Javascript模拟scroll滚动效果脚本

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> 
<html> 
<head> 
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 
<title>Insert title here</title> 
<script type="text/javascript"> 
var oEventUtil = new Object(); 
oEventUtil.addEventHandler = function (oTarget,sType,fnDo){ 
    if(oTarget.addEventListener){ 
        oTarget.addEventListener(sType,fnDo,false) 
    } 
    if(oTarget.attachEvent){ 
        oTarget.attachEvent("on"+sType,fnDo) 
    } 
    else{ 
        oTarget["on"+sType] = fnDo; 
    } 
} 
oEventUtil.removeEventHandler = function (oTarget,sType,fnDo){ 
    if(oTarget.removeEventListener){ 
        oTarget.removeEventListener(sType,fnDo,false) 
    } 
    if(oTarget.detachEvent){ 
        oTarget.detachEvent("on"+sType,fnDo) 
    } 
    else{ 
        oTarget["on"+sType] = null; 
    } 
} 
oEventUtil.getEvent = function() { 
if (window.event) { 
return this.formatEvent(window.event); 
} else { 
return oEventUtil.getEvent.caller.arguments[0]; 
} 
} 
oEventUtil.formatEvent = function (oEvent) 
{ 
oEvent.pageY = oEvent.clientY + document.body.scrollTop; 
return oEvent; 
} 
function areaScroll(){ 
    var scroll_hand_top = parseInt(scrollup.offsetHeight); 
var scroll_hand_bottom = parseInt(category_list.offsetHeight) - parseInt(scrolldown.offsetHeight) - parseInt(scroll_hand.offsetHeight); 
    category_list.scrollTop = (parseInt(category_list.scrollHeight)-parseInt(category_list.offsetHeight) + parseInt(scrollup.offsetHeight))*(parseInt(scroll_hand.style.top)-parseInt(scrollup.offsetHeight))/(parseInt(scroll_hand_bottom)-parseInt(scrollup.offsetHeight))-parseInt(scrollup.offsetHeight);

} 
function clickNarrowup(){ 
var oEvent = oEventUtil.getEvent(); 
    var scroll_hand_top = parseInt(scrollup.offsetHeight); 
    var scroll_hand_bottom = parseInt(category_list.offsetHeight) - parseInt(scrolldown.offsetHeight) - parseInt(scroll_hand.offsetHeight); 
     
    if(parseInt(scroll_hand.offsetTop) <= parseInt(scroll_hand_bottom) && parseInt(scroll_hand.offsetTop) >= parseInt(scroll_hand_top)){ 
        scroll_hand.style.top = parseInt(scroll_hand.offsetTop) - 10 +"px"; 
        if(parseInt(scroll_hand.style.top) >= parseInt(scroll_hand_bottom)){ 
            scroll_hand.style.top = parseInt(scroll_hand_bottom)+"px"; 
        } 
        if(parseInt(scroll_hand.style.top)<=parseInt(scroll_hand_top)){ 
            scroll_hand.style.top = parseInt(scroll_hand_top)+"px"; 
        } 
        areaScroll();         
    }     
} 
function clickNarrowdown(){ 
var oEvent = oEventUtil.getEvent(); 
    var scroll_hand_top = parseInt(scrollup.offsetHeight); 
var scroll_hand_bottom = parseInt(category_list.offsetHeight) - parseInt(scrolldown.offsetHeight) - parseInt(scroll_hand.offsetHeight); 
    if(parseInt(scroll_hand.offsetTop) <= parseInt(scroll_hand_bottom) && parseInt(scroll_hand.offsetTop) >= parseInt(scroll_hand_top)){ 
        scroll_hand.style.top = parseInt(scroll_hand.offsetTop) + 10 +"px"; 
        if(parseInt(scroll_hand.style.top) > parseInt(scroll_hand_bottom)){ 
            scroll_hand.style.top = parseInt(scroll_hand_bottom)+"px"; 
        } 
        if(parseInt(scroll_hand.style.top)<parseInt(scroll_hand_top)){ 
            scroll_hand.style.top = parseInt(scroll_hand_top)+"px"; 
        } 
        areaScroll(); 
    } 
} 
function mousedown(e){ 
var oEvent = oEventUtil.getEvent(); 
y = parseInt(oEvent.pageY) - parseInt(scroll_hand.offsetTop); 
oEventUtil.addEventHandler(scroll_hand, "mousemove", mousemove); 
oEventUtil.addEventHandler(scroll_hand, "mouseup", mouseup); 
oEventUtil.addEventHandler(scroll_hand, "mouseout", mouseout); 
} 
function mousemove(e){ 
    var oEvent = oEventUtil.getEvent(); 
    var scroll_hand_top = parseInt(scrollup.offsetHeight); 
    var scroll_hand_bottom = parseInt(category_list.offsetHeight) - parseInt(scrolldown.offsetHeight) - parseInt(scroll_hand.offsetHeight); 
     
