Technical interview guide

React Fundamentals

The rendering model, state as a snapshot, keys and component identity, effects as synchronization, memoisation, and the client boundary.

Read: 44 min
Practice MCQs: 25
Interview QA: 25
Edition: v3
Editorial status: Reviewed
Relevant for: Frontend Engineer

Scope: React 19, including automatic batching from React 18 and the Server Component model as implemented by React 19 and Next.js 16.

Interview QA

Treat each question like a live interview question: answer out loud first (structure, assumptions, tradeoffs), then open the model answer to spot gaps and rehearse a tighter follow-up.

Curated: 2026-10-05 · Written: 2026-10-05 · Reviewed: 2026-10-04

QA-1

Explain render as a calculation, commit as the write and how it changes a production decision.

QA-2

How would you reason about components as pure functions of props and state in a system you own?

QA-3

Walk through props as a read-only argument, including where engineers most often get it wrong.

QA-4

What does state as a snapshot of one render guarantee, and what does it deliberately leave open?

QA-5

Describe updater functions against captured values and the evidence you would collect before relying on it.

QA-6

A teammate proposes a design that hinges on automatic batching of updates. How do you evaluate it?

QA-7

Where does lazy initial state matter, and where is it irrelevant?

QA-8

Teach derived values computed rather than stored to an engineer who has only seen it as a rule of thumb.

QA-9

Why does React require keys when rendering lists of elements, and what bugs or performance issues occur when keys are omitted or unstable?

QA-10

Under what specific conditions is using an array index as a key problematic in React, and when (if ever) is it acceptable?

QA-11

How does React decide when to preserve or reset state across renders based on tree position and keys?

QA-12

What is the primary purpose of useEffect, and when does its cleanup function execute relative to component lifecycle and re-renders?

QA-13

Why do stale closures occur inside useEffect, and how does the dependency array prevent them?

QA-14

When would you refuse a design because of cleanup and out-of-order responses?

QA-15

How would you explain the effects you do not need without using the usual slogan?

QA-16

What failure would you inject to check a team's understanding of strictMode double invocation in development?

QA-17

How should on-call treat an alert that names useLayoutEffect against useEffect as the cause?

QA-18

What belongs in a runbook section on refs for values that do not render, and what does not?

QA-19

How would you review a pull request whose risk is useMemo and useCallback as a measured trade?

QA-20

What trade-off does memo and the identity of props force that a junior answer usually skips?

QA-21

How would you brief product on why context and who re-renders delays a ship date?

QA-22

What is the smallest experiment that would change your mind about controlled against uncontrolled inputs?

QA-23

How does hooks and the order they are called in interact with rollback, and where do people ignore that?

QA-24

What would you ask a candidate who recites error boundaries and Suspense boundaries but cannot apply it?

QA-25