我要评论核心代码：
<!doctype html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<title>鼠标拖动和拖拽的鼠标指针特效</title>
<style>
#box{width:100px;height:100px;background:#966;position:absolute;left:100px;top:100px;cursor:pointer;}
</style>
</head>
<body>
<div id="box"></div>
<script>
var doc=document;
function getviewport(){
return {width:math.max(document.documentelement.clientwidth,document.documentelement.scrollwidth),
height:math.max(document.documentelement.clientheight,document.documentelement.scrollheight)
};
}
function comparenum(x,minnum,maxnum){
switch(true){
case x<minnum:x=minnum;break;
case x>maxnum:x=maxnum;break;
default:x;
}
return x;
}
box.style.left='100px'
box.style.top='100px'
box.onmousedown=function(e){
var e=e||window.event;
var maxl=getviewport().width-100,
maxt=getviewport().height-100;
this.startl=parseint(this.style.left);
this.startt=parseint(this.style.top);
//alert(e.clientx)
this.startx=e.clientx;
this.starty=e.clienty;
//alert(maxl+':'+maxt+':'+l+':'+t)
doc.onmousemove=function(e){
posxy(e,0,0,maxl,maxt);
};
doc.onmouseup=function(){
doc.onmousemove=null;
doc.onmouseup=null;
};
return false;
}
function posxy(e,minl,mint,maxl,maxt){
var e=e||window.event;
var x=box.startl-(box.startx-e.clientx);
var y=box.startt-(box.starty-e.clienty);
box.style.left=comparenum(x,minl,maxl)+'px'
box.style.top=comparenum(y,mint,maxt)+'px'
}
</script>
</body>
</html>
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