Yesterday:

<nav script="dropdown.js" style="dropdown.css">
  <button onclick="toggleDropdown()">Menu</button>
</nav>

Today:

// index.js
import React from 'react';
import ReactDOM from 'react-dom';
import './global.css';
import App from './App';

ReactDOM.createRoot(document.getElementById('root')).render(<App />);

// App.jsx
import Dropdown from './components/Dropdown';
import './App.css';

export default function App() {
  return (
    <main>
      <Dropdown />
      <p>Hello, world!</p>
    </main>
  );
}

// components/Dropdown.jsx
import { useState } from 'react';
import styles from './Dropdown.module.css';
import ArrowIcon from '../assets/icons/ArrowIcon.jsx';

export default function Dropdown() {
  const [open, setOpen] = useState(false);
  return (
   <div className={styles.dropdown}>
      <button onClick={() => setOpen(!open)}>Menu <ArrowIcon /></button>
      {open && (
        <ul>
          <li>Option 1</li>
          <li>Option 2</li>
        </ul>
      )}
    </div>
  );
}