JavaScript 实现简易动态进度条

<!DOCTYPE html>
<html lang="en">
<head>
 <meta charset="UTF-8">
 <title>Document</title>
 <style>
 div{
 width: 500px;
 height: 50px;
 position: relative;
 border: 2px solid;
 margin: 0 auto;
 }
 aside{
 height: 50px;
 background:red;
 }
 p{
 position: absolute;
 top: 0;
 right: 0;
 }
 </style>
 <script type="text/javascript">
 var i = 0;
 var timer = setInterval(function(){
 document.getElementById("aside").style.width = i++ + 'px';
 document.getElementById("span").innerHTML = parseInt(i*100/500);
 if(i > 500){
 clearInterval(timer);}
 },10)
 </script>
</head>
<body>
 <div>
 <aside id="aside" style="width: 10px;"></aside>
 <p><span id="span">0</span>%</p>
 </div>
</body>
</html>