JS中用for实现n的阶乘

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
</head>
<body>
<p><input type="text" name="jc" );
var i=1;
var sum=1;
for (;i<=x;i++) {
sum=sum*i;
}
res.innerHTML=sum;
}
</script>
</body>
</html>