Issue #3063281: 'Select All' option

js/select2.js

@@ -5,9 +5,49 @@
 (function ($, drupalSettings, Sortable) {
   'use strict';
 
+  $.fn.select2.amd.define("CustomDropdownAdapter", [
+      'select2/utils',
+      'select2/dropdown',
+      'select2/dropdown/attachBody'
+    ],
+    function (Utils, Dropdown, AttachBody) {
+
+      let dropdownWithButton = Utils.Decorate(Dropdown, AttachBody);
+
+      dropdownWithButton.prototype.render = function () {
+        var $rendered = Dropdown.prototype.render.call(this);
+
+        var $selectAll = $(
+          '<button type="button">Select All</button>'
+        );
+
+        var self = this;
+
+        $selectAll.on('click', function (e) {
+          // Get all results that aren't selected.
+          let $results = $rendered.find('.select2-results__option[aria-selected=false]');
+
+          $results.each(function (foo, bar) {
+            // Trigger the select event.
+            self.trigger('select', {
+              data: Utils.GetData(bar, 'data')
+            });
+          });
+          self.trigger('close');
+        });
+
+        $rendered.prepend($selectAll);
+
+        return $rendered;
+      };
+
+      return Utils.Decorate(dropdownWithButton, AttachBody);
+    });
+
   Drupal.behaviors.select2 = {
     attach: function (context) {
       $('.select2-widget', context).once('select2-init').each(function () {
+
         var config = $(this).data('select2-config');
         config.createTag = function (params) {
           var term = $.trim(params.term);
@@ -19,6 +59,8 @@
             text: term
           };
         };
+        config.dropdownAdapter = $.fn.select2.amd.require("CustomDropdownAdapter")
+
         config.templateSelection = function (option, container) {
           // The placeholder doesn't have value.
           if ('element' in option && 'value' in option.element) {
@@ -27,6 +69,8 @@
           }
           return option.text;
         };
+
+
         $(this).data('select2-config', config);
 
         // Emit an event, that other modules have the chance to modify the