上菜：
<html>
<body>
<input id="xxx" />
<script>
function fun()
{
var x = document.getElementById("xxx").value;
if("" == x || isNaN(x))
{
alert("no");
}
else
{
alert("yes");
}
}
</script>
<button οnclick="fun()">judge</button>
</body>
</html>