Use ES6 arrow function.

js/src/modal.js

@@ -568,7 +568,7 @@ const Modal = (($) => {
 
   $.fn[NAME] = Modal._jQueryInterface
   $.fn[NAME].Constructor = Modal
-  $.fn[NAME].noConflict = function () {
+  $.fn[NAME].noConflict = () => {
     $.fn[NAME] = JQUERY_NO_CONFLICT
     return Modal._jQueryInterface
   }