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What is the latest React version, and is 'React 20' real?
What are Actions in React 19?
What does useActionState do?
What does useFormStatus do?
What does useOptimistic do?
What is the use() API?
What are React Server Components (RSC)?
'use client' vs 'use server' — what does each mark?
Async functions passed to a transition or form. React automatically manages the pending state, error handling, optimistic updates, and form resets you used to hand-write.
React 19.2 (19.0 shipped Dec 2024; 19.2 landed Oct 2025). No React 20 exists. React is now stewarded by the independent React Foundation under the Linux Foundation.
Lets a child component read the status (pending, data) of its nearest parent <form> — like context for forms. Classic use: a submit button that disables itself.
Wraps an action and returns [state, formAction, isPending] — replacing hand-rolled loading/error/result state for submissions. Pass formAction straight to <form action={...}>.
Reads a promise or context during render, suspending until the promise resolves. Unlike hooks, use() can be called conditionally and inside loops.
Shows a temporary 'optimistic' value while an async action is in flight, then automatically reverts to the real state when the action completes or fails.
'use client': the boundary where components ship to the browser. 'use server': marks Server Functions callable from the client — NOT server components (those are the default, no directive).
Components that run only on the server and send serialized UI — not JavaScript — to the client. Zero bundle cost. 'use client' marks where the client-side world begins.
What replaced forwardRef?
How does React 19 handle <title> and <meta> tags?
What is the React Compiler?
What is the <Activity> component (19.2)?
What does useEffectEvent do (19.2)?
Name three smaller React 19 quality-of-life changes.
Which headline features are still NOT stable in 19.2?
Interview one-liner: what's the architectural theme of React 19?
Render them anywhere in a component and React hoists them to <head> automatically — native document metadata support, plus stylesheet and async script precedence handling.
Nothing is needed anymore: in React 19, ref is a regular prop on function components. forwardRef is deprecated and codemods remove it.
Wraps UI with modes 'visible' and 'hidden'. Hidden trees keep their state but unmount effects and render at low priority — ideal for tabs or pre-rendering likely-next screens.
A build-time compiler (stable 1.0, late 2025) that auto-memoizes components and values by understanding the Rules of React — retiring most manual useMemo, useCallback, and React.memo.
Context used directly as a provider (<MyContext> instead of .Provider), ref callbacks can return cleanup functions, and hydration errors now show a single readable diff.
Extracts the 'event' part of an effect so it always reads the latest props/state without being a dependency — eliminating stale closures and effects that re-fire too often.
Work moves off the client and out of your hands: RSC shifts rendering to the server, Actions absorb form boilerplate, and the Compiler absorbs memoization — less JS shipped, less code to maintain.
View Transitions and Fragment Refs — real and demoed, but only in Canary/Experimental channels. Saying they're stable in an interview is a red flag.