React Strict Mode intentionally double-invokes certain lifecycles and effects in development to surface impure side effects. It does not run twice in production.
Common Interview Questions
Q: What does Strict Mode double-invoke?
Answer: In React 18+ dev, components mount, unmount, and remount; effects run setup-cleanup-setup. Class constructors and certain hooks are stress-tested.
Q: Why does React do this?
Answer: To ensure effects are idempotent and cleanup works — preparing for concurrent features where React may mount/unmount/resume components.
Q: My API fires twice in dev — bug?
Answer: Often expected in dev without proper cleanup or deduplication. Fix effect cleanup; do not disable Strict Mode as the solution.
<React.StrictMode>
<App />
</React.StrictMode>
Q: Does Strict Mode affect production?
Answer: No double rendering in production builds. Strict Mode checks are development-only warnings and behaviors.
Key Points for Interviews
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Dev-only behavior — not a production perf hit
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Exposes missing effect cleanup
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Helps future-proof for concurrent rendering
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Wrap app root with <StrictMode> in development
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Use idempotent effects and proper abort/cleanup