/react-native

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.

Incorrect (inline object breaks memoization):

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 }}
        />
      )}
    />
  )
}

Incorrect (inline style object):

renderItem={({ item }) => (
  <UserRow
    name={item.name}
    // Bad: new style object on every render
    style={{ backgroundColor: item.isActive ? 'green' : 'gray' }}
  />
)}

Correct (pass item directly or primitives):

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

Correct (pass primitives, derive inside child):

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>
})

Correct (hoist static styles in module scope):

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

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

Passing primitives or stable references allows memo() to skip re-renders when the actual values haven't changed.

Note: If you have the React Compiler enabled, it handles memoization automatically and these manual optimizations become less critical.