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React re-renders a component when its state or props change. The reconciler diffs the previous virtual DOM tree against the new one, and only the changed nodes get updated in the actual DOM. Pure components and React.memo can short-circuit this when nothing meaningful changed.

useMemo caches the result of a computation across renders, recomputing only when its dependency array changes. Use it when the computation is genuinely expensive — for cheap calculations, the bookkeeping overhead can outweigh the savings.

useCallback memoizes a function reference; useMemo memoizes any value (including computed results). In practice, useCallback is just useMemo for functions. Reach for them when you're passing the value to a memoized child or when stable identity matters for an effect dependency.