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

assets/app.js

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+ /*
+ * 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';

assets/bootstrap.js

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+import { startStimulusApp } from '@symfony/stimulus-bridge';
+
+// Registers Stimulus controllers from controllers.json and in the controllers/ directory
+export const app = startStimulusApp(require.context(
+    '@symfony/stimulus-bridge/lazy-controller-loader!./controllers',
+    true,
+    /\.[jt]sx?$/
+));
+
+// register any custom, 3rd party controllers here
+// app.register('some_controller_name', SomeImportedController);

assets/controllers.json

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+{
+    "controllers": [],
+    "entrypoints": []
+}

assets/controllers/hello_controller.js

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+import { Controller } from '@hotwired/stimulus';
+
+/*
+ * This is an example Stimulus controller!
+ *
+ * Any element with a data-controller="hello" attribute will cause
+ * this controller to be executed. The name "hello" comes from the filename:
+ * hello_controller.js -> "hello"
+ *
+ * Delete this file or adapt it for your use!
+ */
+export default class extends Controller {
+    connect() {
+        this.element.textContent = 'Hello Stimulus! Edit me in assets/controllers/hello_controller.js';
+    }
+}

assets/jstest.js

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+export default function (param) {
+    return 'test - '.repeat(param)
+}

assets/styles/app.scss

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+@import "~bootstrap";
+
+body {
+    background-color: lightblue !important;
+}