Rendering Lists Using React Virtualized

Working with data in React is relatively easy because React is designed to handle data as state. The hassle begins when the amount of data you need to consume becomes massive. For example, say you have to handle a dataset which is between 500-1,000 records. This can result in massive loads and lead performance problems. Well, we’re going to look at how we can make use of virtualized lists in React to seamlessly render a long list of data in your application.

We’re going to use the React Virtualized component to get what we need. It will allow us to take large sets of data, process them on the fly, and render them with little-to-no jank.

The setup

React Virtualized already has a detailed set of instructions to get it up and running, so please check out the repo to get started.

We’re going to want data to work with, so we will set up a function which uses faker to create a large data set.

function createRecord(count) {
  let records = [];

  for (let i = 0; i < count; i++) {
    records.push({
      username: faker.internet.userName(),
      email: faker.internet.email()
    });
  }
  return records;
}

Next, we will pass it the number of data records we want to create, like so:

const records = createRecord(1000);

Alright, now we have what we need to work on rendering a list of those records!

Creating a virtualized list

Here’s the list we want to create, sans styling. We could make use of the few presentational styles that the library includes by importing the included CSS file, but we're going to leave that out in this post.

See the Pen React Virtualized 1 by Kingsley Silas Chijioke (@kinsomicrote) on CodePen.

Go ahead and re-run that demo. Crazy fast, right?

You might wonder what the heck React Virtualized is doing behind the scenes to make that happen. Turns out it's a bunch of crazy and cool sizing, positioning, transforms and transitions that allow the records to scroll in and out of view. The data is already there and rendered. React Virtualized creates a window frame that allows records to slide in and out of view as the user scrolls through it.

To render a virtualized list in React Virtualized, we make use of its List component, which uses a Grid component internally to render the list.

First, we start by setting up rowRenderer, which is responsible for displaying a single row and sets up an index that assigns an ID to each record.

rowRenderer = ({ index, isScrolling, key, style }) => {
    return (
      <div key={key} style={style}>
        <div>{this.props.data[index].username}</div>
        <div>{this.props.data[index].email}</div>
      </div>
    );
  };

As you can see, this returns a single div node that contains two additional divs: one for the username and another for the email. You know, a common list pattern to display users.

rowRenderer accepts several parameters. Here’s what they are and what each one does:

• index: The numeric ID of a record.
• isScrolling: Indicates if the scrolling is occurring in the List component.
• isVisible: Determines if a row is visible or out of view.
• key: The records position in the array.
• parent: Defines whether the list is a parent or a child of another list.
• style: A style object to position the row.

Now that we know more about the rowRenderer function, let’s make put it to use in the List component:

<List
  rowCount={this.props.data.length}
  width={width}
  height={height}
  rowHeight={rowHeight}
  rowRenderer={this.rowRenderer}
  overscanRowCount={3}
/>

You may have noticed a few new parameters. Here’s what they are:

• rowCount: This takes the numbers of a row in a list that we pass to calculate the length of our list.
• width: The width of the list.
• height: The height of the list.
• rowHeight: This can be a number or a function that returns a row height given its index.
• rowRenderer: This is responsible for rendering the row. the list is not supposed to be passed directly, so we pass the rowRenderer function that we created in this tutorial.
• overscanRowCount: This is used to render additional rows in the direction the user scrolls. It reduces the chances of the user scrolling faster than the virtualized content is rendered.

At the end, your code should look something like this;

const { List } = ReactVirtualized

...

const height = 700;
const rowHeight = 40;
const width = 800;

class App extends React.Component {
  rowRenderer = ({ index, isScrolling, key, style }) => {
    return (
      <div key={key} style={style}>
        <div>{this.props.data[index].username}</div>
        <div>{this.props.data[index].email}</div>
      </div>
    );
  };

  render() {
    return (
      <div>
        <h2>Details</h2>
        <List
          rowCount={this.props.data.length}
          width={width}
          height={height}
          rowHeight={rowHeight}
          rowRenderer={this.rowRenderer}
          overscanRowCount={3}
        />
      </div>
    );
  }
}

Cell measurer

According to the documentation, a cell measurer is a higher-order component that is used to temporarily render a list. It’s not yet visible to the user at this point, but the data is held and ready to display.

