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'} />
}
```