Whether you're trying to add your own JavaScript to a page, or make use of one of the many popular libraries provided by Drupal core, it's important to understand the asset library system when working with JavaScript in Drupal. By wrapping your JavaScript in an asset library it can be added to every page, selectively included when it's needed, or be easily modified by others.
In addition to the inclusion of third party libraries, Drupal core also contains numerous Drupal-specific JavaScript libraries that make it easier to do things like implement AJAX calls to a Drupal route, localize UI strings, and pass configuration stored in the database to the front-end. Drupal also contains several external projects in core, like jQuery, Backbone, Underscore, and Modernizer.
The addition of the asset library system in Drupal 8, while new, is quite well established. There may be emerging best practices regarding how to split up and structure your custom libraries, but the overall concepts are unlikely to change.
These tutorials cover best practices for integrating JavaScript with Drupal:
These high level tutorials walk through some of the new JavaScript features and libraries that are available in Drupal 8 core:
