表单验证必须为纯数字

<form name="myForm" action="" οnsubmit="return validateForm()" method="post">
姓名: <input type="text" name="fname">
<input type="submit" value="提交">
</form>
<script>
function validateForm()
{
  var x=document.forms["myForm"]["fname"].value;
  if(x==""||isNaN(x)||y==""||isNaN(y))
{
alert("姓名必须为数字！");
return false
}
else
{return true}
}
</script>