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前阵子在公司实现一个常见的需求，就是添加一个返回顶部的按钮，当滑动网页到一定程度后会出现这个按钮，需要获取到元素的滚动高度scrollTop
。结果在pc端的浏览器可以生效，pc端浏览器中的移动端模式可以生效，在苹果手机上的微信浏览器也可以生效，唯独在安卓手机的微信浏览器不能生效。先展示一下核心代码：
// 用vue写的项目，scrollToTop控制按钮显示与否，当滚动高度大于1000像素就出现按钮 handleScroll () { let
scrollTop= document.documentElement.scrollTop if (scrollTop > 1000) { this.
scrollToTop= true } else { this.scrollToTop = false } }
后来发现在安卓手机的微信浏览器中scrollTop的值是0，上网查了一下，解决办法是针对不同的浏览器需要做一个兼容处理：
handleScroll () { let scrollTop = document.documentElement.scrollTop || window.
pageYOffset|| document.body.scrollTop if (scrollTop > 1000) { this.scrollToTop =
true } else { this.scrollToTop = false } }
解决是解决了，但是为什么呢？这几个写法有什么区别呢？
* document.documentElement在页面具有DTD，或者说指定了DOCTYPE 时可以使用，即可以取到scrollTop的值；
* document.body在页面不具有DTD，或者说没有指定了DOCTYPE时使用；
* window.pageYOffset表示当前文档从窗口左上角垂直滚动的像素数。
以后遇到这种类型的兼容性问题一般这样处理就行了。
