Add all app files

app/javascript/application.js

@@ -0,0 +1,3 @@
+// Configure your import map in config/importmap.rb. Read more: https://github.com/rails/importmap-rails
+import "@hotwired/turbo-rails"
+import "controllers"

app/javascript/controllers/application.js

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+import { Application } from "@hotwired/stimulus"
+
+const application = Application.start()
+
+// Configure Stimulus development experience
+application.warnings = true
+application.debug    = false
+window.Stimulus      = application
+
+export { application }

app/javascript/controllers/hello_controller.js

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+import { Controller } from "@hotwired/stimulus"
+
+export default class extends Controller {
+  connect() {
+    this.element.textContent = "Hello World!"
+  }
+}

app/javascript/controllers/index.js

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+// Import and register all your controllers from the importmap under controllers/*
+
+import { application } from "controllers/application"
+import { registerControllersFrom } from "@hotwired/stimulus-importmap-autoloader"
+registerControllersFrom("controllers", application)