在使用PHP和Javascript提交表单时无法检查空白选择字段

我在提交表单时遇到一个问题，无法检查空白选择字段。我在下面解释我的代码。

添加投诉.php：

    <form name="billdata" id="billdata"  enctype="multipart/form-data" method="POST" onSubmit="javascript:return checkForm();">
       <div class="input-group bmargindiv1 col-md-12">
        <span class="input-group-addon ndrftextwidth text-right" style="width:180px"> Name :</span>
         <input type="text" name="u_name" id="name" class="form-control" placeholder="Add Name" onKeyPress="clearField('name');">
        </div> 
        <div class="input-group bmargindiv1 col-md-12">
        <span class="input-group-addon ndrftextwidth text-right" style="width:180px">Email :</span>
        <input type="email" name="u_email" id="emailid" class="form-control" placeholder="Add Email Id" onKeyPress="clearField('emailid');">
         </div>
        <div class="input-group bmargindiv1 col-md-12">
         <span class="input-group-addon ndrftextwidth text-right" style="width:180px">TYPE OF COMPLAINT
     :</span>
        <select class="form-control"  id="compleant" name="complainType" onChange="javascript:checkComField();">
         <option value="">Select types of complaint</option>
         <option value="card">Card Related</option>
         <option value="claim">Claim Related</option>
         <option value="product">Product Related</option>
         <option value="Premimum">Premimum Related</option>
        </select>
        </div>
<input type="submit" class="btn btn-success" name="complainSubmit"  id="addProfileData" value="Add"/>
    </form>
<script>
    function checkForm(){
        var s=document.billdata;
        console.log('compalin',s.complainType.value=='');
        if(s.u_name.value==''){
            alert('Please enter name');
            s.u_name.focus();
            s.u_name.style.borderColor = "red";
            return false;
        }else if(s.u_email.value==''){
            alert('Please enter email');
            s.u_email.focus();
            s.u_email.style.borderColor = "red";
            return false;
        }else if(! validateEmail(s.u_email.value)){
            alert('Please enter a valid email');
            s.u_email.focus();
            s.u_email.style.borderColor = "red";
            return false;
        }else if(s.complainType.value==''){
            alert('Please select the type of complain');
            s.complainType.focus();
            s.complainType.style.borderColor = "red";
            return false;
        }else{
        }
    }
</script>

在这里，我的问题是当我在名称和电子邮件字段中有一些值并且没有从选择字段中选择值时，然后我单击表单正在提交的添加按钮，但空白选择字段无法检查。在这里，我需要在提交之前检查所有空白字段。请帮助我。