React Interview Questions: Basic to Advanced

1What problem does React actually solve, and what does it deliberately leave to you?Basicconceptual30 minIn writing
2What is the Virtual DOM and why does React use it instead of updating the real DOM directly?Basicconceptual5 min
3What does JSX compile to, and why does that matter when you are reading a stack trace?Basicconceptual25 minIn writing
4Walk through everything that happens between calling a state setter and the browser painting new pixels.Intermediateconceptual35 minIn writing
5How would you explain declarative UI to someone who has only ever written jQuery?Basicconceptual25 minIn writing
6Why does React run your component body twice in development, and which bugs does that surface?Intermediatetroubleshooting30 minIn writing
7An interviewer asks you to describe React's rendering model end to end in three minutes. What do you say, and in what order?Intermediateconceptual40 minIn writing
8Check your understanding of rendering, reconciliation, and committing.Basicconceptual15 minIn writing

9What are the practical differences between function and class components, and why did the ecosystem move?Basicconceptual25 minIn writing
10Why are props read-only, and what breaks if you mutate one anyway?Basicconceptual25 minIn writing
11When do you accept children instead of adding another prop, and what does that buy the caller?Basicpractical30 minIn writing
12What are the ways to render conditionally in JSX, and which one silently prints a stray zero?Basicpractical25 minIn writing
13How do you decide where one component ends and the next one begins?Intermediatedesign35 minIn writing
14How do you type a component's props in TypeScript so that misuse fails at compile time rather than in QA?Intermediatepractical35 minIn writing
15Design the props API for a reusable DataTable component on the spot, and defend every prop you include.Intermediatedesign40 minIn writing
16Check your understanding of props, children, and component boundaries.Basicconceptual15 minIn writing

17What does useState actually return, and why can you never assign to the value directly?Basicconceptual30 minIn writing
18Why does reading state immediately after calling its setter give you the old value?Intermediateconceptual35 minIn writing
19When do you need the functional updater form of a setter, and what bug shows up without it?Intermediatepractical30 minIn writing
20How does React batch state updates, and what changed about batching in React 18?Intermediateconceptual30 minIn writing
21How do you decide whether a piece of state belongs in a component, in its parent, or nowhere at all?Basicdesign30 minIn writing
22Why is storing derived data in state usually a bug, and what do you do instead?Intermediatetroubleshooting35 minIn writing
23A handler calls setCount three times but the counter only moves by one. Explain why, then fix it while the interviewer watches.Intermediatescenario40 minIn writing
24Check your understanding of state snapshots, updaters, and batching.Basicconceptual15 minIn writing

25What is useEffect actually for, and what is your test for whether code belongs in one at all?Intermediateconceptual35 minIn writing
26Explain how useEffect's dependency array controls when an effect re-runs, and what goes wrong when a dependency is missing.Intermediatepractical5 min
27What does an effect's cleanup function do, and what leaks when you forget to return one?Intermediatepractical35 minIn writing
28Name three effects that appear in most codebases and should be deleted, and say what replaces each.Advancedtroubleshooting40 minIn writing
29What goes wrong when you fetch inside useEffect, and how do you handle two responses racing each other?Intermediatepractical40 minIn writing
30Your team's dashboard re-fetches search results on every keystroke of a filter input, and the backend team reports the API is being rate-limited. Walk through how you'd fix it.Scenarioscenario5 min
31An interviewer shows you an effect firing in an infinite loop. Walk through your diagnosis out loud.Advancedtroubleshooting45 minIn writing
32Check your understanding of effect timing, dependencies, and cleanup.Intermediateconceptual15 minIn writing

33What are the Rules of Hooks, and what actually breaks if you call one inside a condition?Basicconceptual30 minIn writing
34When do you reach for useReducer over useState, and how do you shape the actions?Intermediatepractical40 minIn writing
35What do useMemo and useCallback really do, and when are they a net loss?Intermediatepractical40 minIn writing
36When should a value live in a ref rather than in state?Intermediateconceptual30 minIn writing
37What problems do useId and useSyncExternalStore solve that you cannot solve with useState?Advancedpractical40 minIn writing
38When must you use useLayoutEffect instead of useEffect, and what does it cost?Advancedpractical35 minIn writing
39How does React know which hook call belongs to which piece of state? Explain the mechanism.Advancedconceptual45 minIn writing
40Check your understanding of the built-in hooks and when each applies.Intermediateconceptual15 minIn writing

