有多个复选框，名字都一样，如何用JS判断有多少个选中的

[one_bird]

如题

[wasuka]

<script language="javascript">
function counter()
{
count = 0
for(c=0;c<f.length;c++)
{
if(f.elements[c].checked) count++
}
alert(count)
}
</script>
<form name="f">
<input type="checkbox" name="a">
<input type="checkbox" name="a">
<input type="button" onclick="counter()">
</form>

[MikeCheers]

function CheckAll(){
for (var i=0;i<form1.elements.length;i++){
var e = form1.elements[i];
if (e.name != 'checkbox'){
e.checked = form1.checkbox.checked;
}
}
}

[clare2003]

<script language="javascript">
function f1()
{
count = 0
var objlen=document.getElementsByName("a").length;
for(var i=0;i<objlen;i++)
{
if(document.getElementsByName("a")[i].checked) count++
}
alert(count)
}
</script>

<input type="checkbox" name="a" ID=Checkbox1>
<input type="checkbox" name="a" ID=Checkbox2>
<input type="button" onclick="f1()" ID=Button1>

[ghtyan]

1 .<INPUT name="chk" type="checkbox"><br>
2 .<INPUT name="chk" type="checkbox"><br>
3 .<INPUT name="chk" type="checkbox"><br>
4 .<INPUT name="chk" type="checkbox"><br>
5 .<INPUT name="chk" type="checkbox"><br>
6 .<INPUT name="chk" type="checkbox"><br>
<input type="button" value="check" onclick="checks()">
<script>
function checks()
{
var chks = document.all('chk');
for(var i=0;i<chks.length;i++)
{
if (chks[i].checked)
alert(i+1);
}
}
</script>