React 19 — Notable New Features You Should Know

React Compiler - Automatic Performance Optimization

React 19 introduces the React Compiler - a tool that automatically memoizes components without requiring useMemo or useCallback. The compiler analyzes your code at build time and adds optimizations automatically.

Before React 19

const MemoizedComponent = React.memo(({ data }) => {
  const processed = useMemo(() => heavyComputation(data), [data]);
  const handler = useCallback(() => doSomething(data), [data]);
  return <div onClick={handler}>{processed}</div>;
});

With React 19

function Component({ data }) {
  const processed = heavyComputation(data);
  const handler = () => doSomething(data);
  return <div onClick={handler}>{processed}</div>;
}
// React Compiler handles optimization automatically

Server Components - Server-First by Default

React 19 brings Server Components into the core. Components render on the server by default, significantly reducing bundle size and improving initial load performance.

Practical Benefits

• Smaller bundle - Server-only code never reaches the client
• Direct database access - Query the DB directly inside a component
• Better SEO - HTML is rendered before JavaScript loads
• Faster TTFB - Reduced Time to First Byte

Actions - Simpler Form Handling

Server Actions let you call server functions directly from client components. No need to create separate API routes for every mutation.

The use() Hook - Read Promises and Context

The new use() hook lets you read promises and contexts more flexibly, even inside conditional rendering.

Conclusion

React 19 is a major step forward, focused on DX and performance. If you're on React 18, start planning your migration. Most breaking changes are handled by codemods.