<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>计时器</title>
<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> Why can’t I write startCount() directly on line 13? Prompt error Uncaught TypeError: Cannot set property 'value' of null. If I write a script tag below the html tag and write startCount() in it, it will work again, or if I write setTimeout(startCount(),1000); on line 13, it will also work. Why?
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
The reason is that the dom has not been completely loaded, so it is recommended to execute js under the body, or use window.onload to execute it in the head.