<form method="POST" name="contact" class="formContact" id="formContact">

    <div class="col-md-12">
        <h1><label class="col-md-12"><?php echo $this->lang->line('index_contact_contact'); ?></label></h1>
    </div>
    <div class="col-md-12"></div>
    <label class="col-md-4">*<?php echo $this->lang->line('index_contact_name'); ?>:</label>
    <div class="col-md-8">
        <input type="text" name="naam" class="form-control" placeholder="<?php echo $this->lang->line('index_contact_yname'); ?>">
    </div>
    <label class="col-md-4">*<?php echo $this->lang->line('index_contact_email'); ?>:</label>
    <div class="col-md-8">
        <input type="text" name="email" class="form-control" placeholder="<?php echo $this->lang->line('index_contact_yemail'); ?>">
    </div>
    <label class="col-md-4"><?php echo $this->lang->line('index_contact_phone'); ?>:</label>
    <div class="col-md-8">
        <input type="text" name="telefoon" class="form-control" placeholder="<?php echo $this->lang->line('index_contact_yphone'); ?>">
    </div>
    <label class="col-md-4">*<?php echo $this->lang->line('index_contact_question'); ?>:</label>
    <div class="col-md-8">
        <textarea id="vragen" name="vraag" style="resize:none" maxlength="500" class="form-control" placeholder="<?php echo $this->lang->line('index_contact_yquestion'); ?>"></textarea>
        <p id="teller"></p>
    </div>
    <div class="col-md-8 col-md-offset-4" style="margin-top:10px;">
        <?php
        //echo $recaptcha->create_box();
        ?>

        <div class="g-recaptcha"
             data-sitekey="......."
             data-callback="onCompleted"
             data-size="invisible">
        </div>

    </div>
    <div class="col-md-12 text-right inputcontact">
        <input type="submit" name="btnSubmit" value="Verstuur"  class="btn btn-default">
    </div>
    <div class="col-md-12">
        <label class="col-md-12 inputcontact"><?php echo $this->lang->line('index_contact_required'); ?></label>
    </div>
</form>

Javascript (jquery)

$("#formContact").validate({

   rules:{
       naam:{
           required:true,
           minlength:2,
           maxlength:40,
           accept:"[a-z A-Z]+"
       },
       email:{
           required:true,
           maxlength:50,
           email:true
       },
       telefoon:{
           required:false,
           accept:"[0-9 -]+",
           maxlength:15
       },
       vraag:{
           required:true
       }
   },
    messages:{
        naam:{
            minlength:"Dit veld moet minimaal {0} tekens bevatten.",
            accept: "Geen cijfers of speciale tekens."
        },
        telefoon:{
            maxlength: "Maximaal {0} tekens",
            accept: "alleen cijfers en het scheidingsteken: - "
        },
        email:{
            maxlength: "Dit veld mag maximaal {0} tekens bevatten.",
            email:"Dit is geen geldig e-mailadres"
        }
    }






});
var validationCheck = false;
$("#formContact").submit(function(event) {


    if (grecaptcha.getResponse()) {
        // 2) finally sending form data
        event.submit();
    }else{
        // 1) Before sending we must validate captcha

        grecaptcha.reset();
        console.log('validation completed.');

        event.preventDefault(); //prevent form submit before captcha is completed
        grecaptcha.execute();

    }

});

onCompleted = function(response) {
    console.log('captcha completed.');
    $("#formContact").submit();
    return true;
};

PHP

if (isset($_POST['btnSubmit'])) {
    die("for testing purposes");
}

question: I'm trying to add a invisible recaptcha to my site, it works but somehow I have the press the submit button twice, once for validation and once for the submit, I only want to press it once to work, when I press the button the first time I get this error: Uncaught (in promise) null

the second one does the thing but I still get this: 

Uncaught TypeError: event.submit is not a function

    at HTMLFormElement.<anonymous> (contact:301)

    at HTMLFormElement.dispatch (VM34 jquery.js:3)

    at HTMLFormElement.r.handle (VM34 jquery.js:3)

Could anyone help me with this?

the recaptcha:  https://developers.google.com/recaptcha/docs/invisible