npx depfixer

git add -A
git commit -m "chore: pre-upgrade checkpoint"

npx depfixer migrate

Select target version:

  > React 19 (Latest)
      -> 19.0.0 - Latest stable

    React 18 (LTS)
      -> 18.3.0 - Long-term support

✨ SUCCESS
──────────────────────────────────────────────────

    ✓ 8 Packages Updated
    ✓ Backup: package.json.bak

    👉 NEXT STEP:
    Run the following command to finalize changes:

    $ npm install

npm install

// Old (React 17)
import ReactDOM from 'react-dom';
ReactDOM.render(<App />, document.getElementById('root'));

// New (React 18+)
import { createRoot } from 'react-dom/client';
const root = createRoot(document.getElementById('root')!);
root.render(<App />);

npx tsc --noEmit

// React 17: Two renders
// React 18: One render (batched)
setTimeout(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
}, 1000);

useEffect(() => {
  const subscription = subscribe();
  return () => subscription.unsubscribe(); // Must clean up!
}, []);

// tsconfig.json
{
  "compilerOptions": {
    "jsx": "react-jsx"
  }
}

// React 18
const Input = forwardRef((props, ref) => (
  <input ref={ref} {...props} />
));

// React 19 (forwardRef not needed)
function Input({ ref, ...props }) {
  return <input ref={ref} {...props} />;
}

cp package.json.bak package.json
rm -rf node_modules
npm install