What are the key features introduced in React 18?

React 18, released in March 2022, introduced several groundbreaking features focused on performance and user experience:

#### 1. Automatic Batching
Batch multiple state updates together (even in async code) to reduce re-renders.

     // Before React 18: Only batched in event handlers
     // After React 18: Batched everywhere
     setTimeout(() => {
       setCount(c => c + 1);
       setFlag(f => !f);
       // Only 1 re-render in React 18!
     }, 1000);
     

#### 2. Concurrent Features

• useTransition: Mark updates as non-urgent
• useDeferredValue: Defer expensive re-renders
• Suspense on Server: SSR with Suspense support

#### 3. New createRoot API

     // Old way (React 17)
     ReactDOM.render(<App />, document.getElementById('root'));

     // New way (React 18)
     import { createRoot } from 'react-dom/client';
     const root = createRoot(document.getElementById('root'));
     root.render(<App />);
     

#### 4. Streaming SSR with Suspense
Stream HTML from server and hydrate components as they arrive.

#### 5. New Hooks

• useId: Generate unique IDs for accessibility
• useSyncExternalStore: Subscribe to external stores
• useInsertionEffect: For CSS-in-JS libraries

#### 6. Strict Mode Improvements
Double-invokes effects in development to catch bugs.