript

below is my  user control code. Not sure why the javascript is not getting fired. Any sugestions please

<asp:TextBox ID="txtAmount" runat="server"
             onkeyup = "keyUP(event.keyCode)" onkeydown = "return isNumeric(event.keyCode);" onpaste = "return false;"
             MaxLength="8" autoComplete = "off"></asp:TextBox>
            <asp:RequiredFieldValidator ID="rfvAmount" runat="server" ControlToValidate="txtAmount"
                Display="Dynamic" ValidationGroup="Rewards" ErrorMessage="*" />

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

  
    var isShift = false;
    function keyUP(keyCode) {
        if (keyCode == 16) {
            isShift = false;
        }
    }

    function isNumeric(keyCode) {
        alert('hi');
        return ((keyCode >= 48 && keyCode <= 57) || keyCode == 8 ||
                (keyCode >= 96 && keyCode <= 105))
    }

</script>