41What makes a function a custom hook, and what does the use prefix actually enforce?Basicpractical30 minIn writing
42Walk through refactoring a component with three tangled effects into custom hooks.Intermediatepractical40 minIn writing
43How do you decide whether a custom hook returns a value, a tuple, or an object?Advanceddesign40 minIn writing
44How do you test a custom hook without mounting an entire page around it?Intermediatepractical35 minIn writing
45Two components call the same custom hook and each gets its own state. When is that a bug, and how do you fix it?Intermediatetroubleshooting35 minIn writing
46What did hooks replace, and are higher-order components or render props ever still the right answer?Intermediateconceptual35 minIn writing
47Design a usePagination hook in the interview. Talk through the API before you write a line of it.Advanceddesign45 minIn writing
48Check your understanding of custom hook extraction and API design.Intermediateconceptual15 minIn writing

49What problem does Context solve, and how does a provider find its consumers?Basicconceptual30 minIn writing
50Prop drilling three levels deep: is that worth a Context? Argue both sides.Intermediatedesign30 minIn writing
51Why does every consumer re-render when one field of a context value changes, and how do you contain that?Advancedtroubleshooting40 minIn writing
52When do you split one context into several, and what does that cost you in provider nesting?Advanceddesign35 minIn writing
53What happens when a consumer renders outside its provider, and how do you make that fail loudly?Intermediatetroubleshooting25 minIn writing
54How do you combine Context with useReducer, and where does that pattern stop scaling?Intermediatepractical40 minIn writing
55A theme context re-renders an entire dashboard on every toggle. Diagnose it live and propose two fixes.Advancedscenario45 minIn writing
56Check your understanding of context propagation and re-render scope.Intermediateconceptual15 minIn writing

57What is a ref, and why does assigning to ref.current not re-render anything?Basicconceptual25 minIn writing
58How do you focus an input or measure an element after render without fighting React's model?Basicpractical30 minIn writing
59How does a parent get a ref to a DOM node inside your component, and what changed about that in React 19?Intermediatepractical35 minIn writing
60When is a callback ref the right tool over useRef, and what is its cleanup story?Advancedpractical35 minIn writing
61When is useImperativeHandle justified, and what does exposing an imperative API cost you later?Advanceddesign35 minIn writing
62How do you integrate a non-React library that owns a DOM node, such as a chart or a map, without leaking it?Scenariopractical45 minIn writing
63An interviewer's ref is null inside an effect on first render. Walk through every reason that can happen.Advancedtroubleshooting40 minIn writing
64Check your understanding of refs versus state versus plain variables.Intermediateconceptual15 minIn writing

65What is the difference between a controlled and an uncontrolled input, and when do you pick each?Basicconceptual30 minIn writing
66How do you hold the state of a twenty-field form without writing twenty useState calls?Intermediatedesign35 minIn writing
67Where does validation belong, and how do you surface errors without punishing someone mid-typing?Intermediatepractical40 minIn writing
68What do form actions, useActionState, and useFormStatus change about how you handle submission in React 19?Advancedpractical40 minIn writing
69Why can a file input never be controlled, and which other inputs behave unusually?Intermediatetroubleshooting30 minIn writing
70When is React Hook Form or a similar library worth the dependency over plain state?Intermediatedesign35 minIn writing
71Build a multi-step wizard whose back button must never lose typed data. Talk through the state design first.Scenarioscenario45 minIn writing
72Check your understanding of controlled inputs, validation, and submission.Intermediateconceptual15 minIn writing

73How do you render a list in React, and what exactly does the key prop tell the reconciler?Basicpractical25 minIn writing
74What goes wrong when you use the array index as a key? Give a concrete failure, not a slogan.Intermediatetroubleshooting35 minIn writing
75How does React decide whether to update a component in place or unmount and remount it?Advancedconceptual40 minIn writing
76Why did a component keep stale state after the data behind it changed, and how do you force a reset?Advancedtroubleshooting40 minIn writing
77Why do fragments exist, and how do you key one inside a list?Basicpractical25 minIn writing
78Ten thousand rows make the page unusable. What do you try before reaching for a virtualization library?Scenariopractical45 minIn writing
79Checkboxes in a sortable table start checking the wrong rows after a sort. Find the cause on the spot.Advancedtroubleshooting40 minIn writing
80Check your understanding of keys, remounting, and state preservation.Intermediateconceptual15 minIn writing

81A component has grown nine boolean props. How do you refactor it, and what do you offer callers instead?Intermediatedesign35 minIn writing
82How do compound components work, and what makes them worth the extra indirection?Advanceddesign40 minIn writing
83When is passing a render function better than passing children or a slot prop?Intermediatepractical35 minIn writing
84How do you design a component that works both controlled and uncontrolled, the way an input does?Advanceddesign40 minIn writing
85How do you build a component that can render as any element without lying to TypeScript?Advancedpractical40 minIn writing
86Your design system component has been forked in four apps. What did the original API get wrong?Scenariodesign45 minIn writing
87Design the API for a Modal used by six teams. Which decisions do you refuse to make on their behalf?Advanceddesign45 minIn writing
88Check your understanding of composition patterns and component API design.Intermediateconceptual15 minIn writing