    if(parseInt(scroll_hand.offsetTop) <= parseInt(scroll_hand_bottom) && parseInt(scroll_hand.offsetTop) >= parseInt(scroll_hand_top)){ 
        scroll_hand.style.top = parseInt(oEvent.pageY) - y + "px"; 
        if(parseInt(scroll_hand.style.top) > parseInt(scroll_hand_bottom)){ 
            scroll_hand.style.top = parseInt(scroll_hand_bottom)+"px"; 
        } 
        if(parseInt(scroll_hand.style.top)<parseInt(scroll_hand_top)){ 
            scroll_hand.style.top = parseInt(scroll_hand_top)+"px"; 
        } 
        areaScroll(); 
    } 
} 
function mouseup(e){ 
    oEventUtil.removeEventHandler(scroll_hand,"mousemove",mousemove); 
    oEventUtil.removeEventHandler(scroll_hand,"mouseup",mouseup); 
} 
function mouseout(e){ 
    oEventUtil.removeEventHandler(scroll_hand,"mousemove",mousemove); 
    oEventUtil.removeEventHandler(scroll_hand,"mouseout",mouseout); 
} 
function displayScrollbar(){ 
    if(category_list.scrollHeight > category_list.offsetHeight){ 
        category_scroll.style.display="block"; 
    } 
    else{category_scroll.style.display="none";} 
} 
window.onload = function () { 
//var category_scroll = document.getElementById("category_scroll"); 
    //var scroll_hand = document.getElementById("scroll_hand"); 
    //var scrollup = document.getElementById("scrollup"); 
    //var scrolldown = document.getElementById("scrolldown"); 
    //var category_list = document.getElementById("category_list"); 
category_list.scrollTop = 0; 
displayScrollbar(); 
oEventUtil.addEventHandler(scroll_hand,"mousedown",mousedown); 
    oEventUtil.addEventHandler(scrollup,"click",clickNarrowup); 
    oEventUtil.addEventHandler(scrolldown,"click",clickNarrowdown); 
} 
</script> 
<style type="text/css"> 
.category_list{width:200px;border:solid 1px #666666;height:300px;position:relative;} 
#category_list{list-style:none;margin:0;padding:0;padding-left:20px;overflow:hidden;height:298px;} 
#category_scroll{height:298px;width:15px;border:green solid 1px;position:absolute;right:0;top:0;font-size:12px;cursor:hand;} 
#scrollup{height:15px;width:15px;border:green solid 1px;} 
#scroll_hand{height:100px;width:15px;border:green solid 1px;position:absolute;} 
#scrolldown{height:15px;width:15px;border:green solid 1px;position:absolute;bottom:0;} 
</style> 
</head> 
<body> 
<div class="category_list"> 
<ul id="category_list"> 
    <li>1something</li> 
    <li>2something</li> 
    <li>3something</li> 
    <li>4something</li> 
    <li>5something</li> 
    <li>6something</li> 
    <li>7something</li> 
    <li>8something</li> 
    <li>9something</li> 
    <li>10something</li> 
    <li>11something</li> 
    <li>12something</li> 
    <li>13something</li> 
    <li>14something</li> 
    <li>15something</li> 
    <li>16something</li> 
    <li>17something</li> 
    <li>18something</li> 
    <li>19something</li> 
    <li>20something</li> 
    <li>21something</li> 
    <li>22something</li> 
    <li>23something</li> 
    <li>24something</li> 
    <li>25something</li> 
    <li>26something</li> 
    <li>27something</li> 
    <li>28something</li> 
    <li>29something</li> 
    <li>30something</li> 
</ul> 
<div id="category_scroll"> 
    <div id="scrollup">上</div> 
            <div id="scroll_hand">滚动滚动滚动</div> 
            <div id="scrolldown">下</div> 
            <script type="text/javascript"> 
                var category_scroll = document.getElementById("category_scroll"); 
                var scroll_hand = document.getElementById("scroll_hand"); 
                var scrollup = document.getElementById("scrollup"); 
                var scrolldown = document.getElementById("scrolldown"); 
                var category_list = document.getElementById("category_list"); 
            </script> 
        </div> 
     </div> 
</body> 
</html>

[Ctrl+A 全选注:如需引入外部Js需刷新才能执行]

现在有个问题就是当点中滚动条后，触发键盘的上下键，来控制上下滚动。然而，一般情况下，键盘事件似乎只针对Form表单对象和Winsow窗体对象起作用（除IE）,那么当创建事件处理函数时该怎们写呢？
oEventUtil.addEventHandler(category_hand,"keyup",test)，似乎是不行，毕竟category_hand代表的是一个DIV
test()函数如下：
function test(){
//alert("RUN");
var oEvent = oEventUtil.getEvent();
if(oEvent.keyCode==38){
alert("UP");
clickNarrowup();
}
if(oEvent.keyCode==40){
clickNarrowdown();
alert("DOWN");
}
}
请高手帮忙解答一下。

疑问二:这段代码中，在定义变量的时候我是在HTML中间插入的代码，用于获取对象，如果这段代码插入到window.onload函数中时，那么很有可能此时这些对象尚未加载，无法读取到而产生程序错误，那么问题就是，如果我想完全将：
<script type="text/javascript">
var category_scroll = document.getElementById("category_scroll");
var scroll_hand = document.getElementById("scroll_hand");
var scrollup = document.getElementById("scrollup");
var scrolldown = document.getElementById("scrolldown");
var category_list = document.getElementById("category_list");
</script>
引入到<head>之间怎们处理比较好呢？
blueidea版主给出的解决方法

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