What new features appeared in React 19? Tell us about Concurrent Mode and startTransition.

React 19 was not officially released at the time of my last update, so there are no precise details about its new features. However, the latest versions of React actively develop concepts such as Concurrent Mode and the startTransition API.

Concurrent Mode is an experimental mode that allows React to work asynchronously, breaking rendering into smaller tasks. This improves interface responsiveness, enabling priority updates (such as user input) to be performed faster without blocking the UI.

startTransition is an API that explicitly marks state updates as low priority. For example, during text input, you can update the display immediately, while heavy computations or rendering large lists can be marked with startTransition so React can pause them and not block input.

Example of using startTransition:

import { startTransition, useState } from 'react';

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

  function handleChange(e) {
    const value = e.target.value;
    setQuery(value);
    startTransition(() => {
      // update that does not block input
      setResults(heavySearch(value));
    });
  }

  return <input value={query} onChange={handleChange} />;
}

Thus, Concurrent Mode and startTransition help create smoother and more responsive interfaces, especially during complex and resource-intensive updates.