Building Bulletproof React Components

React components often break in real-world scenarios like server rendering, hydration, concurrent rendering, and portals. This guide covers 10 defensive patterns to make components resilient: moving browser APIs into useEffect for SSR compatibility, preventing hydration mismatches with synchronous scripts, using useId for multiple instances, deduplicating server queries with React.cache, replacing cloneElement with context for async children, handling portals with ownerDocument.defaultView, wrapping state updates in startTransition for view transitions, disabling styles in Activity components, preventing data leaks with taintUniqueValue, and using useState instead of useMemo when persistence matters.