当滚动条滚到一定距离，div实现实现效果

 1 window.onscroll=function  () {
 2     var sTop=document.body.scrollTop;
 3     var WH=document.body.clientHeight;
 4     var con=document.getElementById(‘con‘);
 5     var dTop=con.offsetTop;
 6
 7     if (sTop>=dTop-(WH-100)) {
 8         //执行动画
 9     };
10 }