<!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;
setTimeout("startCount()",1000)
}
startCount();
</script>
</head>
<body>
<form>
<input type="text" id="count" />
</form>
</body>
</html>在 body前面加个onload="startCount()"
js执行的时候input还没有加载，因此取不到值报错