为什么打开网页后没有弹窗
```javascript
<script type = "text/javascript">
function Student(name,sex,age){
this.name=name;
this.sex=sex;
this.age=age;
}
var student=new Student("周星星","男",26);
with(student)
{
alert("姓名:"+name+"\n 性别:"+sex+"\n 年龄:"+age);
}
</script>