rerender-lazy-state

name	rerender-lazy-state
description	Use lazy state initialization with useState function form. Apply when computing expensive initial values like building search indexes, parsing JSON, or complex calculations.

Use Lazy State Initialization

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect (runs on every render):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items))
  const [query, setQuery] = useState('')

  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.(.() || )
  )

   
}