计时器问题，赋值先后的错误，请大神解答
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>定时器</title>
<script type="text/javascript">
var sum=0;
function c(){
document.getElementById("shu").value=sum;
sum++;
setTimeout(c,1000);
}
</script>
</head>
<body>
<form>
<input type="text" id="shu"></input>
<input type="button" value="开始计数" onclick="c()"></input>
</form>
</body>
</html>
document.getElementById("shu").value=sum;这行如果换成下面的那行为什么就不行呢？
sum=document.getElementById("shu").value;
