add react

app/javascript/application.ts

@@ -1 +0,0 @@
-console.log('Hello, Typescript!')

app/javascript/application.tsx

@@ -0,0 +1,14 @@
+import React from 'react';
+import { createRoot } from 'react-dom/client';
+
+const container = document.getElementById('app');
+
+const App = () => {
+  return (<div>Hello, Rails 7!</div>)
+}
+
+if (container) {
+  const root = createRoot(container); // createRoot(container!) if you use TypeScript
+
+  root.render(<App />);
+}