rerender-derived-state

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):**

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

**Correct (re-renders only when boolean changes):**

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