Question Bank

React Interview Questions

React questions from reconciliation and Fiber internals to hooks, Suspense, concurrent rendering, Server Components, and the React 19 compiler era. Includes the trade-off and debugging scenarios senior interviews lean on, each with an ELI5 explanation and diagram.

67 questions — free to read, with quizzes and AI-scored practice for members.

React67 questions

What is the difference between props and state in React?junior What is the difference between controlled and uncontrolled components in React?junior What are React hooks and why are they used?junior What is JSX and why does React use it?junior Why is the `key` prop important when rendering lists in React?junior What is the `useEffect` hook in React and when do you use it?junior What are the concrete architectural trade-offs between Zustand and Redux Toolkit, and when should a team choose one over the other?mid What is useActionState in React 19, what boilerplate does it eliminate, and how does it integrate with Server Actions and progressive enhancement?mid Explain how React's Reconciliation, Virtual DOM, and Fiber work together.mid What triggers a React component to re-render, and how costly is it?mid Explain the differences and coordination between useMemo, useCallback, and React.memo in React.mid How did ref handling change in React 19 — why was forwardRef needed before, what does the new ref-as-prop model enable, and what is the new ref cleanup API?mid What is automatic batching in React 18, how does it differ from React 17 batching, and when would you need to opt out with flushSync?mid What does React 18's createRoot API unlock compared to legacy ReactDOM.render(), and what is the migration path for an existing app?mid Explain React 18's useTransition hook: what problem it solves, how the concurrent scheduler handles non-urgent updates, and how isPending drives UX.mid What does useFormStatus return, why must it be called in a child of a form element, and how does it eliminate prop-drilling for submit button state?mid What can the use() hook read in React 19, why can it be called conditionally unlike other hooks, and how does it integrate with Suspense for Promises?mid What is useDeferredValue in React 18, how does it keep UI responsive for expensive derived computations, and when should you prefer it over useTransition?mid What is the useId hook in React 18, what SSR hydration problem does it solve compared to Math.random(), and what are valid vs invalid use cases?mid How does useOptimistic in React 19 work, how does it automatically roll back on error, and how does it compare to manually managing optimistic state?mid What are Server Actions in React 19, how are they exposed as HTTP endpoints under the hood, and what security mistakes must you avoid?senior How does Qiankun enable micro-frontend architecture in a React application?senior How does TypeScript enhance React development?senior How does unidirectional data flow in Redux simplify debugging?senior How did you improve conversion rates and DAU for a payments platform?senior When should you choose useReducer over useState in React?senior When should you use useMemo over useCallback in React?senior What are some basic concepts of Web3 that a frontend engineer should know?senior How does EIP-4337 enable smart wallets without changing the Ethereum protocol?senior How would you design a React SDK for third-party developers?senior How should you handle a tri-state variable like isNewCrossCurrencyUI in React?senior How can React components utilize different types of memory layers to manage state and behavior effectively?senior How does TanStack Query manage its query cache, stale time, and background refetching — and how do you tune these for a high-traffic dashboard?senior How do you ensure high-quality UI/UX in a React application?senior How did you achieve the 12% to 15% performance improvement mentioned in your CV using React?senior How would you design an error handling architecture for a payment SDK in React?senior How does the exponential backoff retry pattern improve payment SDK reliability?senior How does the Orchestrator-Workers pattern improve scalability in a React application?senior What is a stale closure in useEffect, why does it occur, and how can you handle it?senior What are the four types of state in React and their suitable tools?senior How would you structure your response using the STAR framework to describe a technical challenge you faced?senior How would you design a React-based frontend to detect and fix bugs using user interactions and backend logs?senior How would you design a React component for a web3 checkout interface?senior How does a cross-chain bridge protocol improve transactions compared to traditional single-chain methods?senior Why does a failed Ethereum transaction still incur a gas cost?senior How would you architect a micro-frontend system using React and Qiankun?senior How does micro-frontend architecture benefit large-scale React applications?senior What are the key differences between EOA and Smart Contract Wallets in terms of control and transaction capabilities?senior Describe a time you took ownership of a challenging project in React.senior How would you design a universal checkout SDK for a payments platform?senior How does React's architecture manage state and flow?senior What does the React Compiler do at build time, which manual memoization patterns does it replace, and what code patterns prevent it from optimising a component?senior How would you design a scalable frontend architecture for a fintech application using React?senior How do Server Components differ from Client Components in React, and when should each be used?senior How do you decide where to store a piece of state in a React application?senior How does the Compound Components pattern improve flexibility and reusability in React?senior What should you prepare for the React Assessment in the interview process?senior How would you implement a debounce function in a React component to handle API requests efficiently?senior What are the key differences between LLM Call, Workflow, and Agent in Anthropic's framework?senior How does the ReAct pattern in React help manage state updates in a loop?senior How can advanced React patterns improve component reusability?senior How do React Suspense and concurrent features like useTransition change how you architect data-fetching and UI responsiveness?senior How would you design an error boundary strategy for a large React application to isolate failures without degrading the full page?senior What causes React hydration mismatches, how do you diagnose them, and how does React 18 selective hydration change the recovery strategy?senior Compare useTransition and useDeferredValue: when do you own the state update vs receive a value, how do their loading signals differ, and can they be combined?senior What is useSyncExternalStore, what is "tearing" in concurrent React, and how does this hook guarantee a consistent snapshot from external stores?senior How does React 18 streaming SSR with renderToPipeableStream and Suspense improve TTFB, FCP, and TTI compared to the legacy renderToString approach?senior

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