🚀 40 React Interview Questions Top Tech Companies Are Asking in 2026 🔥

React in 2026 looks different than React in 2022. The compiler, server components, the use hook, transitions — the surface area is bigger and the bar moved up. Here are 40 questions I've collected from real loops, organized by theme.

A 2026 React interview tests four layers: hooks fundamentals, the React 19 compiler, concurrent rendering / transitions, and Server Components / streaming. Most candidates over-prep hooks and under-prep concurrency.

Section 1 — Hooks fundamentals (10)

1. useState vs useReducer — when each?
2. useEffect cleanup — common bugs.
3. Why is the dependency array so painful?
4. useMemo vs useCallback — when do you actually need them in 2026 (compiler era)?
5. useRef — three legitimate uses.
6. useImperativeHandle — example use.
7. useLayoutEffect vs useEffect.
8. Custom hook design — when to extract?
9. The use hook (React 19) — semantics.
10. Why do hooks need a stable call order?

Section 2 — React 19 compiler (6)

1. What does the compiler do, conceptually?
2. Does the compiler eliminate the need for useMemo?
3. Cases where the compiler bails out.
4. How do you debug a component the compiler skipped?
5. Compiler vs forget hooks — interaction model.
6. When would you opt out of the compiler for a component?

Section 3 — Concurrency and transitions (8)

1. startTransition — when do you reach for it?
2. useDeferredValue — example use.
3. Suspense boundaries — granularity tradeoffs.
4. How does React decide what's urgent vs non-urgent?
5. Streaming SSR with renderToPipeableStream.
6. Selective hydration — what is it?
7. Concurrent rendering — what changed in error boundaries?
8. Tearing — what is it and how does React 18+ avoid it?

Section 4 — Server Components and frameworks (8)

1. Server Components vs Client Components — boundary.
2. Server Actions — when over a fetch handler?
3. How do server components shrink client bundles?
4. Cache layers in Next.js 15 / 16 with React Server Components.
5. Why can't server components use hooks?
6. Streaming a server-rendered list with Suspense.
7. Forms with Server Actions — error handling patterns.
8. SEO with Server Components — what to know.

Section 5 — Performance and tricky gotchas (8)

1. Re-renders — three common causes in 2026.
2. React DevTools Profiler — what to look for.
3. Why is the key prop so important in lists?
4. Memoizing entire trees — when does it backfire?
5. Context overuse — symptoms and fixes.
6. Hydration mismatch — common causes.
7. INP optimization — how React's scheduler helps.
8. State management in 2026 — why most apps don't need Redux anymore.

How to drill

Don't memorize. Build a small app touching every primitive. Use the coding copilot to verify hook patterns against current best practice.

Mock rhythm

• Monday: hooks fundamentals
• Tuesday: compiler
• Wednesday: concurrency
• Thursday: server components
• Friday: perf gotchas
• Saturday: full mock with practice mode

Common interview mistakes

1. Saying "useMemo everywhere" in 2026
2. Confusing server components with SSR
3. Not knowing what use does
4. Underestimating Suspense boundary granularity
5. Mixing class components into the conversation when nothing requires them

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