Drupal 8 Add library JS

add_library.php

<?php
/////// ADD LIBRARY /////////

// helloworld.libraries.yml
helloworlds:
  version: 1.x
  js:
    js/script.js: {}
  dependencies:
    - core/jquery

/**
 * FOR ALL PAGES
 * hook_page_attachments().
 */
function MODULENAME_page_attachments(array &$attachments) {
  $attachments['#attached']['library'][] = 'modulename/helloworlds';
}

// FOR CUSTOM /hello page
// In custom controller /helloWorld/src/Controller/HelloWorldController.php
public function helloWorld() {
   $output = array();

   $output['#title'] = 'HelloWorld page title';
   $output['#markup'] = 'Hello World!';
   $output['#attached']['library'][] = 'helloworld/helloworlds';

   return $output;
}

// add library from TWIG
{{ attach_library('MYMODULE/helloworlds') }}

// Attach external library, ex: VexJS

helloworld.libraries.yml

helloworld:
  version: 1.x
  js:
    js/script.js: {}
  dependencies:
    - core/jquery

vexjs:
  remote: https://github.com/HubSpot/vex
  version: 2.3.2
  license:
      name: MIT
      url: https://github.com/HubSpot/vex/blob/master/LICENSE
      gpl-compatible: true
  css:
    theme:
      https://cdn.rawgit.com/HubSpot/vex/master/css/vex.css: { type: external, minified: false }
      https://cdn.rawgit.com/HubSpot/vex/master/css/vex-theme-default.css: { type: external, minified: false }
  js:
    https://cdn.rawgit.com/HubSpot/vex/master/js/vex.combined.min.js: { type: external, minified: true }
  dependencies:
    - core/jquery

HelloWorldController.php

<?php

namespace Drupal\helloworld\Controller;

use Drupal\Core\Controller\ControllerBase;

class HelloWorldController extends ControllerBase {

  public function helloWorld() {
    
    $output = array();

    $output['#title'] = 'HelloWorld page title';
    $output['#markup'] = 'Hello World!';
    $output['#attached']['library'][] = 'helloworld/vexjs';
    $output['#attached']['library'][] = 'helloworld/helloworld';

    return $output;
  }
  
}

script.js

/**
 * @file
 * Simple JavaScript hello world file.
 */

(function ($, Drupal, settings) {

  "use strict";

  Drupal.behaviors.helloworld = {
    attach: function (context) {
      vex.defaultOptions.className = 'vex-theme-default';
      vex.dialog.confirm({
        message: 'Are you absolutely sure you want to destroy the alien planet?',
        callback: function(value) {
          return console.log(value ? 'Successfully destroyed the planet.' : 'Chicken.');
        }
      });
    }
  }

})(jQuery, Drupal, drupalSettings);

https://cdn.rawgit.com seems to be soon removed, https://cdn.jsdelivr.net/gh can be used instead