React & Frontend Engineer Interview Questions & Screening Guide
Essential screening questions for agency recruiters vetting React.js, Next.js, and TypeScript developers.
What causes unnecessary re-renders in React, and how do you diagnose and optimize them?
Understanding of object reference equality, state colocation, useMemo, useCallback, and React Profiler.
"Unnecessary re-renders typically occur when parent components re-render or when non-primitive props create new object references. I diagnose using React DevTools Profiler and resolve by pushing state down, memoizing expensive callbacks, and structuring state immutably."
Claims putting useMemo around every single variable is best practice.
Explain how React handles event delegation under the hood.
SyntheticEvent system and delegation at the root root/container node.
"React attaches a single listener to the root container rather than individual DOM nodes, wrapping browser events in SyntheticEvent for cross-browser consistency."
Has no knowledge of event bubbling or thinks each JSX onClick creates a native DOM listener.
Describe a situation where a frontend deployment broke production. How did you handle it?
Accountability, blameless post-mortem culture, and rapid rollback capability.
"A broken API contract slipped through staging. I immediately triggered an automated rollback via CI/CD, diagnosed the mismatch with Sentry error logs, and added automated TypeScript contract tests to prevent recurrence."
Blames backend team entirely and demonstrates zero initiative to stabilize user experience.
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