React 18 brought major changes to how React works under the hood, focusing on better user experience and performance with concurrent rendering.

Key Features in React 18

• Automatic Batching
  Multiple state updates inside events, timeouts, or native handlers are now batched by default. This reduces unnecessary re-renders.

setCount(c => c + 1)
setFlag(f => !f)
// These are now batched automatically

• startTransition
  Marks updates as non-urgent, so React can keep the UI responsive.

import { startTransition } from 'react'

startTransition(() => {
  setSearchQuery(input)
})

Use this for things like filtering lists, background tab updates, or lazy-loaded components.

• Improved Suspense
  Suspense works more broadly now. It’s especially useful with streaming server rendering in frameworks like Next.js.

<Suspense fallback={<Loading />}>
  <Comments />
</Suspense>

• New Root API
  You must now use createRoot from react-dom/client.

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

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

• Concurrent Rendering
  Under the hood, React 18 introduces a concurrent renderer. It enables React to interrupt rendering to prioritize urgent tasks and continue where it left off.

This means:

• React can render updates in the background.
• Low-priority updates won’t block more important ones.
• UIs feel faster and more responsive.

Breaking Changes to Know

• useEffect Timing
  Effects now run after the layout and paint phase, closer to how the browser handles rendering. This change avoids blocking the browser's ability to paint updated content.

• Third-Party Library Compatibility
  Some older React libraries may need updates to fully support concurrent features.

Final Thoughts

React 18 doesn’t force you to change your code, but to take full advantage of the new features, you’ll need to gradually adopt the new APIs. If you're using a framework like Next.js or Remix, many of these features are already baked in.