Source: modules/media_library/js/media_library.view.es6.js

/**
 * @file media_library.view.es6.js
 */
(($, Drupal) => {
  /**
   * Adds checkbox to select all items in the library.
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches behavior to select all media items.
   */
  Drupal.behaviors.MediaLibrarySelectAll = {
    attach(context) {
      const $view = $(
        '.js-media-library-view[data-view-display-id="page"]',
        context,
      ).once('media-library-select-all');
      if ($view.length && $view.find('.js-media-library-item').length) {
        const $checkbox = $(Drupal.theme('checkbox')).on(
          'click',
          ({ currentTarget }) => {
            // Toggle all checkboxes.
            const $checkboxes = $(currentTarget)
              .closest('.js-media-library-view')
              .find('.js-media-library-item input[type="checkbox"]');
            $checkboxes
              .prop('checked', $(currentTarget).prop('checked'))
              .trigger('change');
            // Announce the selection.
            const announcement = $(currentTarget).prop('checked')
              ? Drupal.t('All @count items selected', {
                  '@count': $checkboxes.length,
                })
              : Drupal.t('Zero items selected');
            Drupal.announce(announcement);
          },
        );
        const $label = $(
          '<label class="media-library-select-all"></label>',
        ).text(Drupal.t('Select all media'));
        $label.prepend($checkbox);
        $view.find('.js-media-library-item').first().before($label);
      }
    },
  };
})(jQuery, Drupal);