React Guideline: Effects — Use refs for non-reactive values. Description: Store values that don't trigger re-renders in refs. Do: useRef for interval IDs DOM elements. Don't: useState for values that don't need render. Good Example: const intervalRef = useRef(null). Bad Example: const [intervalId, setIntervalId] = useState(). Severity: Medium. Docs: https://react.dev/reference/react/useRef.