怎么样通过JS模拟滚动条效果代码？

var scrollMoveObj = null, scrollPageY = 0, scrollY = 0;
var scrollDivList = new Array();
// obj需要添加滚动条的对象 w滚动条宽度 className滚动条样式名称
// obj元素 必须指定高度，并设置overflow:hidden;
// 如要兼容IE6 必须给obj元素 指定 overflow:hidden; 
function jsScroll(obj, w, className)
{
 if(typeof(obj) == 'string') {
  obj = document.getElementById(obj);
 }
 //当内容未超出现在高度时，不添加滚动条 
 if(!obj || obj.scrollHeight <= obj.clientHeight || obj.clientHeight == 0) {
  return;
 }
 obj.scrollBarWidth = w||6;
 obj.style.overflow = 'hidden';
 obj.scrollBar = document.createElement('div');
 document.body.appendChild(obj.scrollBar);
 obj.scrollBarIndex = document.createElement('div');
 obj.scrollBar.appendChild(obj.scrollBarIndex);
 obj.scrollBar.style.position = 'absolute';
 obj.scrollBarIndex.style.position = 'absolute';
 obj.scrollBar.className = className || '';
 if(!className) {
  obj.scrollBar.style.backgroundColor = '#ddd';
  obj.scrollBarIndex.style.backgroundColor = '#aaa';
 }
 scrollDivList.push(obj);
 scrollResetSize(obj);
 //使用鼠标滚轮滚动
 obj.scrollBar.scrollDiv = obj;
 obj.scrollBarIndex.scrollDiv = obj;
 obj.onmousewheel = scrollMove;
 obj.scrollBar.onmousewheel = scrollMove;
 obj.scrollBarIndex.onmousewheel = scrollMove;
 //拖动滚动条滚动
 obj.scrollBarIndex.onmousedown = function(evt){
  evt = evt || event;
  scrollPageY = evt.clientY;
  scrollY = this.scrollDiv.scrollTop;
  isScrollMove = true;
  document.body.onselectstart = function(){return false};
  scrollMoveObj = this.scrollDiv;
  if(this.scrollDiv.scrollBar.className == '') {
   this.scrollDiv.scrollBarIndex.style.backgroundColor = '#888';
  }
  return false;
 }
}
//当页面大小发生变化时，重新计算滚动条位置
window.onresize = function(){
 for(var i=0; i<scrollDivList.length; i++) {
  scrollResetSize(scrollDivList[i]);
 }
}
//计算滚动条位置
function scrollResetSize(o) {
 if(o.scrollHeight <= o.clientHeight) {
  o.scrollTop = 0;
  o.scrollBar.style.display = 'none';
 } else {
  o.scrollBar.style.display = 'block';
 }
 var x=0, y=0;
 var p = o;
 while(p) {
  x += p.offsetLeft;
  y += p.offsetTop;
  p = p.offsetParent;
 }
 var borderTop = parseInt(o.style.borderTopWidth||0);
 var borderBottom = parseInt(o.style.borderBottomWidth||0);
 o.scrollBar.style.width = o.scrollBarWidth + 'px';
 o.scrollBar.style.height = o.clientHeight + 'px';
 o.scrollBar.style.top = y + borderTop + 'px';
 o.scrollBar.style.left = x + o.offsetWidth - o.scrollBarWidth + 'px';
 o.scrollBarIndex.style.width = o.scrollBarWidth + 'px';
 var h = o.clientHeight - (o.scrollHeight - o.clientHeight);
 //当滚动条滑块最小20个像素
 if(h < 20) {
  h = 20;
 }
 o.scrollBarHeight = h;
 o.scrollBarIndex.style.height = h + 'px';
 o.scrollBarIndex.style.left = '0px';
 setScrollPosition(o);
}
function setScrollPosition(o) {
 o.scrollBarIndex.style.top = (o.clientHeight - o.scrollBarHeight) * o.scrollTop / (o.scrollHeight - o.clientHeight) + 'px';
}
document.documentElement.onmousemove = function(evt){
 if(!scrollMoveObj)return;
 evt = evt || event;
 var per = (scrollMoveObj.scrollHeight - scrollMoveObj.clientHeight) / (scrollMoveObj.clientHeight - scrollMoveObj.scrollBarHeight)
 scrollMoveObj.scrollTop = scrollY - (scrollPageY - evt.clientY) * per;
 setScrollPosition(scrollMoveObj);
}
document.documentElement.onmouseup = function(evt){
 if(!scrollMoveObj)return;
 if(scrollMoveObj.scrollBar.className == '') {
  scrollMoveObj.scrollBarIndex.style.backgroundColor = '#aaa';
 }
 scrollMoveObj = null;
 document.body.onselectstart = function(){return true};
}
// 鼠标滚轮滚动
function scrollMove(evt){
 var div = this.scrollDiv || this;
 if(div.scrollHeight <= div.clientHeight) return true;
 evt = evt || event;
 var step = 20;
 if(evt.wheelDelta < 0) {
  if(div.scrollTop >= (div.scrollHeight - div.clientHeight)) return true;
  div.scrollTop += step;
 } else {
  if(div.scrollTop == 0) return true;
  div.scrollTop -= step;
 }
 setScrollPosition(div);
 return false;
}