<form action="../json/data.json" name="my-form">
    <div data-formjs-question class="form-group">
        <div class="answers-box label-move-up">
            <input name="name" type="text" data-length="[3,20]" class="form-control" required />
            <label>Name</label>
        </div>
        <div class="field-error-message small">Write your name! It must be between 3 and 20 chars.</div>
    </div>
    <div data-formjs-question class="form-group">
        <div class="answers-box label-move-up">
            <input name="surname" type="text" data-length="[3,20]" class="form-control" data-validate-if-filled />
            <label>Surname</label>
        </div>
        <div class="field-error-message small">Surname is not mandatory but, if filled, it must be between 3 and 20 chars!</div>
    </div>
    <div data-formjs-question class="form-group">
        <div class="answers-box label-move-up">
            <input name="email" type="email" class="form-control" required />
            <label>Email</label>
        </div>
        <div class="field-error-message small">Check your email address, it is not valid!</div>
    </div>
    <div data-formjs-question class="form-group">
        <p class="mb-2 small">Select your contact preferences <br />( 2 answers max )</p>
        <div class="answers-box">
            <div class="form-check">
                <input name="contactPrefCheck" type="checkbox" value="sms" class="form-check-input" id="checkbox-1-a" required data-checks="[1,2]" />
                <label class="form-check-label" for="checkbox-1-a">SMS</label>
            </div>
            <div class="form-check">
                <input name="contactPrefCheck" type="checkbox" value="email" class="form-check-input" id="checkbox-2-a" required />
                <label class="form-check-label" for="checkbox-2-a">Email</label>
            </div>
            <div class="form-check">
                <input name="contactPrefCheck" type="checkbox" value="app-notiifcation" class="form-check-input" id="checkbox-3-a" required />
                <label class="form-check-label" for="checkbox-3-a">App Notification</label>
            </div>
        </div>
        <div class="field-error-message small">
            You must select at least 1 and at most 2 answers!
        </div>
    </div>
    <div data-formjs-question class="form-group">
        <div class="answers-box">
            <div class="small">
                I agree with the <a href="#" target="_blank">Privacy Policy</a>.
            </div>
            <div class="form-check form-check-inline">
                <input class="form-check-input" type="radio" name="privacyCheck" id="privacyCheck-1-a" value="accepted" required />
                <label class="form-check-label" for="privacyCheck-1-a">Yes</label>
            </div>
            <div class="form-check form-check-inline">
                <input class="form-check-input" type="radio" name="privacyCheck" id="privacyCheck-2-a" value="" required />
                <label class="form-check-label" for="privacyCheck-2-a">No</label>
            </div>
        </div>
        <div class="field-error-message small">You must agree!</div>
    </div>
    <div class="text-center">
        <button type="submit" class="btn btn-light">SEND</button>
    </div>
    <div class="d-none alert mt-5 mb-4" role="alert" data-formjs-global-feedback></div>
</form>

Form.addValidationErrors({
    email: function( string ){
        const obj = {};
        if( string.indexOf('@') === -1 ){ obj.atChar = true; }
        else if( string.split('@').filter(function(i){ return !!i; }).length < 2 ){ obj.incomplete = true; }
        return obj;
    }
});

const $form = document.querySelector('form');

$form.querySelector('[type="email"]').addEventListener('fjs.field:validation', function(event){
    if( event.detail.errors && Object.keys(event.detail.errors).length > 0 ){
        console.log('EMAIL CUSTOM ERRORS', event.detail.errors);
    }
});

const formInstance = new Form( $form );