js判断radio，checkbox是否选中

从数据库循环数据，多选按钮数组

function type_1(){
//多选
var b= document.getElementsByName('service_zj_ids[]'); 
var a = document.getElementsByName('service_zj_ids[]').length;
str = 0;
for (var i=0; i<a; i++) {
if(b[i].checked == true){
str = str+1;
}
};
if(str == 0){
alert('请选择');
return false;
}

//单选
var g = $(".ddd").length;
aa = 0;
for (var i =0; i < g; i++) {
if ($("#trades_zj_" + i).is(':checked')){
aa= aa+1; //判断aa是0或者1
}
};
if(aa == 0){
alert('请选择');
return false;
}

}

//多选数组

<?php foreach ($caojia as $key => $value):?>

<li><span>
<input type="checkbox" name="service_zj_ids[]" id="service_zj_ids" value="<?php echo $value['service_id'];?>" style="display:none;" class="serchec" />
<?php echo $value['service_name'];?>
</span></li>
<?php endforeach;?>

//单选数组

<?php foreach ($trades as $key => $value):?>
<li class="gcxz"><span>
<input type="radio" id="trades_zj_<?php echo $key;?>" name="trades_id" class="ddd" value="<?php echo $value['trades_id'];?>" >
<label for="trades<?php echo $value['trades_id'];?>"><?php echo $value['trades_name'];?></label>
</span></li>
<?php endforeach;?>