用 JavaScript 操作 DOM 時出現如下錯誤：
Uncaught TypeError: Cannot set property 'value' of null
Uncaught TypeError: Cannot read property 'id' of undefined
例如：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<script>
var div = document.getElementById("測試1");
alert(div.id);
alert(div.className);
alert(div.title);
</script>
<title>測試</title>
</head>
<body>
<div id="測試1" class="測試2" title="測試3">
<span>0</span>
<span>1</span>
<span>2</span>
<span>3</span>
</div>
</body>
</html>
運行時出現如下錯誤：
問題出在 JS 運行的時候你的頁面還沒有加載完成，所以你的 JS 代碼找不到你的頁面元素，就會拋出這個問題。解決辦法就是把 JavaScript 代碼放在 body 的最后，例如：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>測試</title>
</head>
<body>
<div id="測試1" class="測試2" title="測試3">
<span>0</span>
<span>1</span>
<span>2</span>
<span>3</span>
</div>
<script>
var div = document.getElementById("測試1");
alert(div.id);
alert(div.className);
alert(div.title);
</script>
</body>
</html>
