判断滚动条是否到达底端

 1         //取窗口滚动条高度 
 2         function getScrollTop() {
 3             var scrollTop = 0;
 4             if (document.documentElement && document.documentElement.scrollTop) {
 5                 scrollTop = document.documentElement.scrollTop;
 6             }
 7             else if (document.body) {
 8                 scrollTop = document.body.scrollTop;
 9             }
10             return scrollTop;
11         }
12 
13 
14         // 取窗口可视范围的高度 
15         function getClientHeight() {
16             var clientHeight = 0;
17             if (document.body.clientHeight && document.documentElement.clientHeight) {
18                 var clientHeight = (document.body.clientHeight < document.documentElement.clientHeight) ? document.body.clientHeight : document.documentElement.clientHeight;
19             }
20             else {
21                 var clientHeight = (document.body.clientHeight > document.documentElement.clientHeight) ? document.body.clientHeight : document.documentElement.clientHeight;
22             }
23             return clientHeight;
24         }
25 
26         // 取文档内容实际高度 
27         function getScrollHeight() {
28             return Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
29         }

滚动条到达底部的条件：getScrollTop() + getClientHeight() == getScrollHeight()