checkbox判断不为空

@foreach($seals as $v)
    <input type="checkbox" name="seal_id[]" value="{{$v[‘id‘]}}" attr="attr" >{{$v[‘name‘]}}&nbsp;&nbsp;
@endforeach

            //判断checkbox公章是否选择
        var str=document.getElementsByName("seal_id[]");
        var objarray=str.length;
        var chestr="";
        for (i=0;i<objarray;i++)
        {
            if(str[i].checked == true)
            {
                chestr+=str[i].value+",";
            }
        }
        if(chestr == "")
        {
            alert("请选择至少一个公章");
            return false;
        }