Subchapter 4.17

List Performance Item Expensive

rules/list-performance-item-expensive.mdMarkdown2 KBView on GitHub

Keep List Items Lightweight

List items should be as inexpensive as possible to render. Minimize hooks, avoid queries, and limit React Context access. Virtualized lists render many items during scroll—expensive items cause jank.

Incorrect (heavy list item):

List Performance Item Expensive · Vercel React Native Skills · vercel-labs/agent-skills · Skills Docs

function ProductRow({ id }: { id: string }) {
  // Bad: query inside list item
  const { data: product } = useQuery(['product', id], () => fetchProduct(id))
  // Bad: multiple context accesses
  const theme = useContext(ThemeContext)
  const user = useContext(UserContext)
  const cart = useContext(CartContext)
  // Bad: expensive computation
  const recommendations = useMemo(
    () => computeRecommendations(product),
    [product]
  )

  return <View>{/* ... */}</View>
}

function ProductRow({ name, price, imageUrl }: Props) {
  // Good: receives only primitives, minimal hooks
  return (
    <View>
      <Image source={{ uri: imageUrl }} />
      <Text>{name}</Text>
      <Text>{price}</Text>
    </View>
  )
}

// Parent fetches all data once
function ProductList() {
  const { data: products } = useQuery(['products'], fetchProducts)

  return (
    <LegendList
      data={products}
      renderItem={({ item }) => (
        <ProductRow name={item.name} price={item.price} imageUrl={item.image} />
      )}
    />
  )
}

// Incorrect: Context causes re-render when any cart value changes
function ProductRow({ id, name }: Props) {
  const { items } = useContext(CartContext)
  const inCart = items.includes(id)
  // ...
}

// Correct: Zustand selector only re-renders when this specific value changes
function ProductRow({ id, name }: Props) {
  // use Set.has (created once at the root) instead of Array.includes()
  const inCart = useCartStore((s) => s.items.has(id))
  // ...
}