react

Concurrent rendering lets React interrupt and prioritize work — Suspense provides declarative loading states, useTransition marks updates as non-urgent, and useDeferredValue defers expensive re-renders.

Error boundaries are class components that catch JavaScript errors during rendering and lifecycle methods, displaying a fallback UI instead of crashing the entire app — but they don't catch errors in event handlers, async code, or SSR.

React's reconciliation algorithm diffs virtual DOM trees in O(n) using two heuristics — different element types tear down the subtree, same types compare attributes — while the Fiber architecture makes this work interruptible so the browser stays responsive during large updates.

React patterns evolved from mixins to HOCs to render props to hooks — compound components share implicit state for flexible APIs, while custom hooks have replaced most logic-sharing patterns.

React.memo skips re-renders when props haven't changed, useMemo caches computed values, and useCallback caches function references — but these are targeted optimizations, not defaults for every component.

JSX auto-escapes values to prevent XSS by default, but dangerouslySetInnerHTML, href='javascript:', and localStorage token storage are common vulnerability vectors that require explicit mitigation.

Redux provides predictable state with a single store, actions, and reducers (RTK simplifies this with createSlice). Zustand offers a lighter alternative. Choose based on app complexity — most apps can start with useState + Context.