What is React.StrictMode and what does it do?

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React.StrictMode is a development-only wrapper that highlights potential problems in your component tree. It doesn't render anything visible and has no effect in production builds. Double-invokes render and effects — runs functional components twice (in dev only) to surface impurities and effect bugs. Warns about deprecated APIs — legacy lifecycle methods, legacy context, string refs, find DOM node. Detects unsafe side effects — effects with missing cleanup, mutations during render. Future-proofing — ensures your code is compatible with concurrent React features. Wrap your root — typically in main.jsx ; can also wrap subtrees during migration. Production = no-op — zero runtime cost when shipped. The most common interview question about StrictMode is 'why does my useEffect run twice?' Explain that it is development-only behavior to catch missing cleanups and impure functions. It does NOT happen in production.