What version of jQuery does Drupal 8 use? How can I find out what version of jQuery a particular release of Drupal 8 core is using?
If you want to save some time, you can go to the Javascript console and type jQuery.fn.jquery which works on any site using jQuery, not only Drupal sites.

At the moment Drupal 8 core uses jQuery 2.1.4, but it looks like jQuery 3 has been implemented in Drupal 8.4.x.

https://www.drupal.org/node/2533498