window.onscroll=function(){
//变量scrollTop是滚动条滚动时,滚动条上端距离顶部的距离
var scrollTop = document.documentElement.scrollTop||document.body.scrollTop;
//变量windowHeight是可视区的高度
var windowHeight = document
window.onscroll=function(){
//变量scrollTop是滚动条滚动时,滚动条上端距离顶部的距离
var scrollTop = document.documentElement.scrollTop||document.body.scrollTop;
//变量windowHeight是可视区的高度
var windowHeight = document