React interview questions assess your understanding of component architecture, hooks, state management, rendering behavior, and performance optimization. Interviewers test both conceptual knowledge and practical skills, from explaining the virtual DOM to building custom hooks and debugging re-render issues.
Conceptual / Theory Questions (30)
Master React fundamentals: JSX, virtual DOM, reconciliation, the Fiber architecture, and the core concepts every React developer must understand.
easy What is React?
What interviewers evaluate: fundamentals
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easy What is JSX? How does it work and why is it used?
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easy What is the Virtual DOM and how does React use it?
What interviewers evaluate: rendering
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easy What is the difference between the Real DOM and the Virtual DOM?
What interviewers evaluate: rendering
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easy What is the difference between an Element and a Component in React?
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easy What is the difference between functional components and class components?
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easy What are props in React? How are they different from state?
What interviewers evaluate: state
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easy What is state in React and how do you update it?
What interviewers evaluate: state
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easy What is the difference between controlled and uncontrolled components?
What interviewers evaluate: state
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easy What is the purpose of the key prop in React? Why is it important in lists?
What interviewers evaluate: rendering
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medium What happens if you use array index as a key? What bugs can it cause?
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easy What are React Fragments and why are they used?
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medium What are synthetic events in React?
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medium What is reconciliation in React?
What interviewers evaluate: rendering
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hard What is React Fiber and what is its main goal?
What interviewers evaluate: architecture
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hard What is the diffing algorithm in React?
What interviewers evaluate: rendering
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easy Explain one-way data flow in React.
What interviewers evaluate: patterns
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medium What is prop drilling? How do you avoid it?
What interviewers evaluate: patterns
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medium Why does React recommend against mutating state?
What interviewers evaluate: state
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medium What is the difference between React and ReactDOM? Why are they separated?
What interviewers evaluate: fundamentals
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easy What is the children prop in React?
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easy Why should component names start with a capital letter?
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medium What is the difference between createElement and cloneElement?
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medium What are Pure Components in React?
What interviewers evaluate: rendering
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medium What does re-rendering mean in React? What triggers a re-render?
What interviewers evaluate: rendering
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medium What is the difference between re-render and re-mount?
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easy Why can't browsers read JSX directly?
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medium What is the difference between Shadow DOM and Virtual DOM?
What interviewers evaluate: rendering
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easy What are default props? How do you set them?
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easy How does React handle event handling differently from HTML?
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Hooks Questions (30)
Cover useState, useEffect, useMemo, useCallback, useRef, useContext, useReducer, and how to write custom hooks that follow React's rules.
easy What are React Hooks? Why were they introduced?
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easy What are the rules of Hooks and why are they important?
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easy How does the useState hook work?
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easy What is useEffect and what is the role of its dependency array?
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medium What is the difference between useEffect and useLayoutEffect?
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medium What is the difference between useMemo and useCallback?
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medium What is the useRef hook and when should you use it?
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medium What is the difference between useState and useRef?
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medium When should you use useReducer instead of useState?
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medium What is the useContext hook and when should you use it?
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medium How do you create and use custom hooks in React?
What interviewers evaluate: hooks
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hard What are "stale closures" in hooks and how do you fix them?
What interviewers evaluate: hooks
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medium How do you properly cancel async requests inside useEffect?
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medium Why can't you write useEffect(async () => { ... })?
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medium What happens if you return a Promise from useEffect?
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medium Why does useEffect run twice in development mode (React 18 Strict Mode)?
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easy Can you have multiple useEffect hooks in a single component?
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medium How do you prevent infinite loops with useEffect?
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hard What is useImperativeHandle and when would you use it?
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medium What is the useId hook and when should it be used?
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medium Can Hooks replace Redux?
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medium Do Hooks replace render props and higher-order components?
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easy Can Hooks be used in class components?
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medium Can useState take a function as an initial value (lazy initialization)?
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medium Is useState synchronous or asynchronous?
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medium What is the purpose of the callback function format of setState?
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hard How does React batch multiple state updates? What changed in React 18?
What interviewers evaluate: hooks
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hard What is useTransition and how does it improve UI responsiveness?
What interviewers evaluate: hooks
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hard What is useDeferredValue and when would you use it?
What interviewers evaluate: hooks
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medium How do you share hook logic across components without prop drilling?
