React 19 introduces ground-breaking paradigms like Server Actions and automatic memoization via the React Compiler.

Optimistic State UI Update Pattern

import { useOptimistic, useTransition } from 'react';

export function CommentThread({ comments, onAddComment }) {
  const [isPending, startTransition] = useTransition();
  const [optimisticComments, addOptimisticComment] = useOptimistic(
    comments,
    (state, newComment) => [...state, { text: newComment, pending: true }]
  );

  async function handleSubmit(formData) {
    const text = formData.get('comment');
    startTransition(async () => {
      addOptimisticComment(text);
      await onAddComment(text);
    });
  }

  return (
    <form action={handleSubmit}>
      <input name="comment" required />
      <button type="submit" disabled={isPending}>Post Comment</button>
    </form>
  );
}