JavaScript -- 操作input CheckBox 全选框

<!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=gb2312" />
<title>无标题文档</title>

<script type="text/javascript">

function getSum()
{
	var itemNodes = document.getElementsByName("item");
	var sum = 0;
	for(var i=0; i<itemNodes.length; i++)
	{
		if(itemNodes[i].checked)
		{
			sum += parseInt(itemNodes[i].value);
		}
	}

	var spanNode = document.getElementById("spanId");
	var str = sum + "元";
	spanNode.innerHTML = str.fontsize(10);
}

function checkAll(checkallNode)
{
	var itemNodes = document.getElementsByName("item");
	for(var i=0; i<itemNodes.length; i++)
	{
		itemNodes[i].checked = checkallNode.checked;
	}
}

</script>

</head>

<body>
<input type="checkbox" name="all" onclick="checkAll(this)"/>全选<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="checkbox" name="item" value="3000" />笔记本电脑：3000元<br />
<input type="button" value="计算金额" onclick="getSum(this)"/> <span id="spanId"></span>
</body>
</html>