js 模拟鼠标绘制方块

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
    *{margin:0;padding:0;}
    #box1{width:500px;height:500px;border:1px solid #000;position:absolute;left:400px;}
    .cBox{border:1px solid #0C6; width:30px;position:absolute;}
</style>
<script>
 window.onload = function(){
     var oDiv = document.getElementById('box1');
     
     oDiv.onmousedown = function(ev){
         var ev = ev || event;
         var initX = ev.clientX;
         var initY = ev.clientY;
         var cDiv = document.createElement('div');
         cDiv.className = 'cBox';
         cDiv.style.left = (initX-oDiv.offsetLeft)+'px';
         cDiv.style.top = (initY)+'px';
         oDiv.appendChild(cDiv);
         
         document.onmousemove = function(ev){
             
             var ev = ev || event;    
             var disX = 0;    
             if(ev.clientX >(oDiv.offsetLeft + oDiv.offsetWidth)){
                disX = oDiv.offsetLeft + oDiv.offsetWidth;
             }else{
                disX = ev.clientX;
             }
             var disY = 0;
             if(ev.clientY >(oDiv.offsetTop + oDiv.offsetHeight)){
                disY = oDiv.offsetTop + oDiv.offsetHeight;
             }else{
                disY = ev.clientY;
             }
             var cBoxWidth = disX - initX-2; /*需要减去cDiv的两个边框*/
             var cBoxHeight = disY - initY-2;         
             cDiv.style.width = cBoxWidth+'px';
             cDiv.style.height = cBoxHeight+'px';
          };
          
         document.onmouseup = function(){
             document.onmousemove = document.onmouseup = null;
            
        };
         return false;
    };

};
</script>
</head>

<body style="2000px;">
<div id='box1'>
</div>
</body>
</html>