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Use useRef for Transient Values

When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in useRef instead of useState. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.

Incorrect (renders every update):

function Tracker() {
  const [lastX, setLastX] = useState(0);

  useEffect(() => {
    const onMove = (e: MouseEvent) => setLastX(e.clientX);
    window.addEventListener('mousemove', onMove);
    return () => window.removeEventListener('mousemove', onMove);
  }, []);

  return (
    <div
      style={{
        position: 'fixed',
        top: 0,
        left: lastX,
        width: 8,
        height: 8,
        background: 'black',
      }}
    />
  );
}

Correct (no re-render for tracking):

function Tracker() {
  const lastXRef = useRef(0);
  const dotRef = useRef<HTMLDivElement>(null);

  useEffect(() => {
    const onMove = (e: MouseEvent) => {
      lastXRef.current = e.clientX;
      const node = dotRef.current;
      if (node) {
        node.style.transform = `translateX(${e.clientX}px)`;
      }
    };
    window.addEventListener('mousemove', onMove);
    return () => window.removeEventListener('mousemove', onMove);
  }, []);

  return (
    <div
      ref={dotRef}
      style={{
        position: 'fixed',
        top: 0,
        left: 0,
        width: 8,
        height: 8,
        background: 'black',
        transform: 'translateX(0px)',
      }}
    />
  );
}