<virtual-scroller>

<virtual-scroller> is a web component that provides a way to render a large number of elements in a scrollable list without negatively affecting overall UI performance. The component achives high performance by only rendering elements that are currently visible in its "viewport" and virtualizes elements not visible outside of it.

The <virtual-scroller> component is technology agnostic allowing you to use it with whichever rendering libary that you are already using in an existing project, e.g. React, lit, Vue...

Performance

<virtual-scroller> has great performance since it takes full advantage of the browser's ability to batch DOM updates which minimizes reflow and repaint. It also has a very small footprint allowing you to keep your bundles small for faster page loads.

Install

The <virtual-scroller> web component can be installed from NPM:

# NPM
npm install @holmberd/virtual-scroller

# Yarn
yarn add @holmberd/virtual-scroller

Example Usage

See react-virtual-scroller for React wrapper.

React

  import VirtualScroller, { Layout, VISIBLE_RANGE_CHANGE_EVENT } from 'virtual-scroller';

  const getItemHeight = (index) => index % 2 === 0 ? 50 : 100;
  const listItems = Array.from(Array(10000).map((index) => ({
    id: index,
    height: getItemHeight(index),
  }));

  function List() {
    const [items, setItems] = useState([]);

    useEffect(() => {
      if (!scrollerRef?.current) {
        return;
      }

      scrollerRef.current.addEventListener(VISIBLE_RANGE_CHANGE_EVENT, ({ detail }) => {
        const { startIndex, stopIndex, offsetIndex } = detail;
        setItems(listItems.slice(startIndex, stopIndex + 1));
      });

      scrollerRef.current.init(listItems.length, getItemHeight, {
        offsetVisibleIndex: 0,
        layout: Layout.VERTICAL,
      });
    }, [])

  return (
    <virtual-scroller style={{ width: 400, height: 400 }} ref={scrollerRef}>
      {items.map(item => <div key={item.id} style={{ height: item.height }}>{item.id}</div>)}
    </virtual-scroller>
  );
}

Methods

init(itemCount: string, getItemLength: function, options: object): void

Once called the virtual-scroller will calculate the visible range and dispatch a visible-range-change event. You can call this multiple times to reset the items scroll index, e.g. to increase item-count when a user scrolls down the list or when the height of an item changes.

Arguments:

• itemCount: number: The total number of top-level items.
• getItemLength(index: number): number: Function to calculate and return the length(height or width) of each item by index.

Options:

• offsetVisibleIndex = 0: Number of extra items to be rendered before/after the visible range.
• layout = 'vertical': Set whether to use vertical or horizontal layout for virtualization.
• enableResizeObserver = false: Set wether to update visible item indexes on element resize.
• disableVirtualization = false: Set to disable virtualization, (visible-range-change will still be dispatched).

resetOnIndex(index: number = 0, shouldUpdate: boolean = true): void

Rebuilds the items cached scrollOffset index on and after the specified index when called. Useful when the size of an item changes in your list, e.g. expanded/collapsed. By default calling this method will trigger an update, use shouldUpdate to override this behaviour.

scrollToItem(index: number): void

Scrolls to the specified item index when called. (The item aligns to the beginning of the list).

Properties

itemCount = 0

Set the total number of top-level items.

getItemLength = (index) => 0

Set function to calculate and return the length(height or width) of each item by index.

offsetVisibleIndex = 0

Set the number of extra items to be rendered before/after the visible range.

enableResizeObserver = false

Set wether to update visible item indexes on element resize.

layout = 'vertical'

Get/Set current layout virtualization mode.

Events

visible-range-change: CustomEvent

Fired when the visible range of item indexes changes.

event.detail.startIndex: number
event.detail.stopIndex: number
event.detail.offsetIndex: number

Browser Support

<virtual-scroller> supports es2020 JavaScript features for desktop and mobile browsers and builds upon standard web platform APIs so that the performance, capabilities and compatibility of the library get better as the web evolves.

Development (TBD)

The following commands are available when developing <virtual-scroller>:

Command	Description
npm run build	Builds all <virtual-scroller> distributable files.
npm run test	Run <virtual-scroller> unit tests.
npm run dev	TBD.

Contributing

If you'd like to contribute to <virtual-scroller>, please reach out.