Installed webpack encore and configured js and css to use it.

assets/app.js

@@ -0,0 +1,18 @@
+ /*
+ * Welcome to your app's main JavaScript file!
+ *
+ * We recommend including the built version of this JavaScript file
+ * (and its CSS file) in your base layout (base.html.twig).
+ */
+
+// any CSS you import will output into a single css file (app.scss in this case)
+import './styles/app.scss';
+
+
+import 'bootstrap';
+import getNiceMessage from './jstest';
+
+console.info(getNiceMessage(3));
+
+// start the Stimulus application
+import './bootstrap';