JavaScript API overview
Since Drupal 8, the available JavaScript files, which were referenced in .info files in Drupal 7, are now referenced in .yml files
Accessibility tools for JavaScript in Drupal 8
These tools are under active development in Drupal 8. This documentation page may be out of date
Add JavaScript to Your Theme or Module
Attach JavaScript to your modules or themes following these easy steps.
AJAX Forms
Using AJAX callbacks to dynamically react on user input, alter form elements, show dialog boxes or run custom Javascript functions.
Messages API
Display status messages by the JavaScript API from Drupal core.
Theming in JavaScript
Allow HTML added by JavaScript to be overridden by themes.