JavaScript fundamentals (closures, promises, async/await) / React (hooks, lifecycle methods, component architecture) / TypeScript (types, interfaces, generics) / State management (Redux, Context API) / Performance optimization (lazy loading, memoization, virtual DOM) / Testing (unit testing, integration testing, Jest) / REST APIs and GraphQL / Version control (Git, branching strategies) / Code review process / Handling edge cases and debugging / Responsive design and CSS-in-JS / Web security (XSS, CSRF, CORS).