Javascript Function to Allow Only Numbers in Textbox

 function isNumber(evt)
    {
        evt = (evt) ? evt : window.event;
        var charCode = (evt.which) ? evt.which : evt.keyCode;
        if (charCode > 31 && (charCode < 48 || charCode > 57)) {
            return false;
        }
        return true;
    }


<asp:TextBox ID="txt_AdmnNo" runat="server" ontextchanged="TextBox1_TextChanged" onkeyup="TextBox1_TextChanged" AutoPostBack="true"
    onkeypress="return isNumber(event)"></asp:TextBox>