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Performance & Optimization Questions (15)
Learn React.memo, useMemo, useCallback, code splitting, lazy loading, virtualization, and how to debug re-render issues in production React apps.
medium How do you prevent unnecessary re-renders in React?
What interviewers evaluate: rendering
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medium What techniques would you use to optimize a slow React application?
What interviewers evaluate: rendering
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medium How does React.memo help improve performance? When does it NOT prevent re-renders?
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medium What is code splitting and lazy loading in React? How do you implement them?
What interviewers evaluate: architecture
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hard What is the windowing technique (virtualization) for rendering large lists?
What interviewers evaluate: rendering
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medium Why do inline functions or objects in JSX hurt performance?
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hard How would you optimize the performance of React contexts to reduce re-renders?
What interviewers evaluate: rendering
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medium What is the difference between React.PureComponent and React.Component?
What interviewers evaluate: rendering
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medium How do useMemo and useCallback help improve performance?
What interviewers evaluate: hooks
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medium How can you optimize state updates for performance in React?
What interviewers evaluate: state
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medium What causes an infinite re-render loop and how do you debug it?
What interviewers evaluate: rendering
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medium What are some performance tips when rendering large lists in React?
What interviewers evaluate: rendering
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medium What causes the "Too many re-renders" error?
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hard Why does a child component lose input focus after typing, and how do you fix it?
What interviewers evaluate: rendering
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hard How does useLayoutEffect cause layout thrashing?
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State Management Questions (20)
Compare Context, Redux, Zustand, Jotai, and the trade-offs of each. Cover when to use local state, lifted state, and global state management.
medium Explain the Context API in React. What are its use cases?
What interviewers evaluate: state
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medium When should you use Redux over the Context API?
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medium What is the difference between React Context and React Redux?
What interviewers evaluate: state
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hard How do you decide between using React state, context, and external state managers?
What interviewers evaluate: state
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medium What is Redux? What are its core principles?
What interviewers evaluate: state
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medium What is the Flux pattern? How is Redux different from Flux?
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medium What is the difference between local state and global state?
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hard What is the difference between client state and server state?
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medium What are some popular state management solutions in React?
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medium What is MobX and how does it differ from Redux?
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medium What are Redux selectors and why should you use them (Reselect)?
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medium What is Redux Thunk vs Redux Saga? What are the differences?
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medium How do you handle persistent state in React apps?
What interviewers evaluate: state
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hard What is the difference between derived state and computed state?
What interviewers evaluate: state
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medium How do you handle asynchronous state updates in React?
What interviewers evaluate: state
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medium What's a common pitfall when using useContext with objects?
What interviewers evaluate: state
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medium How do you combine useReducer with useContext for state management?
What interviewers evaluate: state
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medium How do you update objects and nested objects inside state?
What interviewers evaluate: state
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medium How do you update arrays inside state without mutation?
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medium What is the Immer library and how does it help with state updates?
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Practical / Coding Questions (24)
Build common React patterns: forms, lists, debounced search, infinite scroll, and the practical coding problems React interviewers ask.
medium How do you handle forms in React?
What interviewers evaluate: patterns
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medium How do you handle multiple input fields in a single form?
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easy How do you prevent default form submission behavior in React?
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medium How do you pass data between sibling components?
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medium How do you lift state up in a React application and why is it necessary?
What interviewers evaluate: state
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easy How do you conditionally render components?
What interviewers evaluate: rendering
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easy How do you render a list of items in React?
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medium How do you re-render the view when the browser is resized?
What interviewers evaluate: patterns
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medium How do you fetch data with React Hooks?
What interviewers evaluate: hooks
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medium How do you implement a debounced search input in React?
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medium How do you implement infinite scroll in React?
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medium How would you implement a custom hook for data fetching?
What interviewers evaluate: hooks
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medium How do you implement protected/private routes in React Router?
What interviewers evaluate: routing
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medium How do you reset a component's state in React?
What interviewers evaluate: state
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easy How do you focus an input element on page load?
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medium How do you create a switching component for displaying different pages?
What interviewers evaluate: patterns
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medium How do you perform an automatic redirect after login?
What interviewers evaluate: routing
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medium How do you implement route-level code splitting (lazy loading routes)?
What interviewers evaluate: routing
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medium Why does console.log(state) show the old value right after setState?
What interviewers evaluate: state
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medium Why do components re-render even when props "look the same"?
What interviewers evaluate: rendering
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medium What happens when you update state based on previous state incorrectly?
