请问为什么document.body.scrollHeight高度为0
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<script type="text/javascript">
var w = document.documentElement.scrollWidth;
var h = document.documentElement.scrollHeight;
var wb = document.body.scrollWidth;
var hb = document.body.scrollHeight;
document.write(w+"/"+h+"<br/>");
document.write(wb+"/"+hb);
</script>
</body>
</html>
hb输出结果为0请问为什么呢?