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Rerender Derived State

Source: .agents/references/coding-standard/vercel-react-best-practices/rules/rerender-derived-state.md

Metadata​

  • title: Subscribe to Derived State
  • impact: MEDIUM
  • impactDescription: reduces re-render frequency
  • tags: rerender, derived-state, media-query, optimization

Content​

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):

function Sidebar() {
  const width = useWindowWidth()  // updates continuously
  const isMobile = width < 768
  return <nav className={isMobile ? 'mobile' : 'desktop'} />
}

Correct (re-renders only when boolean changes):

function Sidebar() {
  const isMobile = useMediaQuery('(max-width: 767px)')
  return <nav className={isMobile ? 'mobile' : 'desktop'} />
}