Frontend system thinking
React Interview Questions
Rendering, state, effects, hooks and performance. The questions that come up when the interviewer asks why, not just how.
40 questions8 easy23 medium9 hard0/40 learned
01What is React and why would you use it?Easyreact · components · declarative UI02How does the virtual DOM and reconciliation work?Mediumvirtual DOM · reconciliation · rendering03What is JSX and how does it work?EasyJSX · elements · syntax04What is the difference between a React element and a component?Easyelements · components · rendering05What is the difference between props and state?Easyprops · state · data flow06Why is React state described as a snapshot?Mediumstate · rendering · closures07What is state batching in React?Mediumstate · batching · rendering08When should you use useReducer instead of useState?MediumuseState · useReducer · state09When do you need a functional state update?Mediumstate · updater · batching10What is the difference between controlled and uncontrolled components?Easyforms · controlled · uncontrolled11What does lifting state up mean?Easystate · composition · data flow12What is derived state and why is duplicating it a problem?Mediumderived state · state · rendering13Why are keys important when rendering lists?Mediumkeys · lists · reconciliation14Why does React prefer composition over inheritance?Easycomposition · children · reuse15What is a custom hook and when should you create one?Mediumcustom hooks · reuse · stateful logic16What are the rules of hooks and why do they exist?Mediumhooks · rules of hooks · call order17What is useEffect actually for?EasyuseEffect · side effects · synchronization18How do you think about useEffect dependencies?MediumuseEffect · dependencies · reactive values19Why does useEffect return a cleanup function?MediumuseEffect · cleanup · subscriptions20What is a stale closure in React and how do you fix it?Hardclosures · useEffect · stale state21When should you use useRef instead of useState?MediumuseRef · useState · mutable values22How do you use refs to interact with DOM nodes?MediumuseRef · DOM · focus management23When would you use useLayoutEffect instead of useEffect?HarduseLayoutEffect · useEffect · layout24What is the difference between useMemo and useCallback?MediumuseMemo · useCallback · memoization25How does React.memo work and when does it help?MediumReact.memo · rendering · props26How do you approach React performance optimization?Hardperformance · profiling · rendering27When should you use Context instead of prop drilling?MediumContext · prop drilling · state sharing28Why do all context consumers re-render when the context value changes?HardContext · re-renders · performance29What is an error boundary and what does it catch?Mediumerror boundaries · resilience · errors30How do Suspense and React.lazy work for code splitting?MediumSuspense · React.lazy · code splitting31What is concurrent rendering and what problem does it solve?Hardconcurrent rendering · startTransition · responsiveness32When would you use useTransition versus useDeferredValue?HarduseTransition · useDeferredValue · concurrent UI33What does React Strict Mode do and why does it double-invoke code?MediumStrictMode · development · purity34What is a React portal and when should you use one?Mediumportals · modals · DOM35How do refs and useImperativeHandle expose imperative component APIs?Mediumrefs · useImperativeHandle · imperative API36What are Actions in React 19 and how do they handle form updates?HardReact 19 · Actions · forms · useActionState37What are React Server Components and how do they differ from SSR?HardServer Components · RSC · SSR38How does server-side rendering and hydration work in React?HardSSR · hydration · rendering39How do you test React components with Testing Library?Mediumtesting · Testing Library · accessibility40How do you build accessible React interfaces?Mediumaccessibility · semantics · keyboard
Reading is step one
Now say it out loud.
Understanding an answer and delivering it under pressure are different skills. Rehearse this topic in a live React mock interview and get scored feedback on depth, clarity and confidence.