89What actually causes a component to re-render? List every trigger you can name.Intermediateconceptual35 minIn writing
90How would you diagnose and fix unnecessary re-renders in a large React tree that's causing visible jank?Advanceddesign5 min
91When does React.memo help, and what silently defeats it?Intermediatepractical35 minIn writing
92Walk through profiling a slow interaction with React DevTools. What do you look at, and in what order?Advancedtroubleshooting40 minIn writing
93A single component takes 200ms to render. What are your options, ranked from cheapest to most invasive?Advanceddesign40 minIn writing
94What does the React Compiler change about writing memoization by hand?Advancedconceptual35 minIn writing
95Typing in a search box drops frames on a page with a large table. Diagnose it end to end for the interviewer.Scenarioscenario50 minIn writing
96Check your understanding of re-render triggers and memoization trade-offs.Intermediateconceptual15 minIn writing

97What does it mean that React rendering is interruptible, and what did that unlock for users?Advancedconceptual40 minIn writing
98What does useTransition do, and how is it different from a debounce?Advancedpractical40 minIn writing
99When do you reach for useDeferredValue instead of useTransition?Advancedpractical35 minIn writing
100What does Suspense actually suspend on, and where do you place the boundary?Intermediateconceptual35 minIn writing
101What can an error boundary catch, what can it never catch, and where do you put them?Intermediatepractical35 minIn writing
102Design the loading and failure states for a dashboard with four independent widgets.Scenariodesign45 minIn writing
103Switching tabs freezes the UI for half a second. Explain how a transition fixes it and what you trade away.Advancedscenario45 minIn writing
104Check your understanding of transitions, deferred values, and Suspense boundaries.Intermediateconceptual15 minIn writing

105What is a Server Component, and how is it different from server-side rendering?Advancedconceptual45 minIn writing
106What does the use client directive actually mark, and what is allowed to cross that boundary?Advancedconceptual40 minIn writing
107How does data fetching change when the component itself runs on the server?Intermediatepractical40 minIn writing
108What does the use hook do with a promise, and how does it break the rules every other hook follows?Advancedpractical40 minIn writing
109What is a server action, and what must you never trust about its inputs?Advancedpractical40 minIn writing
110How would you introduce Server Components into an existing client-only React application?Scenariodesign50 minIn writing
111An interviewer asks where you would draw the client boundary on a product page. Defend the line you pick.Advanceddesign45 minIn writing
112Check your understanding of Server Components and the client boundary.Intermediateconceptual15 minIn writing

113When has an application outgrown useState and Context? Name the signals you actually look for.Intermediatedesign35 minIn writing
114What is the difference between server state and client state, and why does it change your tooling?Intermediateconceptual35 minIn writing
115How does Redux Toolkit work, and what does a slice buy you over hand-rolled reducers?Intermediatepractical40 minIn writing
116How does Zustand differ from Redux, and when is that difference the deciding factor?Intermediatepractical35 minIn writing
117How does React Query's cache work across staleness, refetching, and invalidation, and what code does it delete?Advancedpractical45 minIn writing
118How do you implement an optimistic update that rolls back correctly when the request fails?Advancedpractical40 minIn writing
119Choose the state architecture for an app with live prices, a form wizard, and a user session. Justify each choice.Scenariodesign50 minIn writing
120Check your understanding of server state, client state, and store selection.Intermediateconceptual15 minIn writing

121What does Testing Library mean by testing behaviour rather than implementation, and where is the line?Intermediatepractical40 minIn writing
122How do you test a component that fetches, and what makes those tests flaky?Intermediatepractical40 minIn writing
123Which accessibility problems does React not solve for you? Name the ones you check on every pull request.Intermediatepractical40 minIn writing
124How do you manage focus when a modal opens and closes? Walk through the entire flow.Advancedpractical40 minIn writing
125A bug reproduces only in production, including hydration mismatches. What is your toolkit, in order?Advancedtroubleshooting45 minIn writing
126Capstone: design and defend a complete production React application end to end, covering component boundaries, state, data, performance budget, testing, and accessibility.Scenariodesign240 minIn writing
127You inherit a React app with no tests, visible jank, and accessibility complaints. What do you do in your first two weeks?Scenarioscenario50 minIn writing
128Check your understanding of testing strategy, accessibility, and production debugging.Advancedconceptual15 minIn writing