回到顶部的功能

<a class="totop" id="to_top"></a>

<script>
/*回到顶部的功能*/
window.onload = function(){
var oTop = document.getElementById("to_top");
var screenh = document.documentElement.clientHeight || document.body.clientHeight;
oTop.style.top = screenh - oTop.offsetHeight + "px";
window.onscroll = function(){
var scrolltop = document.documentElement.scrollTop || document.body.scrollTop;
oTop.style.top = screenh - oTop.offsetHeight + scrolltop +"px";
}
oTop.onclick = function(){
document.documentElement.scrollTop = document.body.scrollTop =0;
}
}
</script>