怎样用JS判断一个CheckBOX是否被选上？

<input type="checkbox" name="chk" value="1" />
<input type="checkbox" name="chk" value="2" />
<input type="checkbox" name="chk" value="3" />
<input type="checkbox" name="chk" value="4" />
<input type="checkbox" name="chk" value="5" />
<input type="checkbox" name="chk" value="6" />

<input type="button" value="check" onclick="alert(check());" />

<script type="text/javascript">
function check()
{
var d = document.getElementsByName("chk");
for(var i = 0; i < d.length; i++)
{
if(d[i].checked)return true;
}
return false;
}
</script>