Modern React: Concurrent Features

45 min

React 18 introduced concurrent features that allow React to interrupt, pause, and resume work as needed.

`startTransition` allows you to mark updates as non-urgent, which can be interrupted by more urgent updates.

`useDeferredValue` creates a new copy of the value that may be 'lagging behind' the original value.

Concurrent features enable React to keep the UI responsive even during expensive updates by prioritizing urgent updates over non-urgent ones.

These features are part of React's concurrent rendering model, which allows React to work on multiple versions of the UI simultaneously.

Understanding concurrent features helps build more responsive applications, especially when dealing with large lists, complex calculations, or slow network requests.

Key Concepts

Concurrent features allow React to prioritize urgent updates.
startTransition marks updates as non-urgent and interruptible.
useDeferredValue creates a deferred version of a value.
These features keep UIs responsive during expensive operations.
Concurrent rendering enables better user experience.

Learning Objectives

Master

Using startTransition for non-urgent updates
Implementing useDeferredValue for deferred rendering
Understanding when to use concurrent features
Improving app responsiveness with concurrent rendering

Develop

Performance optimization thinking
Understanding React's rendering priorities
Building responsive user interfaces

Tips

Use startTransition for updates that can wait (like filtering large lists).
Use useDeferredValue for values that don't need immediate updates.
Keep urgent updates (like typing in an input) outside of transitions.
Monitor performance improvements with React DevTools Profiler.

Common Pitfalls

Wrapping all updates in startTransition (only non-urgent ones).
Using concurrent features when simpler optimizations would suffice.
Not understanding the difference between urgent and non-urgent updates.
Overusing these features without measuring actual performance gains.

Summary

Concurrent features prioritize urgent updates over non-urgent ones.
startTransition marks updates as interruptible.
useDeferredValue defers non-critical value updates.
These features improve app responsiveness and user experience.

Exercise

Use startTransition to handle non-urgent updates.

import { useState, useTransition } from 'react';

function SearchResults() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();

  const handleSearch = (newQuery) => {
    setQuery(newQuery);
    
    startTransition(() => {
      // Simulate expensive search
      const searchResults = Array.from({ length: 1000 }, (_, i) => 
        `Result ${i} for ${newQuery}`
      );
      setResults(searchResults);
    });
  };

  return (
    <div>
      <input value={query} onChange={(e) => handleSearch(e.target.value)} />
      {isPending && <div>Searching...</div>}
      <ul>
        {results.slice(0, 10).map((result, index) => (
          <li key={index}>{result}</li>
        ))}
      </ul>
    </div>
  );
}

Exercise Tips

Keep the input update outside startTransition for immediate feedback.
Add debouncing to reduce the frequency of searches.
Use isPending to show loading states.
Compare performance with and without startTransition.

Modern React: Concurrent Features