Rerender Derived State

---
description: Imported skill rerender_derived_state from vercel
name: rerender_derived_state
signature: 1c326bb67b01fb084eb00c8911b0a7cff54681c2315b629f88e49b64eaa6481d
source: /a0/tmp/skills_research/vercel/skills/react-best-practices/rules/rerender-derived-state.md
---

---
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'} />
}
```