前言

app.js

import React from 'react';

const App = () => {
    return (
        <div>
            我是App
        </div>
    );
};

export default App;

index.js

import React from 'react';
import ReactDOM from 'react-dom/client';
import App from "./App";

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(
  <React.StrictMode>
    <App/>
  </React.StrictMode>
);