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profiles/opencode/skill/vercel-react-best-practices/rules/rerender-use-ref-transient-values.md

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+---
+title: Use useRef for Transient Values
+impact: MEDIUM
+impactDescription: avoids unnecessary re-renders on frequent updates
+tags: rerender, useref, state, performance
+---
+
+## 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):**
+
+```tsx
+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):**
+
+```tsx
+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)',
+      }}
+    />
+  )
+}
+```