Agent Ambo · Upskill

React Developer interview questions, and how to answer them

React interviews test JavaScript fundamentals first, then how well you understand rendering, state and performance in real apps.

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React Developer interview questions

Why does React need keys in a list?
What they’re testing: Reconciliation understanding.
How to answer: Keys let React match items between renders. Using the index breaks state when items are reordered or removed; use a stable ID.
Explain the dependency array in useEffect.
What they’re testing: Hooks in depth.
How to answer: It controls when the effect re-runs. Missing dependencies cause stale values; an empty array runs once on mount. Return a cleanup function for subscriptions.
What is a closure? Give a real example.
What they’re testing: JavaScript fundamentals.
How to answer: A function that remembers variables from where it was created, such as a counter factory or a stale state value inside a setTimeout.
Build a search box that calls an API as the user types.
What they’re testing: Machine coding and debouncing.
How to answer: Debounce the input (about 300 ms), cancel stale requests with AbortController, and handle loading and error states.
How would you find why a page re-renders too often?
What they’re testing: Performance debugging.
How to answer: Use React DevTools Profiler, check for new object or function props each render, then apply memo, useMemo or useCallback where it helps.

How the interview usually runs

Round 1JavaScript and React online test
Round 2JavaScript fundamentals round
Round 3React machine-coding round (build a component)
Round 4Project and HR round

Must-know topics

Closures, this, promises and the event loopHooks: useState, useEffect, useMemo, useCallback, custom hooksRendering, reconciliation and keysState management: Context, Redux or ZustandPerformance: memoisation, code splitting, lazy loadingTesting (Jest, React Testing Library) and accessibility

The follow-up they’ll use to test you

“What happens if the user types faster than the API responds?”

Race conditions are a favourite follow-up. Talk about cancelling requests or ignoring out-of-order responses.

React Developer interview preparation course

Ambo builds your React Developer course from your resume and the role you want: every must-know topic above, Interview Q&A at the end of each lesson and a deep dive on your own projects. Pick a crash course (10 days), interview perspective (14) or depth learning (20).

How Upskill works

React Developer certification

TickBig Certified React Developer. Three camera-proctored rounds: an objective test, hands-on tasks for the role and a voice interview with Ambo. Pass all three to earn a seal recruiters can verify, valid for 12 months.

Your projects will be grilled

Explain how your app's state is organised, one performance problem you fixed and how you would scale the codebase.

React Developer resume guide

How Ambo trains you for your React Developer interview

1Ambo reads your resumeYour skills, projects, years and the role you want next.
2Then where you’re interviewingAnd how interviewers there question people like you.
3It maps every topic they could askMust-know first, your own projects included, built into short daily lessons.
4You practise, then get certifiedInterview Q&A after every lesson, then three proctored rounds ending in a voice interview with Ambo.

Questions, answered.

What is a machine-coding round in React interviews?

You build a small working component in 60 to 90 minutes, such as a todo list, autocomplete or modal, judged on structure, state handling and edge cases.

Should I learn Next.js for React interviews?

Many job descriptions now list it. Knowing server-side rendering and routing basics is a plus for most roles.

How is this different from a list of interview questions?

A list is the same for everyone. Ambo builds your course from your resume, your target role and where you're interviewing, then writes every lesson from the interviewer's point of view: what they'll ask, a strong answer, and the follow-up they'll use to test you.

What is the certification test?

Three camera-proctored rounds that unlock one after another: an objective round, a hands-on round (coding or practical tasks for your role) and a voice interview with Ambo, who asks live follow-ups on your answers. Pass all three within 14 days to become TickBig Certified for 12 months.

Walk into your React Developer interview ready.

Ambo builds your course from your resume and trains you the way they’ll question you.

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