📰 What’s New in React Version 19 — A Complete Guide with Examples

React 19 is one of the most significant releases in the history of React.
It introduces new APIs, improves the rendering model, enhances Server Components, and simplifies patterns that previously required complex workarounds.

This article covers all major features, explained simply with real examples so you can start using them immediately.

🟦 1. React’s New Component

React 19 introduces a totally new concept: the Activity component.

It allows you to control visibility, mounting, and prioritization of UI.

Before React 19, if you wrote:

{isVisible && <Page />}

When isVisible becomes false:

• component unmounts
• all state is lost
• async work cancels
• any scroll position disappears

React 19 solves this.

How works

<Activity mode={isVisible ? "visible" : "hidden"}>
  <Page />
</Activity>

Example: Tab Switching Without Losing State

Before (old React):

function App() {
  const [tab, setTab] = useState("profile");

  return (
    <>
      <button onClick={() => setTab("profile")}>Profile</button>
      <button onClick={() => setTab("settings")}>Settings</button>

      {tab === "profile" && <Profile />}
      {tab === "settings" && <Settings />}
    </>
  );
}

Switching tabs unmounts the other screen → state resets.

After (React 19):

function App() {
  const [tab, setTab] = useState("profile");

  return (
    <>
      <button onClick={() => setTab("profile")}>Profile</button>
      <button onClick={() => setTab("settings")}>Settings</button>

      <Activity mode={tab === "profile" ? "visible" : "hidden"}>
        <Profile />
      </Activity>

      <Activity mode={tab === "settings" ? "visible" : "hidden"}>
        <Settings />
      </Activity>
    </>
  );
}

Now both screens stay mounted → state persists.

🟦 2. useEffectEvent — Fixing Common useEffect Problems

This new hook solves one of the most annoying issues:
effects re-running because your callback depends on changing state.

Problem Before:
You often wrote messy code like:

useEffect(() => {
  const handler = () => {
    console.log(count); // stale or re-runs effect
  };

  connection.on("message", handler);
  return () => connection.off("message", handler);
}, [count]); // causes re-registering every time

This caused:

• unnecessary cleanup
• unnecessary re-runs
• stale closures

⭐ Solution: useEffectEvent

const onMessage = useEffectEvent((message) => {
  console.log("Latest count:", count);
});

useEffect(() => {
  connection.on("message", onMessage);
  return () => connection.off("message", onMessage);
}, []);

✔ Effect runs only once

✔ Yet your callback always has the latest state

🟦 3. New cacheSignal API for Server Components

React 19 brings better control to Server Components (RSC).
The new cacheSignal() API gives you an AbortSignal tied to React’s rendering lifecycle.

Why this matters?

If rendering is canceled (e.g., user navigates away), you can stop fetch calls early.

import { cache, cacheSignal } from "react";

const getUser = cache((id) => {
  return fetch(`/api/user/${id}`, { signal: cacheSignal() });
});

Now React can cancel the fetch if the render becomes unnecessary.

🟦 4. Improvements to Server Rendering (SSR)
React 19 introduces:

✔ Partial Prerendering

Render static parts first → dynamic parts later.

✔ Resumable Rendering

React can “pause” rendering on the server and “resume” later.

✔ Web Streams Support

Better streaming of server-rendered UI.

These unlock performance for frameworks like Next.js, Remix, Hydrogen, Expo Router, etc.

🟦 5. Improved Suspense Behavior
Suspense boundaries now:

show content sooner
avoid flickering
batch reveals to reduce layout jumps
This makes SSR + Suspense much smoother.

🟦 6. Updated useId() Prefix
React 19 updates the prefix of auto-generated IDs:

Old:

:r:
New:

r
Why?

• Better compatibility with CSS View Transitions
• XML-safe naming
• Cleaner ID structure

🟦 7. DevTools Enhancements
React 19 adds new Performance tracks in Chrome DevTools.

You can now see:

• React scheduler activity
• Transition priority work
• Component render timings
• Effect execution patterns
• Perfect for optimizing apps.

🟦 8. Many Small Fixes & Quality-of-Life Updates
Some notable small improvements:

• Better hydration and re-suspend behavior
• Better error messages
• Improved stringification of contexts
• Fixes for useDeferredValue infinite loops
• ARIA 1.3 attributes now allowed
• Fewer hydration mismatches
• All these make React apps more stable and predictable.

Final Summary: What’s Big in React 19?