Senior React/Next.js hiring guide: questions and practical tasks

Enterprises that hire React developers via slashdev.io discover difference between “can ship” and “can scale.” This guide helps you probe depth: architecture, performance audits for web apps, and end-to-end release management across web and app stores.

Foundational questions that separate seniors from strong mids

• React architecture: Describe how you would structure a Next.js 14 app using the app directory, Server Components, and Client Components. What belongs on the server boundary and why?
• Data fetching and caching: Explain revalidatePath, revalidateTag, and route segment caching. When do you prefer ISR over full SSR or static exports?
• Concurrency and Suspense: Show how Suspense streaming reduces TTFB vs. TTI. What pitfalls arise with waterfalls, and how do you avoid them?
• State strategy: Compare Redux Toolkit, Zustand, and React Query. Which state should live on the server, which in URL/search params, and which in client stores?
• TypeScript rigor: Demonstrate how you model API responses with zod, narrow unknown, and enforce exhaustive switch with never.
• Testing signal: Outline a strategy using Vitest/Jest, React Testing Library, and Playwright for critical flows and a11y checks.

Performance audits for web apps-turn interviews into proof

Great candidates treat performance as a discipline, not folklore. Ask them to walk a full audit: collect Web Vitals (LCP, INP, CLS), run Lighthouse and WebPageTest, profile with the React Profiler, and inspect bundle size using next build and source-map-explorer. Expect a plan to instrument RUM, set budgets, and prevent regressions via CI.

• Given a flame chart, reduce INP spikes caused by oversized context and unnecessary renders. Show memoization and selector extraction.
• Fix hydration mismatch from locale-specific Date formatting by moving logic server-side and freezing time for SSR.
• Cut bundle bloat by extracting admin tools behind dynamic(() => import(…)) with suspense and prefetch on intent.
• Improve images using next/image, AVIF/WebP, priority hints, and proper sizes to stabilize CLS.
• Eliminate CSS blocking with critical CSS, module scoping, and Tailwind’s content-based purge.
• Virtualize long lists using react-window and keep item keys stable to avoid reflow storms.

Next.js server-side mastery

• RSC boundaries: Identify components that must remain server-only due to secrets or heavy data joins.
• Streaming SSR: Implement route handlers that stream above-the-fold content while awaiting slower subtrees.
• Edge vs Node runtimes: Choose where to run auth, i18n, and bot detection; explain tradeoffs and caching layers.
• ISR cadence: Propose tags and on-demand revalidation triggered by CMS webhooks without stampedes.
• Security: Apply CSP, Secure headers, and sanitize user content. Demonstrate SSRF and CSRF mitigations.

A four-hour take-home that actually reveals seniority

Provide a minimal headless API and ask the candidate to build a micro storefront in Next.js 14 with TypeScript, RSC, and a small design system.

• Catalog page uses ISR with tag-based revalidation; product detail uses streaming SSR.
• Client cart with optimistic updates, persisted per user, and URL-driven filters.
• Performance budgets: LCP under 2.5s on 4G, INP under 200ms; block merges that regress budgets.
• Observability: Add Web Vitals to an endpoint, trace server actions, and log cache hits/misses.
• Accessibility: Keyboard traps avoided, focus management on route change, labels, and color contrast.
• Tests: Unit coverage for utilities, integration tests for cart, and Playwright smoke on CI.
• Short README: architecture decisions, tradeoffs, and known risks.

App store deployment and release management

Even web-first teams need release discipline. Probe cross-channel fluency: how they would ship a React Native or Capacitor companion app, and how web releases coordinate with mobile cadences.

• Mobile: Configure Fastlane, semantic versioning, TestFlight/internal testing, phased release, and over-the-air updates with Expo EAS or CodePush while respecting store policies.
• Web: Trunk-based development, feature flags, blue-green or canary deploys, and instant rollbacks with database migration guards.
• Compliance: Handling app review guidelines, privacy manifests, and crash symbolication pipelines.

Red flags and signal boosters

• Red flag: Talks about “performance” without numbers. Signal: Quotes concrete metrics, shows before/after profiles, and enforces budgets in CI.
• Red flag: Centralizes all state in Redux by default. Signal: Pushes state to the server and URL when appropriate; uses lightweight client stores.
• Red flag: Treats Next.js caching as magic. Signal: Explains cache keys, revalidation triggers, and stampede protection.
• Red flag: Ignores release management. Signal: Presents a rollback plan, SLOs, and can explain incident postmortems.
• Red flag: Weak a11y. Signal: Builds accessible components and includes automated and manual checks.

Structured onsite exercise

Seed a repo with a regression: a client component wraps the home page, causing CLS and INP spikes. Ask them to:

• Refactor to Server Components with Suspense and segment caching.
• Fix layout shift by reserving image space and deferring widgets.