What are the new features in React 19? How do they improve developer experience and performance?

What are the new features in React 19? How do they improve developer experience and performance?

React 19 introduces several significant features that enhance both developer experience and application performance.

New Features:

1. React Compiler:

// Automatic memoization - no need for useMemo/useCallback
function ExpensiveComponent({ items, filter }) {
  // React Compiler automatically optimizes this
  const filteredItems = items.filter(item => item.category === filter);
  const expensiveValue = filteredItems.reduce((sum, item) => sum + item.value, 0);
  
  return <div>{expensiveValue}</div>;
}

// Before React 19, you'd need:
function ExpensiveComponentOld({ items, filter }) {
  const filteredItems = useMemo(() => 
    items.filter(item => item.category === filter), [items, filter]
  );
  const expensiveValue = useMemo(() => 
    filteredItems.reduce((sum, item) => sum + item.value, 0), [filteredItems]
  );
  
  return <div>{expensiveValue}</div>;
}

1. Actions and useActionState:

import { useActionState } from 'react';

// Server actions with built-in state management
async function updateUser(prevState, formData) {
  try {
    const result = await fetch('/api/users', {
      method: 'POST',
      body: formData
    });
    
    if (!result.ok) {
      return { error: 'Failed to update user' };
    }
    
    return { success: true, user: await result.json() };
  } catch (error) {
    return { error: error.message };
  }
}

function UserForm() {
  const [state, formAction, isPending] = useActionState(updateUser, {});
  
  return (
    <form action={formAction}>
      <input name="name" required />
      <button disabled={isPending}>
        {isPending ? 'Updating...' : 'Update User'}
      </button>
      {state.error && <div className="error">{state.error}</div>}
      {state.success && <div className="success">User updated!</div>}
    </form>
  );
}

1. use() Hook:

import { use } from 'react';

function UserProfile({ userPromise }) {
  // Can unwrap promises directly
  const user = use(userPromise);
  
  return (
    <div>
      <h1>{user.name}</h1>
      <p>{user.email}</p>
    </div>
  );
}

// Usage with async data
function App() {
  const userPromise = fetch('/api/user/123').then(res => res.json());
  
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <UserProfile userPromise={userPromise} />
    </Suspense>
  );
}

1. Document Metadata:

import { Title, Meta, Link } from 'react';

function BlogPost({ post }) {
  return (
    <>
      <Title>{post.title}</Title>
      <Meta name="description" content={post.excerpt} />
      <Meta property="og:title" content={post.title} />
      <Link rel="canonical" href={`/posts/${post.slug}`} />
      
      <article>
        <h1>{post.title}</h1>
        <div dangerouslySetInnerHTML={{ __html: post.content }} />
      </article>
    </>
  );
}

1. Ref as Prop:

// ref can now be passed as a regular prop
function MyInput({ ref, ...props }) {
  return <input ref={ref} {...props} />;
}

function Form() {
  const inputRef = useRef(null);
  
  return (
    <form>
      <MyInput ref={inputRef} placeholder="Enter text..." />
      <button onClick={() => inputRef.current?.focus()}>
        Focus Input
      </button>
    </form>
  );
}

Performance Improvements:

• Automatic memoization reduces unnecessary re-renders
• Better tree-shaking and bundle optimization
• Improved hydration performance
• Enhanced concurrent features