--- a/js/select2.js
+++ b/js/select2.js
@@ -15,6 +15,11 @@
     return text == null ? "" : text.trim();
   };
 }
+if (!jQuery.isFunction) {
+  jQuery.isFunction = function (value) {
+    return typeof value === 'function';
+  };
+}

 (function ($, drupalSettings, Sortable, once) {
   'use strict';