Drupal - What version of jQuery does Drupal 8 use?

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

This is probably not the best question because the answer could change over time based on the specific version of Drupal.

The most accurate way to find what version of a JavaScript library is used by Drupal core is to look directly at the core/core.libraries.yml file for a given version of Drupal. One of the easiest ways to do this is to browse Drupal's git repository online:

https://git.drupalcode.org/project/drupal/blob/HEAD/core/core.libraries.yml#n334

jquery:
  remote: https://github.com/jquery/jquery
  version: VERSION_HERE
  license:
    name: MIT
    url: https://github.com/jquery/jquery/blob/VERSION_HERE/MIT-LICENSE.txt
    gpl-compatible: true
  js:
    assets/vendor/jquery/jquery.min.js: { minified: true, weight: -20 }

For Drupal 6-7, without jquery_update module installed, the version of Drupal can be most easily discovered by going into the misc/ directory, and looking at the top of the jquery.js file. This is v1.2.6 and v1.4.4 respectively.

For Drupal 5, the version of jquery is not readily apparent, but the handy git blame misc/jquery.js reveals the commit 0d57bae updated jQuery to v1.0.4.