<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html;
charset=utf-8">
<title>计时器</title>
</head>
<script type="text/javascript">
var num=0;
function startCount() {
document.getElementById('count').value=num;
num=num+1;
}
setInterval(startCount,1000);
</script>
</head>
<body>
<form>
<input type="text" id="count" />
</form>
</body>
</html>
嗯（其实我是水回复的）
其实本节的练习不如例子好，例子可以更好看清setTimeout()与setInterval()的区别