As it sadly took me some time to find out and it's not very well documented, here's how you close a Drupal 8+ dialog (in this case the Off-Canvas) using JavaScript:
Drupal.dialog(document.getElementById('drupal-off-canvas')).close();You may need to change the ID selector for other modal types!
The funny part is: Via AJAX it's quite simple in Drupal by using the Commands:
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Ajax%21C…
