Concurrent Rendering is a foundational change in React 18’s rendering mechanism. Instead of rendering being a single, synchronous, uninterruptible task, React can now prepare multiple versions of the UI simultaneously. It allows React to interrupt a long-running render (e.g., rendering a complex data grid) to handle a higher-priority update (like user input), pause it, resume it later, or even abandon the in-progress work if it becomes stale (e.g., data changes before the render completes). This makes apps more responsive by prioritizing user interactions over background rendering tasks.

How it Improves Performance & Responsiveness:

  • Non-blocking UI: Prevents the UI from freezing during expensive renders.
  • Better Resource Utilization: Works on lower-priority tasks during idle time.
  • Enables New Features: Powers useTransition, useDeferredValue, Suspense for data fetching.

Example – using useTransition:

const [isPending, startTransition] = useTransition();
// High priority update (e.g., typing in an input)
setInputValue(newValue);
// Low priority update (can be interrupted)
startTransition(() => { setSearchResults(newValue); // Heavy operation
});
// Show loading state: {isPending && <Spinner />}