Subchapter 4.16

List Performance Inline Objects

rules/list-performance-inline-objects.mdMarkdown2 KBView on GitHub

Avoid Inline Objects in renderItem

Don’t create new objects inside renderItem to pass as props. Inline objects create new references on every render, breaking memoization. Pass primitive values directly from item instead.

List Performance Inline Objects · Vercel React Native Skills · vercel-labs/agent-skills · Skills Docs

function UserList({ users }: { users: User[] }) {
  return (
    <LegendList
      data={users}
      renderItem={({ item }) => (
        <UserRow
          // Bad: new object on every render
          user={{ id: item.id, name: item.name, avatar: item.avatar }}
        />
      )}
    />
  )
}

function UserList({ users }: { users: User[] }) {
  return (
    <LegendList
      data={users}
      renderItem={({ item }) => (
        // Good: pass the item directly
        <UserRow user={item} />
      )}
    />
  )
}

renderItem={({ item }) => (
  <UserRow
    id={item.id}
    name={item.name}
    isActive={item.isActive}
  />
)}

const UserRow = memo(function UserRow({ id, name, isActive }: Props) {
  // Good: derive style inside memoized component
  const backgroundColor = isActive ? 'green' : 'gray'
  return <View style={[styles.row, { backgroundColor }]}>{/* ... */}</View>
})

const activeStyle = { backgroundColor: 'green' }
const inactiveStyle = { backgroundColor: 'gray' }

renderItem={({ item }) => (
  <UserRow
    name={item.name}
    // Good: stable references
    style={item.isActive ? activeStyle : inactiveStyle}
  />
)}