What interviewers evaluate: state
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medium How do you handle error handling in a React application?
What interviewers evaluate: patterns
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medium How do you handle dynamic addition, removal, and sorting of list items?
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medium How do you prevent navigation when a form has unsaved changes?
What interviewers evaluate: routing
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Routing Questions (12)
Cover React Router covering nested routes, protected routes, dynamic params, navigation guards, and the SPA routing patterns React apps use.
easy What is React Router and why is it used?
What interviewers evaluate: routing
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medium What is the difference between BrowserRouter and HashRouter?
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medium How do nested routes work? Why do we need Outlet?
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medium What are the major changes between React Router v5 and v6?
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medium What is the difference between useNavigate and the Navigate component?
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medium How do you navigate programmatically in React Router?
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medium How do you handle 404 Not Found routes?
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medium How do you get query parameters in React Router?
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medium How do you pass data during navigation without exposing it in the URL?
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medium What is the difference between client-side routing and server-side routing?
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hard How do loaders and actions work in React Router (data routers)?
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medium How do you persist scroll position or scroll-to-top on route change?
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Advanced / Architecture Questions (19)
Tackle forwardRef, Portals, Suspense, error boundaries, render props, HOCs, and the advanced React patterns experienced developers must know.
medium What are Higher-Order Components (HOC)? What can you do with them?
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medium What are render props in React?
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medium Explain the composition pattern in React.
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medium Explain the presentational vs container component pattern.
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medium What are error boundaries in React? In which scenarios do they not catch errors?
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medium What is forwardRef() in React and what is it used for?
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medium What are React Portals and what are they used for?
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hard What is React Suspense and what does it enable?
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hard Explain server-side rendering (SSR) in React and its benefits.
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hard What is static site generation and how does it differ from SSR?
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hard What is React hydration?
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medium What is Strict Mode in React? Why does it render twice?
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hard What are some common React anti-patterns?
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easy What are the different ways to style a React component?
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easy What is the role of PropTypes in React? What are alternatives for type checking?
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medium What are the benefits of using TypeScript with React?
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medium Describe the component lifecycle methods in class components.
What interviewers evaluate: lifecycle
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medium What are the different phases of the React component lifecycle?
What interviewers evaluate: lifecycle
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medium What are styled-components? How does CSS-in-JS work?
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React 18+ / Modern React Questions (12)
Master React 18 features: concurrent rendering, automatic batching, useTransition, useDeferredValue, Suspense for data fetching, and Server Components.
hard What is Concurrent Mode / Concurrent Rendering in React? How does it improve performance?
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hard How does React handle concurrent rendering with multiple updates and prioritize them?
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hard What is automatic batching in React 18? How is it different from React 17?
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hard What are React Server Components?
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hard When should you use client components vs server components?
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hard What is useTransition and how does it help with expensive state updates?
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hard What is useDeferredValue and how does it differ from useTransition?
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medium What is the useId hook introduced in React 18?
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hard Is it possible to prevent automatic batching in React 18?
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hard How does Suspense work with data fetching in React 18+?
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medium What is the new JSX transform and what are its benefits?
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hard What are capture phase events in React?
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Testing Questions (10)
Cover React Testing Library, Jest, snapshot tests, mocking hooks, testing async behavior, and the testing patterns that catch real bugs.
medium How do you test React applications?
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medium What is Jest and how is it used for testing React?
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medium What is React Testing Library and how do you use it?
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medium How do you test asynchronous code in React components?
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medium How do you mock API calls in React component tests?
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medium How do you test custom hooks in React?
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medium How do you test components that use context or Redux?
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medium What is Snapshot Testing?
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medium What is the difference between shallow rendering and full DOM rendering?
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hard How do you handle long-running tasks without blocking the UI?
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Next.js / Framework Questions (8)
Cover Next.js routing, SSR vs SSG vs ISR, API routes, App Router, server components, and the framework-specific React knowledge interviewers expect.
medium What is Next.js and what are its major features?
What interviewers evaluate: architecture
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hard What are the differences between the Pages Router and App Router in Next.js?
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medium How do you implement routing in Next.js vs React Router?
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medium How do you localize React applications?
What interviewers evaluate: patterns
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medium How do you handle data persistence in a React application?
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medium What are common folder structures for React projects?
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medium How do you ensure a user remains authenticated on page refresh using Context API?
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easy What is the difference between React Native and React?
What interviewers evaluate: fundamentals
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