window.setInterval()设置时间间隔执行其他.

<script>

var div = document.getElementById('div');

window.setInterval(function(){

   var scrolltop = document.documentElement.scrollTop;

   alert(scrolltop);

   if(scrolltop >= 30) div.style.top = scrolltop+'px'

   else div.style.top = '30px'

},1000)

</script>