Q: What is a React 19 Action? A: An Action is a function passed to <form action> or created with useActionState. React tracks its pending state, handles the form data, and manages error/success states automatically — no manual useState for loading spinners needed.

Q: What does useActionState return? A: It returns [state, formAction, isPending]. state holds the current result/error from the action, formAction is the wrapped function to pass to <form>, and isPending tells you if the action is still running.

Q: How is useFormStatus different from useActionState? A: useFormStatus is called inside a form component and reads the parent form’s pending state. useActionState wraps the action itself and provides both state and pending status to the component that defines the action.

Q: Can Actions be used outside of forms? A: Yes. You can pass an Action to startTransition or use it as an event handler. React will track the pending state and handle error boundaries automatically.

Q: What problem does useOptimistic solve? A: It enables optimistic UI updates — showing the expected result immediately before the server confirms. If the server request fails, the UI automatically reverts to the real state.

Q: What are the arguments to useOptimistic? A: useOptimistic(initialState, updateFn). initialState is the real data. updateFn(currentState, optimisticValue) returns the new optimistic state.

Q: When would you NOT use optimistic updates? A: For financial transactions, irreversible operations (like permanent deletion), or when the UI would be confusing if it had to revert. Always show a clear indicator when using optimistic state.

Q: How is use() different from useContext? A: use() can read Context or Promises. Unlike useContext, it can be called conditionally and inside loops. It also triggers Suspense when reading a Promise.

Q: When would you use use() instead of useEffect for data fetching? A: When you want Suspense integration — use() suspends the component, showing a fallback while data loads, without manual loading states or useEffect boilerplate.

Q: What happens if use() receives a rejected Promise? A: The nearest Error Boundary catches it. You don’t need try/catch or error state — just wrap the component in an Error Boundary.

Q: What is the difference between Server and Client Components? A: Server Components run only on the server — they can access databases directly and ship zero JavaScript to the browser. Client Components ("use client") run in the browser with full hooks support.

Q: Can a Server Component have useState or useEffect? A: No. Server Components are async functions that render to static HTML. They cannot use any hooks. For interactivity, compose a Client Component ("use client") inside a Server Component.

Q: How do Server Components improve performance? A: They eliminate JavaScript bundles for non-interactive content, reduce client-side processing, and can stream content progressively using Suspense boundaries.

Q: What does the React Compiler do? A: It automatically memoizes components and hooks at build time. You no longer need useMemo, useCallback, or React.memo — the compiler adds them automatically.

Q: Is the React Compiler required? A: No. It’s an opt-in optimization. Existing code works without it. When enabled, it recompiles your components to add automatic memoization, reducing manual optimization work.

Q: What changed with ref in React 19? A: ref is now a regular prop — you can pass it directly to function components without forwardRef. The forwardRef wrapper is no longer needed (but still works for backward compatibility).

Q: How does React 19 improve Suspense? A: React 19 adds stable support for Suspense with data fetching. The new use() hook suspends components natively, and <Suspense> integrates with Actions for seamless loading states.

Q: What are the four key features of React 19? A: (1) Actions & useActionState for form handling, (2) useOptimistic for optimistic UI, (3) use() for reading Promises and Context, and (4) the React Compiler for automatic memoization.

• React 19 Announcement
• React Docs: Actions
• React Compiler