FlatListのdata、renderItem、keyExtractor

ScrollView renders all its children at once, which becomes slow when displaying hundreds of items. FlatList virtualizes the list — it only renders the items currently visible on screen plus a small buffer, dramatically reducing memory usage and improving scroll performance for large datasets.

Pass your array of items to the data prop. FlatList iterates over this array and calls renderItem for each element. The array can contain any kind of JavaScript objects. Changes to the data array trigger a re-render of affected list items.

The renderItem prop is a function that receives an object with an item property (the current array element) and must return a React element. This is where you define how each row looks. Keep it as a stable reference using useCallback to avoid unnecessary re-renders.

keyExtractor tells FlatList how to extract a unique string key from each item. React uses these keys to track which items changed during re-renders. Without a proper key extractor FlatList falls back to the array index, which can cause subtle bugs during data mutations.

Combining data, renderItem, and keyExtractor gives you a fully functional, scrollable, virtualized list. You can style the rows inside renderItem just like any other component. FlatList automatically handles scroll and touch events.

Use ListHeaderComponent to render content above the list (such as a search bar or page title) and ListFooterComponent for content below (such as a loading spinner when fetching more data). These components scroll with the list, unlike a view placed outside FlatList.

When the data array is empty, FlatList renders nothing by default. Provide a ListEmptyComponent to display a placeholder message or illustration when there are no items. This is much cleaner than wrapping FlatList in a conditional.

Instead of adding border styles directly to item rows, use ItemSeparatorComponent to render a separator between items. React Native only renders the separator between items, not before the first or after the last, which avoids double borders at the edges.

Set the horizontal prop to true to render a horizontally scrolling list. This is useful for carousels, category chips, or story rows. The showsHorizontalScrollIndicator prop can hide the scroll bar for a cleaner look.

Use the ref on FlatList and call flatListRef.current.scrollToIndex({ index, animated: true }) to jump to a specific item. You can also use scrollToOffset for pixel-level control or scrollToEnd to scroll to the bottom of the list.

Avoid defining renderItem as an anonymous arrow function directly in JSX (renderItem={() => ...}). This creates a new function reference on every render, causing every row to re-render unnecessarily. Define it outside the JSX or memoize it with useCallback. Also never use an array index as the key — always use a stable unique ID.

Test your understanding of FlatList data, renderItem, and keyExtractor from this lesson.

In this lesson you learned: FlatList virtualizes large lists by only rendering visible items, renderItem defines the appearance of each row, and keyExtractor provides stable unique keys for efficient React reconciliation. Next up we explore pull-to-refresh and infinite scroll with load more.