//获取滚动条距离顶部位置
function getScrollTop() {
 var scrollTop = 0;
 if (document.documentElement && document.documentElement.scrollTop) {
  scrollTop = document.documentElement.scrollTop;
 } else if (document.body) {
  scrollTop = document.body.scrollTop;
 }
 return scrollTop;
}
//获取当前可视范围的高度
function getClientHeight() {
 var clientHeight = 0;
 if (document.body.clientHeight && document.documentElement.clientHeight) {
  clientHeight = Math.min(document.body.clientHeight, document.documentElement.clientHeight);
 } else {
  clientHeight = Math.max(document.body.clientHeight, document.documentElement.clientHeight);
 }
 return clientHeight;
}
//获取文档完整的高度
function getScrollHeight() {
 return Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
}
//判断滚动条是否达到底部
getScrollTop() + getClientHeight() == getScrollHeight()