This is how you add Drupal 8 JavaScript and CSS to a certain page in your custom theme instead of the methods we are used to in Drupal 7 drupal_add_js and drupal_add_css.

Drupal 8 add javascript to a page

In the following example i will show you how to add jquery-selectall a jQuery plugin witten by me for selecting check-boxes and a custom javascript file jquery-selectall-instance.js to your drupal 8 website.

Once you downloaded the plugin you need and/or created your custom javascript place the files in your theme. I will place the files in
MYTHEME/vendor/jquery-selectall/jquery-selectall.min.js
MYTHEME/js/jquery-selectall-instance.js
Now to add the javascript to the theme in 2 steps.

  1. Define it as a custom library in your MYTHEME.libraries.yml file like so 
    jquery-selectall:
  js:
    vendor/jquery-selectall/jquery-selectall.min.js: {}
    js/jquery-selectall-instance.js: {}

    OPTIONAL: You could technically add CSS as well if you would create a css: tag at the same level where js: is like so

    jquery-selectall:
  js:
    vendor/jquery-selectall/jquery-selectall.min.js: {}
    js/jquery-selectall-instance.js: {}
  css:
    theme:
      css/jquery-selectall.css: {}
  2. Now to attach the library to a certain page. In your MYTHEME.theme file create a custom logic in the preprocess_page and attach the library like so:
    function MYTHEME_preprocess_page(&$vars){

  // get current path
  $current_path = Drupal::request()->getRequestUri();

  // check for a path
  if (!empty($current_path) && (substr( $current_path, 0, 6 ) === "/user/")) {
    // attach our library
    $vars['#attached']['library'][] = 'pbow/jquery-selectall';
  }

}

I hope you found this helpful, if you want to know more on defining libraries in drupal 8 visit https://www.drupal.org/docs/8/theming-drupal-8/adding-stylesheets-css-and-javascript-js-to-a-drupal-8-theme

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