React 18 introduces several Concurrent Features that allow React to work in a more efficient and responsive way by rendering in the background, prioritizing updates, and keeping the UI interactive. These features are designed to make React apps feel faster and more fluid, especially when dealing with complex and resource-intensive UIs.

Key Concurrent Features in React 18:

1. Concurrent Rendering (Concurrent Mode):

   • What it is: React’s new rendering engine (Concurrent Rendering) allows React to interrupt rendering work and prioritize updates based on their importance. Instead of blocking the UI while rendering, React can pause work, do more urgent updates, and then come back to finish the remaining work.
   • How it works: React can break up long-running tasks into smaller chunks, allowing the app to remain responsive. For example, while React is rendering a list, it can pause to handle user input like clicks or typing, and then resume rendering the rest of the list afterward.
   • How to enable it: In React 18, concurrent rendering is opt-in. You can enable it by wrapping your app with ReactDOM.createRoot().
   const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);


2. Suspense for Data Fetching:

   • What it is: React Suspense, which was initially introduced for code splitting, now supports data fetching as well. It allows components to “wait” for something (like data) before rendering. This helps avoid blocking the UI when you’re waiting for asynchronous data (e.g., from an API or a server).
   • How it works: You can use Suspense to wrap components that depend on asynchronous data. While the data is being fetched, a fallback (like a loading spinner) is shown, and once the data is available, the component renders.
   • How to use:

     import React, { Suspense } from 'react';
     
     const DataComponent = React.lazy(() => fetchData());
     
     function App() {
     return (
     <Suspense fallback={<div>Loading...</div>}>
     <DataComponent />
     </Suspense>
     );
     }

3. Automatic Batching of Updates:

   • What it is: React 18 introduces automatic batching of updates, meaning that React can group multiple updates into a single render. This reduces the number of re-renders and makes your app more efficient.
   • How it works: With automatic batching, even updates triggered by event handlers, promises, or timeouts will be batched together. This is beneficial because it prevents React from performing multiple re-renders when multiple state updates happen simultaneously.
   • Example:
     function Counter() {
     const [count, setCount] = useState(0);
     
     const handleClick = () => {
     setCount(count + 1);
     setCount(count + 2);
     };
     return <button onClick={handleClick}>Count: {count}</button>;
     }

4. Transition API:

   • What it is: The Transition API allows you to mark certain updates as transitions. Transitions are updates that can be delayed until more urgent updates (like user interactions) are completed. This helps in cases like navigating through pages or updating non-critical UI elements without blocking user interactions.
   • How it works: You can use the startTransition function to wrap state updates that you consider non-urgent. React will prioritize urgent updates (like typing or clicking) over transitions.
   • Example:

     import { useState, startTransition } from 'react';
     
     function Search() {
     const [query, setQuery] = useState('');
     const [results, setResults] = useState([]);
     
     const handleChange = (event) => {
     const newQuery = event.target.value;
     setQuery(newQuery);
     
     startTransition(() => {
     // Simulate fetching results for the search query
     fetchResults(newQuery).then((data) => {
     setResults(data);
     });
     });
     };
     
     return (
     <div>
     <input type="text" value={query} onChange={handleChange} />
     <div>Results: {results.length}</div>
     </div>
     );
     }

5. Concurrent Features in useDeferredValue:

   • What it is: The useDeferredValue hook helps delay rendering of certain components until more important updates have been processed. This is especially useful in situations like search input or filters, where the UI should remain responsive and not update too quickly while typing.
   • How it works: You wrap a value inside useDeferredValue to allow React to defer its update and prioritize more urgent UI updates (like typing). The deferredValue will be updated at a later time, preventing unnecessary re-renders.
   • Example:
     import { useState, useDeferredValue } from 'react';
     
     function FilterList() {
     const [query, setQuery] = useState('');
     const deferredQuery = useDeferredValue(query);
     
     const filteredItems = items.filter(item =>
     item.name.toLowerCase().includes(deferredQuery.toLowerCase())
     );
     
     return (
     <div>
     <input type="text" value={query} onChange={(e) => setQuery(e.target.value)} />
     <ul>
     {filteredItems.map(item => (
     <li key={item.id}>{item.name}</li>
     ))}
     </ul>
     </div>
     );
     }
     

Summary of Concurrent Features:

1. Concurrent Rendering: React can prioritize updates, interrupt and resume work, leading to faster, more responsive UIs.
2. Suspense for Data Fetching: React can suspend rendering until data is ready, making it easier to manage async data.
3. Automatic Batching: React automatically batches updates, reducing unnecessary re-renders and improving performance.
4. Transition API: Allows non-urgent updates (like animations or search results) to be delayed while urgent updates (like typing) are processed first.
5. useDeferredValue: Defers updates to values (like search queries) to prevent frequent re-renders, improving performance during user input.

How to Enable Concurrent Features:

In React 18, Concurrent Mode and other concurrent features are opt-in and require using ReactDOM.createRoot() to create a root, as opposed to the older ReactDOM.render() method.

Conclusion:

React 18’s concurrent features allow you to build more performant and responsive applications by enabling fine-grained control over updates. These features make it easier to manage complex UIs and asynchronous tasks, improving the user experience and app performance.