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>