Number Validation Using Javascript

<script language="javascript">      
function isNumberKey(evt)      
{
  var charCode = (evt.which) ? evt.which : event.keyCode        
  if (charCode > 31 && (charCode < 48 || charCode > 57))
  {
   alert("Enter Only Numbers");            
   return false;
  }          
   return true;      
}        
</script >  

 <asp:Button ID="btnSubmit" Text="Submit" onkeypress="return isNumberKey(event)" runat="server" OnClick="Submit_Onclick" />