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Introduction React.js is a powerful JavaScript library for building user interfaces, but developers often encounter errors that can disrupt workflow and degrade user experience. This whitepaper categorizes common React.js errors and provides solutions to resolve them efficiently.  1. Rendering Errors 1.1 Cannot Read Property ‘map’ of Undefined Cause: Attempting to map over an undefined or non-array variable.  Solution: Use optional chaining (`?.`) or provide a fallback:  1.2, Text Content Does Not Match Server HTML (Hydration Error) Cause: Mismatch between server-rendered and client-rendered content.  Solution: 2. State Management Errors 2.1. Too Many Re-renders  Cause: Updating state unconditionally in the render phase.  Solution: 2.2, Cannot Update a Component While Rendering a Different Component Cause: State update triggering another component’s render mid-cycle.  Solution: 3. Hooks-Related Errors  3.1, Hooks Can Only Be Called Inside the Body of a Function Component Cause: Using hooks in class components or outside React functions.  Solution: 3.2, React Hook useEffect Has Missing Dependencies  Cause: Unlisted dependencies in `useEffect`.  Solution:  Include all dependencies in the dependency array:  4. Key Prop Errors 4.1, Each Child in a List Should Have a Unique Key Prop Cause: Missing or duplicate `key` props in dynamically rendered lists.  Solution: Use unique and stable identifiers (e.g., `id`):  5. SEO-Friendly Practices in React 5.1, Dynamic Content & Search Engine Crawling Issue: SPAs may not be properly indexed.  Solution: 5.2 Lazy Loading & Performance Issue: Slow page load affects SEO rankings.  Solution: Use `React.lazy` and `Suspense`:  Conclusion Understanding and resolving common React.js errors improves code reliability and user experience. By following best practices in rendering, state management, hooks usage, and SEO optimization, developers can build robust and search-engine-friendly applications.  For further debugging, leverage React DevTools and error boundary components.