famapp/.agents/skills/react-best-practices/rules/rerender-derived-state.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Subscribe to Derived State	MEDIUM	reduces re-render frequency	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"} />;
}

Correct (re-renders only when boolean changes):

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