React Guideline: Effects — Avoid unnecessary effects. Description: Don't use effects for transforming data or events. Do: Transform data during render handle events directly. Don't: useEffect for derived state or event handling. Good Example: const filtered = items.filter(...). Bad Example: useEffect(() => setFiltered(items.filter(...))). Severity: High. Docs: https://react.dev/learn/you-might-not-need-an-effect.