title: Callback after Ajax form submit by Drupal 8 form
body: <p>I have one Drupal 8 form (ID: regform) and I have to done some JS triggers after the form AJAX submit.</p>
  
  <p>For example form contains choose plan field, after form submit, I have to display some calculated price in the other block in the same page, So I am trying to pick a callback function after Ajax submit. What I have to do ?</p>