group stimulus scripts in an asset subfolder

app/assets/stimulus/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);

app/assets/stimulus/controllers.json

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

app/assets/stimulus/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/stimulus/controllers/hello_controller.js';
+    }
+}