为什么用for in输出，会多输出6组undefined
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<body>
<div>
javascript
<p>javascript</p>
<div>jQuery</div>
<h5>PHP</h5>
</div>
<script type="text/javascript">
var div = document.getElementsByTagName('div')[0].childNodes;
for(var a in div){
document.write(div[a].nodeName+' '+div[a].nodeValue+" "+div[a].nodeType+'<br/>');
}
</script>
</body>
</html>
以下为输出：
javascript
javascript
jQuery
PHP
#text javascript 3
P null 1
#text 3
DIV null 1
#text 3
H5 null 1
#text 3
undefined undefined undefined
undefined undefined undefined
undefined undefined undefined
undefined undefined undefined
undefined undefined undefined
undefined undefined undefined
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JavaScript