js判断checkbox是否选中个数

<SCRIPT LANGUAGE="JavaScript">
//判断同类型
function checkboxnum(){
var form = document.forms['formbj1'];
var i,j=0;
for (i=0; i<form.length; i++){
var e=form[i];
if (e.checked && e.type=='checkbox' && e.name=='bjid[]') {
j++;
if(j==3){
alert("最多只能选择2项！");
return false;
break;
}
}
}
}
</script>

<form action="zhdk_bj.php" method="post" name="formbj1" id="formbj1">

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="1" />

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="2" />

<input name="bjid[]" id="bjid[]" type="checkbox" onclick="return checkboxnum()" value="3" />

</form>