HTML中遍历Table表格中的Checkbox

<!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>
<title>Untitled Page</title>
</head>
<body onload="f()">
<script type="text/javascript">
function f()
{
var t=document.getElementById("tb").childNodes.item(0);
for(var i=0;i< t.childNodes.length;i++)
{
alert(t.childNodes(i).firstChild.firstChild.nodeValue);
alert(t.childNodes(i).childNodes[1].firstChild.checked);
}
}
</script>
<table >
<tr>
<td >
aaa</td>
<td >
<input type="checkbox" /></td>
<td >
</td>
</tr>
<tr>
<td >
bbb</td>
<td >
<input type="checkbox" checked="CHECKED" /></td>
<td >
</td>
</tr>
<tr>
<td >
ccc</td>
<td >
<input type="checkbox" /></td>
<td >
</td>
</tr>
</table>
</body>
</html>