title: ajax:success callback doesn't fire after conversion from Coffeescript to ES6
body: <p>I have some ajax Form on my page. 
  I create a class to manage the behaviour of the buttons, and I have to change the class after the ajax success.</p>
  
  <pre><code>export class Following {
    constructor(element) {
      element  = $(element);
  
      this.follow   = element.find('.follow');
      this.unfollow = element.find('.following');
  
      this.followForm   = this.follow.parents('form');
      this.unfollowForm = this.unfollow.parents('form');
  
      this.bindClicks();
  
    }
  
    bindClicks() {
      this.followForm.on('ajax:error', this.trySignIn);
  
      this.followForm.on('ajax:success', event =&gt; {
        console.log('follow success')
      });
  
      this.unfollowForm.on('ajax:success', event =&gt; {
        console.log('unfollow success')
      });
    }
  
    static init() {
      $('.follow-unfollow-buttons').each((i, el) =&gt; new Following($(el)));
    }
  };
  </code></pre>
  
  <p>When I submit the form, the ajax call is a success, but I cannot intercept the callback. Why?</p>