React app broken in production.Fixed this session.

PM assigned in 10 minutes. React/Next.js engineer starts diagnosing within the hour. Next.js production crashes are typically caused by: hydration mismatches between server and client rendering, missing environment variables breaking data fetching on cold starts, module import errors from incompatible dependencies after an upgrade, or App Router data caching issues serving stale or wrong data. The engineer uses Sentry traces, Vercel function logs, or your observability stack to isolate the exact failure. Most Next.js crashes hydration errors, build failures, runtime 500s fixed within a single 4-hour session.

Yes. Next.js 14 App Router migrations are one of our most common requests. Breaking changes the engineer handles in a single session: Pages Router to App Router restructuring, getServerSideProps and getStaticProps conversion to Server Components and async fetch, client-component boundary errors from using hooks in Server Components, Metadata API migration from Head, route handler syntax changes (GET/POST exports), and third-party library incompatibilities that have not yet updated for App Router. PM scopes the migration complexity in the first 15 minutes and gives you a realistic timeline before work starts.

Yes. PM scopes testing into the session as a standard deliverable. For React bug fixes, the engineer writes component-level tests using React Testing Library and integration tests using Playwright or Cypress covering the exact failure scenario. For Next.js App Router pages, tests cover: route rendering, data fetching, error boundaries, and hydration behavior. Test files are added to your existing test suite and configured to run in your CI pipeline. The goal is a failing test for the bug before the fix, then a passing test after proving the fix works and preventing regression automatically on every future commit.

Hydration errors are among the most common React requests we receive and are resolved same session in the vast majority of cases. Hydration mismatches occur when server-rendered HTML does not match what React expects to hydrate on the client caused by: using window or document in server-rendered components, date or random value differences between server and client, conditional rendering that depends on client state, third-party libraries that are not SSR-safe, or strict mode double-invocation exposing timing bugs. The engineer traces the exact component tree discrepancy using React DevTools and the error boundary stack trace, isolates the SSR-unsafe pattern, and fixes the root cause rather than suppressing the error.

Every QuickHire React engineer passes: a live React debugging exercise on a broken Next.js App Router application (real environment, time-limited), a written assessment covering React rendering, hooks, performance patterns (memo, useMemo, useCallback), and Server Component mental model, plus a portfolio review of production applications they have built and maintained at scale. Only the top 3% of applicants pass all stages. PM matches engineers to your specific stack Next.js version, state management library (Zustand, Redux, Jotai), styling system (Tailwind, CSS Modules, Styled Components), and testing framework before the session starts.