Why should you care about this? The popular use case is a situation where the value of your rowHeight is dynamic. React Virtualized can render the height of the row on render then cache that height so it no longer needs to calculate as data scrolls out of view — it's always the right height, no matter the content it contains!

First, we create our cache, which can be done in our component’s constructor using CellMeasurerCache:

constructor() {
  super()
  this.cache = new CellMeasurerCache({
    fixedWidth: true,
    defaultHeight: 100
  })
}

We make use of the cache when we set up the List component;

<List
  rowCount={this.props.data.length}
  width={rowWidth}
  height={listHeight}
  deferredMeasurementCache={this.cache}
  rowHeight={this.cache.rowHeight}
  rowRenderer={this.renderRow}
  overscanRowCount={3}
/>

The value passed to deferredMeasurementCache will be used to temporarily rendering the data, then — as the calculated value for rowHeight comes in — additional rows will flow in like they were always there.

Next, though, we will make use of React Virtualized’s CellMeasurer component inside our rowRenderer function instead of the div we initially set up as a placeholder:

rowRenderer = ({ index, parent, key, style }) => {
  return (
    <CellMeasurer
      key={key}
      cache={this.cache}
      parent={parent}
      columnIndex={0}
      rowIndex={index}
    >
      <div style={style}>
        <div>{this.props.data[index].username}</div>
        <div>{this.props.data[index].email}</div>
      </div>
    </CellMeasurer>
  );
  };

Now the data is fetched, cached and ready to display in the virtual window at will!

Virtualized table

Yeah, so the main point of this post is to cover lists, but what if we actually want to render data to a table instead? React Virtualized has you covered on that front, too. In this case, we will make use of Table and Column components that come baked into React Virtualized.

Here’s how we would put those components to use in our primary App component:

class App extends React.Component {
  render() {
    return (
      <div>
        <h2>Details</h2>
        <Table
          width={500}
          height={300}
          headerHeight={20}
          rowHeight={40}
          rowCount={this.props.data.length}
          rowGetter={({ index }) => this.props.data[index]}
        >
          <Column
            label='Username'
            dataKey='username'
            width={100}
          />
            
          <Column
            width={200}
            label='Email'
            dataKey='email'
          />
        </Table>
      </div>
    );
  }
}

The Table component accepts the following parameters:

• width: The width of the table.
• height: The height of the table.
• headerHeight: The table header height.
• rowHeight: The height of a row given its index.
• rowCount: This is the initial number of rows we want in the table. It’s the same as the way we defined the number of records we wanted to start with in the List component example.
• rowGetter: This returns the data of a specific row by its index.

If you take a look at the Column component, you will notice that we put a dataKey parameter to use. That passes the data for each column we called in the dataKey, which receives a unique identifier for that data. Remember that in the function where we create our random data, we make use of two keys; username and email. This is why we have the dataKey of one column set as username and the other set as email.

In conclusion

Hopefully, this walkthrough gives you a good idea of what React Virtualized is capable of doing, how it can make rendering large data sets into lists and tables super fast, and how to put it to use in a project.

We've only scratched the surface here. The library is capable of handling a lot of other use cases, like generating placeholders for the data records on scroll, an infinite loading component to fetch and cache data in real-time, a method for allowing arrow keys to navigate through the data, and a slick grid and masonry layouts that we didn't even cover here.

That should give you a lot to play around with!

Plus, the package is highly maintained. In fact, you can join the Slack group to keep up with the project, contribute to it, and generally get to connect with other folks.

It’s also worth noting that React Virtualized has it own tag in StackOverflow and that can be a good resource to find questions other people have asked about it, or even post your own questions.

Oh, and if you’ve put React Virtualized to use on a project, we’d love to know it! Share it with us in the comments with some notes on how you approached it or what you learned from it.

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