jquery submit form and stay on same page not working

The below code is supposed to submit the form and stay on the current page. It does submit the form, but it doesn’t stay on the same page as it redirects to the form processing page. I have tried using event.preventDefault(); and return false; – but neither are stopping the redirect. I tried them one at a time then added both at the same time and at different locations in the functin, but the redirect still happens.

  function submitForm() {
    var $subForm = $('#signupForm')[0] ;
    if (!$subForm.checkValidity()) {
      $subForm.find(':submit').click() ;
      return ;
    }
    
    $subForm.submit(function(event){
      event.preventDefault(); // not working  here
      $.post($(this).attr('action'), $(this).serialize(), function(response){
            // do something here on success
        console.log(response) ;
      },'json');
      return false;  // not working here
    });
    return false ;  // not working here
  }

My form is defined as:

<form method="POST" id="signupForm" action="submitSignup.php" enctype="multipart/form-data" validate>
    ....
    <button type="button" onclick='submitForm();' id="ccInfoButton" style="" class="btn btn-primary buttonSignup" disabled >CREATE ACCOUNT NOW<i class="iconRequired icon-chevron-right"></i></button>
</form>