求助：移动“DIV元素“

Question

<script>
var flag=0;
var divtime;
function divmove()
{
var moveboj=document.getElementById("div_element")if(flag==0)
{
var x1=moveboj.style.left;
var x=eval(Number(x1.substring(0,x1.length-2))+10); 
moveboj.style.left=x+"px";
flag=(screen.width<=(moveboj.offsetLeft+moveboj.offsetWidth+10))?1:0; 
}
else
{
var x1=moveboj.style.left;
var x=eval(Number(x1.substring(0,x1.length-2))-10); 
moveboj.style.left=x+"px";
flag=(moveboj.offsetLeft<=0)?0:1; }divtime=setTimeout("divmove()",100)
}function f1()
{
var buttonobj=document.all.Button1;
buttonobj.value=(buttonobj.value=="开始")?"停止":"开始"
if (buttonobj.value=="停止")
{
divmove();
}
else
{
clearTimeout(divtime);
}
}</script>
<input type="button" value="开始" ID=Button1 onclick=f1()>
<div id="div1"></div>
<div id="div_element" style="position:absolute;top:300;left:10;width:100;height:80;background-color:#0000ff;border:#ff0000 solid 3"><font color="#ffffff">DIV元素</font></div>
</body>
</html>