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5chan/.claude/skills/vercel-react-best-practices/rules/rerender-derived-state.md
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Tommaso CasaburiandClaude Opus 4.6 189d1c2165 add .claude config folder with Claude-equivalent agent and hook settings
Mirrors .cursor and .codex structure with Claude model assignments:
- haiku for lighter tasks (translator, browser-check, profiler)
- sonnet for standard tasks (code-quality, plan-implementer, etc.)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
2026-03-29 16:35:21 +07:00

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---
title: Subscribe to Derived State
impact: MEDIUM
impactDescription: reduces re-render frequency
tags: rerender, derived-state, media-query, optimization
---
## Subscribe to Derived State
Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
**Incorrect (re-renders on every pixel change):**
```tsx
function Sidebar() {
const width = useWindowWidth() // updates continuously
const isMobile = width < 768
return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```
**Correct (re-renders only when boolean changes):**
```tsx
function Sidebar() {
const isMobile = useMediaQuery('(max-width: 767px)')
return <nav className={isMobile ? 'mobile' : 'desktop'} />
}
```