就是我们常见的网页，当我们往下浏览的时候先是隐藏的元素
开始可见然后执行动画效果
这种效果是具体怎么实现的楠？我怎么知道我往下浏览的时候 到什么位置开始元素不隐藏了 然后执行动画。并且只要我不刷新页面第二次往下浏览的时候 动画不在执行。因为现在页面元素已经不是隐藏状态了。
ringa_lee
JS determines the position, adds class and deletes class
Determine the height of the scroll bar. If you need the animated element to appear on the screen, add a class and use css3 transition to animate it
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
JS determines the position, adds class and deletes class
Determine the height of the scroll bar. If you need the animated element to appear on the screen, add a class and use css3 transition to animate it