React Concurrent Mode – Next-Gen Features for Responsive UI (2025 Guide)

Introduction – What is React Concurrent Mode?

React’s Concurrent Mode is a set of next-generation features introduced in React 18+ to make UIs more responsive, interruptible, and smooth under load.

It allows React to:

• Interrupt renders
• Prioritize urgent updates
• Pause and resume rendering
• Keep the UI snappy even when processing heavy logic

In this guide, you’ll learn:

• What Concurrent Mode is and how it works
• Key features like startTransition, useDeferredValue, Suspense
• When to use them and why
• How to upgrade safely and optimize interactivity

1. What Is Concurrent Mode?

Concurrent Mode is not a mode you “turn on”, but a set of features enabled by default in React 18+ via the new Concurrent Renderer.

It allows React to pause rendering a component and continue later, giving priority to more important UI updates (like user input).

React 18 introduced automatic batching, startTransition(), useDeferredValue(), and improved Suspense capabilities.

2. Setup – Use Concurrent Features in React 18+

Entry File:

import { createRoot } from 'react-dom/client';

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

createRoot enables concurrent rendering features
Old ReactDOM.render() is legacy and doesn’t support concurrency

3. startTransition – Prioritize Updates

Use startTransition() to mark non-urgent updates (like filtering, sorting, autocomplete).

import { startTransition } from 'react';

function SearchBox() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);

  const handleChange = (e) => {
    const value = e.target.value;
    setQuery(value);

    startTransition(() => {
      const filtered = bigList.filter((item) =>
        item.includes(value)
      );
      setResults(filtered);
    });
  };

  return (
    <>
      <input value={query} onChange={handleChange} />
      <ul>{results.map((r) => <li key={r}>{r}</li>)}</ul>
    </>
  );
}

UI stays responsive while searching large data sets
Urgent updates (typing) take precedence over heavy rendering

4. useDeferredValue – Delay Expensive UI Updates

Use useDeferredValue() to defer non-urgent updates and prevent UI lag.

const deferredQuery = useDeferredValue(query);
const filteredItems = useMemo(() =>
  items.filter(item => item.includes(deferredQuery)), [deferredQuery]
);

Input stays fast while filtering updates in the background

5. Suspense Improvements in Concurrent Mode

Concurrent Mode improves data fetching and UI fallback rendering with Suspense.

const ProductPage = React.lazy(() => import('./ProductPage'));

<Suspense fallback={<Loader />}>
  <ProductPage />
</Suspense>

Suspense now works for:

• Dynamic imports (React.lazy)
• Server Components (Next.js)
• Streaming SSR
• Data fetching libraries (Relay, React Query)

6. Automatic Batching

React 18 automatically batches state updates across events like timeouts, promises, and native events.

fetchData().then(() => {
  setLoading(false);
  setData(newData); //  Only one re-render
});

Previously, only React events were batched
Now async callbacks are too

7. Concurrent Mode Limitations

Best Practices

Use startTransition for non-blocking updates (search, filters)
Use useDeferredValue for smooth UX during typing
Combine with useMemo and React.memo for max performance
Use Suspense + lazy loading for routes, modals, and data
Use the React Profiler to measure improvements

Summary – Recap & Next Steps

React Concurrent Mode enables smarter rendering and better interactivity in modern apps. By letting React pause, prioritize, and resume rendering, your UI feels faster, lighter, and more responsive.

Key Takeaways:

• startTransition for background updates (filtering, sort)
• useDeferredValue for delayed value-based rendering
• Suspense enables smarter loading states
• Automatic batching reduces re-renders
• Upgrade via createRoot() for React 18+

Real-World Relevance:
Used in apps like Figma, Facebook, and Notion to deliver smooth search, real-time dashboards, and instant navigation without blocking the UI thread.

FAQ Section

Is Concurrent Mode enabled by default in React 18?
Yes, if you’re using createRoot() from react-dom/client.

What’s the difference between startTransition and useDeferredValue?
startTransition defers state updates, while useDeferredValue defers value rendering.

Do I need a special config to use Concurrent Mode?
No extra config—just use React 18+ with createRoot() and built-in APIs.

Can I use Concurrent Mode with class components?
Only Suspense is supported. Other concurrent features like useDeferredValue are Hook-specific.

Does React Query support Concurrent Mode?
Yes. React Query v5+ works seamlessly with Suspense and transitions.