# 308 Rerender Derived State 1c326bb6

> Subscribe to Derived State

- Skill: `tools-only/308-rerender-derived-state-1c326bb6` (Agent Skill, multi-file: 3 files)
- Install (CLI): `npx skillmds@latest add tools-only/308-rerender-derived-state-1c326bb6`
- Raw SKILL.md: https://api.skillmd.com/api/skills/tools-only/308-rerender-derived-state-1c326bb6/raw
- Safety review: pending (external: skill-scanner PASS, skillspector PASS)
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Coding & Dev Tools
- Author: tools-only (https://skillmd.com/u/tools-only)
- Updated: 2026-09-22
- Page: https://skillmd.com/skills/tools-only/308-rerender-derived-state-1c326bb6

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## 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'} />
}
```