Optimizing Large Lists with React Window

When dealing with large lists or data-heavy views in React, rendering all the items at once can significantly hurt performance, especially in terms of memory usage and rendering time. One of the most effective ways to optimize the rendering of large lists is by virtualization—only rendering the visible items in the list and reusing DOM elements for other items as the user scrolls.

React Window is a lightweight library designed for virtualizing lists and tabular data, making it an excellent choice for optimizing large lists in React.

This guide will walk you through using React Window to optimize large lists, improve performance, and ensure smooth user experiences.

1. What is React Window?

React Window is a React library that helps you render only the visible portion of large lists or grids by implementing virtualization. Instead of rendering every item in the list at once, React Window only renders the items that are visible on the screen, improving both performance and memory usage.

Key Features:

Lightweight: React Window is designed to be smaller and faster than similar libraries like react-virtualized.
Efficient: It only renders the visible items in the list and reuses the DOM elements as the user scrolls.
Flexible: It supports both vertical and horizontal lists, grids, and even dynamic item sizes.

2. Installing React Window

To get started with React Window, you need to install it via npm or yarn.

npm install react-window

yarn add react-window

3. Basic Usage of React Window

Let’s start with the simplest case: rendering a large vertical list using FixedSizeList from React Window. FixedSizeList is the most commonly used component for static-sized items in a list.

Example: Basic Usage of `FixedSizeList`

import React from 'react';
import { FixedSizeList as List } from 'react-window';

const data = new Array(1000).fill("Item").map((_, index) => `Item ${index + 1}`);

const MyList = () => {
  return (
    <List
      height={400}           // Height of the visible area
      itemCount={data.length} // Total number of items
      itemSize={35}           // Height of each list item
      width={300}             // Width of the list container
    >
      {({ index, style }) => (
        <div style={style}>{data[index]}</div>
      )}
    </List>
  );
};

export default MyList;

Explanation:

height: The height of the viewport (visible area) where the list is displayed.
itemCount: The total number of items in the list.
itemSize: The fixed height of each item in the list.
width: The width of the list container.
The style passed to each list item ensures that the list is positioned correctly within the scrollable area.

4. Using Dynamic Item Sizes

In some cases, the items in your list might have dynamic heights (i.e., each item could have a different size). React Window provides the VariableSizeList component for this scenario.

Example: Using `VariableSizeList`

import React, { useCallback } from 'react';
import { VariableSizeList as List } from 'react-window';

const data = new Array(1000).fill("Item").map((_, index) => `Item ${index + 1}`);

const getItemSize = (index) => {
  return 50 + (index % 5) * 10; // Dynamic item size
};

const MyList = () => {
  return (
    <List
      height={400}
      itemCount={data.length}
      itemSize={getItemSize} // Provide a function to calculate dynamic item size
      width={300}
    >
      {({ index, style }) => (
        <div style={style}>{data[index]}</div>
      )}
    </List>
  );
};

export default MyList;

Explanation:

itemSize: Instead of passing a fixed value, here we provide a function that calculates the size for each item dynamically based on its index.

5. Horizontal Lists

React Window can also be used for horizontal scrolling. For horizontal lists, you can use FixedSizeGrid or VariableSizeGrid depending on whether the items have fixed or dynamic sizes.

Example: Horizontal Scrolling with `FixedSizeGrid`

import React from 'react';
import { FixedSizeGrid as Grid } from 'react-window';

const data = new Array(1000).fill("Item").map((_, index) => `Item ${index + 1}`);

const MyGrid = () => {
  return (
    <Grid
      columnCount={100}
      columnWidth={100}   // Width of each grid item
      height={200}
      rowCount={10}
      rowHeight={50}      // Height of each grid item
      width={1000}
    >
      {({ columnIndex, rowIndex, style }) => (
        <div style={style}>
          {data[rowIndex * 100 + columnIndex]}
        </div>
      )}
    </Grid>
  );
};

export default MyGrid;

Explanation:

FixedSizeGrid is similar to FixedSizeList, but it works for grids (both rows and columns).
You define columnCount and rowCount to specify how many items are in the grid.
columnWidth and rowHeight specify the dimensions of each grid cell.

6. Advanced Features

React Window provides several advanced features to further optimize the rendering process:

initialScrollOffset: Allows you to specify an initial scroll position, which is useful for restoring scroll position or starting at a specific point in the list.
onItemsRendered: A callback that is triggered when the visible items are rendered. You can use it to fetch more data or track the current scroll position.

Example: Using `onItemsRendered`

import React from 'react';
import { FixedSizeList as List } from 'react-window';

const data = new Array(1000).fill("Item").map((_, index) => `Item ${index + 1}`);

const MyList = () => {
  const onItemsRendered = ({ visibleStartIndex, visibleStopIndex }) => {
    console.log('Visible items:', visibleStartIndex, visibleStopIndex);
  };

  return (
    <List
      height={400}
      itemCount={data.length}
      itemSize={35}
      width={300}
      onItemsRendered={onItemsRendered}
    >
      {({ index, style }) => (
        <div style={style}>{data[index]}</div>
      )}
    </List>
  );
};

export default MyList;

7. Best Practices for Using React Window

Measure Your Items Carefully: If your items have dynamic sizes, always ensure that you are calculating the size correctly to avoid rendering issues.
Avoid Re-Renders: Avoid unnecessary re-renders by memoizing data passed to the list and using React.memo() for list items when applicable.
Batch Updates: Use batching to handle large datasets efficiently, especially if you’re fetching data in chunks or asynchronously.
Use onItemsRendered for Fetching More Data: If you’re building an infinite scrolling list, use onItemsRendered to trigger data fetching when the user reaches the end of the list.
Test Performance: Always measure performance with tools like React DevTools to ensure the optimizations are making a noticeable impact.