In many applications, we often need to render large lists of data. Rendering all the items at once can lead to performance issues, especially in cases where the list contains hundreds or even thousands of items. To optimize this, we can use a technique called virtualization, which only renders the items that are currently visible on the screen.

In this blog post, we will create a custom React hook called useVirtualList that will help us efficiently render large lists by leveraging virtualization. Let’s dive in!

Table of Contents

• What is virtualization?
• Implementing the useVirtualList hook
• Using the useVirtualList hook
• Final thoughts

What is virtualization?

Virtualization is the technique of dynamically rendering only the visible portion of a large list, instead of rendering all the items at once. This ensures smooth scrolling and better performance, as only a small subset of the list needs to be rendered and updated at any given time.

To achieve virtualization, we need to know which items are currently visible on the screen and their positions. This allows us to render only the visible items and efficiently manage updates as the user scrolls through the list.

Implementing the useVirtualList hook

Let’s start by implementing the useVirtualList hook. This hook will take in the total number of items and the height of each item and return the visible subset of items that should be rendered.

import { useState, useEffect } from 'react';

const useVirtualList = (totalItems, itemHeight) => {
  const [visibleItems, setVisibleItems] = useState([]);
  const [scrollTop, setScrollTop] = useState(0);

  useEffect(() => {
    const handleScroll = () => {
      setScrollTop(window.pageYOffset || document.documentElement.scrollTop);
    };

    window.addEventListener('scroll', handleScroll);
    return () => {
      window.removeEventListener('scroll', handleScroll);
    };
  }, []);

  useEffect(() => {
    const visibleItemCount = Math.ceil(window.innerHeight / itemHeight);
    const startIndex = Math.floor(scrollTop / itemHeight);
    const endIndex = startIndex + visibleItemCount;

    setVisibleItems(Array.from({ length: totalItems }).slice(startIndex, endIndex));
  }, [totalItems, itemHeight, scrollTop]);

  return visibleItems;
};

export default useVirtualList;

In this hook, we use the scrollTop value to determine the current scroll position of the window. We calculate the visible range of items based on the scroll position and the height of each item. Finally, we return the subset of visible items.

Using the useVirtualList hook

To use the useVirtualList hook, we can import it into our component and invoke it with the total number of items and the height of each item. The hook returns the visible subset of items, which can then be used to render the list.

import React from 'react';
import useVirtualList from './useVirtualList';

const LargeList = () => {
  const data = Array.from({ length: 1000 }).map((_, index) => `Item ${index}`);
  const itemHeight = 30;

  const visibleItems = useVirtualList(data.length, itemHeight);

  return (
    <div>
      <div style=>
        {/* Placeholder element to maintain scroll height */}
      </div>
      <ul style=>
        {visibleItems.map((item) => (
          <li key={item} style=>
            {item}
          </li>
        ))}
      </ul>
    </div>
  );
};

export default LargeList;

In this example, we create a large list with 1000 items. We pass the data.length and itemHeight to the useVirtualList hook to get the visible subset of items. We use this subset to render only the visible items within the list.

Final thoughts

By implementing the useVirtualList hook, we have leveraged virtualization to efficiently render large lists by rendering only the visible subset of items. This technique can greatly improve the performance of applications that deal with large amounts of data.

Remember to carefully consider how you implement virtualization, as different scenarios may require different optimizations. Experiment with different windowing techniques, such as windowed rendering or infinite scrolling, to find the best solution for your specific use case.

#development #react