为什么点击按钮开始就不可以
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
</head>
</head>
<body>
<form>
<input type="text" id="count" />
<button onclick="startCount()">点击开始</button>
</form>
<script type="text/javascript">
var num=0;
function startCount() {
document.getElementById('count').value=num;
num++;
setTimeout(startCount,1000) ;
}
</script>
</body>
</html>
