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74 lines
1.7 KiB
Markdown
74 lines
1.7 KiB
Markdown
---
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title: Use useRef for Transient Values
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impact: MEDIUM
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impactDescription: avoids unnecessary re-renders on frequent updates
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tags: rerender, useref, state, performance
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---
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## Use useRef for Transient Values
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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.
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**Incorrect (renders every update):**
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```tsx
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function Tracker() {
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const [lastX, setLastX] = useState(0)
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useEffect(() => {
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const onMove = (e: MouseEvent) => setLastX(e.clientX)
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window.addEventListener('mousemove', onMove)
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return () => window.removeEventListener('mousemove', onMove)
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}, [])
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return (
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<div
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style={{
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position: 'fixed',
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top: 0,
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left: lastX,
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width: 8,
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height: 8,
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background: 'black',
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}}
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/>
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)
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}
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```
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**Correct (no re-render for tracking):**
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```tsx
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function Tracker() {
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const lastXRef = useRef(0)
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const dotRef = useRef<HTMLDivElement>(null)
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useEffect(() => {
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const onMove = (e: MouseEvent) => {
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lastXRef.current = e.clientX
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const node = dotRef.current
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if (node) {
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node.style.transform = `translateX(${e.clientX}px)`
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}
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}
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window.addEventListener('mousemove', onMove)
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return () => window.removeEventListener('mousemove', onMove)
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}, [])
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return (
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<div
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ref={dotRef}
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style={{
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position: 'fixed',
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top: 0,
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left: 0,
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width: 8,
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height: 8,
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background: 'black',
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transform: 'translateX(0px)',
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}}
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/>
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)
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}
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```
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