ginnoir
3d825fba20
Adds the Vercel skill under .agents with famapp path patterns. Includes skills-lock.json pin.
30 lines
732 B
Markdown
30 lines
732 B
Markdown
---
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title: Subscribe to Derived State
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impact: MEDIUM
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impactDescription: reduces re-render frequency
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tags: rerender, derived-state, media-query, optimization
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---
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## Subscribe to Derived State
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Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
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**Incorrect (re-renders on every pixel change):**
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```tsx
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function Sidebar() {
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const width = useWindowWidth(); // updates continuously
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const isMobile = width < 768;
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return <nav className={isMobile ? "mobile" : "desktop"} />;
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}
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```
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**Correct (re-renders only when boolean changes):**
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```tsx
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function Sidebar() {
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const isMobile = useMediaQuery("(max-width: 767px)");
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return <nav className={isMobile ? "mobile" : "desktop"} />;
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}
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```
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