React Performance: Rerender — Functional setState. Keywords: functional setstate callback. Platform: React/Next.js. Description: Use functional setState updates for stable callbacks and no stale closures. Do: Use functional form: setState(curr => ...). Don't: Reference state directly in setState. Good Example: setItems(curr => [...curr, newItem]). Bad Example: setItems([...items, newItem]) // items in deps. Severity: Medium.