JavaScript实现滚轮控制模拟滚动条

<!DOCTYPE html> 
<html> 
 <head> 
  <meta charset="UTF-8"> 
  <title></title> 
  <style type="text/css"> 
   *{ 
    margin: 0; 
    padding: 0; 
   } 
   #boxwrap{ 
    position: relative; 
    width: 15px; 
    height: 500px; 
    margin: 50px auto; 
    box-sizing: border-box; 
    border: 1px solid gainsboro; 
    border-radius: 6px; 
   } 
   #box{ 
    position: absolute; 
    left: 0px; 
    top: 0px; 
    width: 13px; 
    height: 30px; 
    background: gray; 
    border-radius: 6px; 
   } 
  </style> 
  <script type="text/javascript"> 
   window.onload = function (){ 
    var boxwrp = document.getElementById('boxwrap'); 
    var box = document.getElementById('box'); 
    //兼容firefox 
    if(boxwrp.addEventListener){ 
     document.addEventListener("DOMMouseScroll", fn, false); 
    } 
    document.onmousewheel = fn;//兼容IE、chrome 
     
    function fn(ev){ 
     var ev = ev||event; 
     var bool = false; 
     //IE、chrome 向上：120，向下：-120 
     if(ev.wheelDelta){ 
      bool= ev.wheelDelta > 0? true : false; 
     } 
     //firefox 向上：-3，向下：3 
     else{ 
      bool= ev.detail < 0? true : false; 
     } 
      
     if(bool){ 
      if(box.offsetTop>=10){ 
       box.style.top = box.offsetTop - 10 + "px"; 
      } 
      else{ 
       box.style.top = 0; 
      } 
       
     } 
     else{ 
      if(box.offsetTop<=boxwrp.offsetHeight-box.offsetHeight-10){ 
       box.style.top = box.offsetTop + 10 + "px"; 
      } 
      else{ 
       box.style.top = boxwrp.offsetHeight - box.offsetHeight + "px"; 
      } 
     } 
    } 
   } 
  </script> 
 </head> 
 <body> 
  <div id="boxwrap"> 
   <div id="box"></div> 
  </div> 
 </body> 
</html>