@innei/virtual-scroller

VirtualScroller

A universal open-source implementation of Twitter's VirtualScroller component: a component for efficiently rendering large lists of variable height items. Automatically measures items as they're rendered and supports items of variable/dynamic height. Also includes a React component for those who're using React. Also provides a low-level component that supports any type of rendering engine, not just DOM.

Demo

DOM (no frameworks):

• Basic
• Dynamically loaded

React:

• Basic
• Dynamically loaded

Grid Layout:

• Basic
• Dynamically loaded

Rationale

Rendering really long lists in HTML can be performance intensive which sometimes leads to slow page load times and wasting mobile users' battery. For example, consider a chat app rendering a list of a thousand of the most recent messages: when using React the full render cycle can take up to 100 milliseconds or more on a modern PC. If the chat message component is complex enough (rich text formatting, pictures, videos, attachments, buttons) then it could take up to a second or more (on a modern PC). Now imagine users viewing the website on their aged low-tier smartphones and it quickly results in annoying user experience resulting in them closing the website and the website losing its user base.

In 2017 Twitter completely redesigned their website with responsiveness and performance in mind using the latest performance-boosting techniques available at that time. They wrote an article about it where they briefly mentioned this:

On slower devices, we noticed that it could take a long time for our main navigation bar to appear to respond to taps, often leading us to tap multiple times, thinking that perhaps the first tap didn’t register. It turns out that mounting and unmounting large trees of components (like timelines of Tweets) is very expensive in React. Over time, we developed a new infinite scrolling component called VirtualScroller. With this new component, we know exactly what slice of Tweets are being rendered into a timeline at any given time, avoiding the need to make expensive calculations as to where we are visually.

However, Twitter didn't share the code for their VirtualScroller component (unlike Facebook, Twitter doesn't share much of their code). This library is an attempt to create an open-source implementation of such VirtualScroller component for anyone to use in their projects.

There's also an "RFC" for a native VirtualScroller component where they try to formulate what is a VirtualScroller component and how it should behave.

How it works

VirtualScroller works by measuring each list item's height as it's being rendered, and then, as the user scrolls, it hides the items that are no longer visible, and shows the now-visible items as they're scrolled to. The hidden items at the top are compensated by setting padding-top on the list element, and the hidden items at the bottom are compensated by setting padding-bottom on the list element. The component listens to scroll / resize events and re-renders the currently visible items as the user scrolls (or if the browser window is resized).

To observe list item elements being dynamically mounted and unmounted, go to the demo page, open Developer Tools ("Elements" tab), find <div id="root"/> element, expand it, see <div id="messages"/> element, expand it and observe the changes to it while scrolling the page.

To add some inter-item spacing, one could use margin-top / margin-bottom or border-top / border-bottom: see the Gotchas section for more details on how to do that properly.

Install

npm install virtual-scroller --save

If you're not using a bundler then use a standalone version from a CDN.

Core

The default export is a low-level VirtualScroller class: it implements the core logic of a VirtualScroller component and can be used for building a VirtualScroller component for any UI framework or even any rendering engine other than DOM. Hence, the core component is not meant to be used in applications directly: instead, use the high-level components exported from virtual-scroller/dom or virtual-scroller/react packages.

State

The core VirtualScroller component works by dynamically updating its state as the user scrolls the page. The state provides the calculations on which items should be rendered (and which should not) depending on the current scroll position. A high-level wrapper around VirtualScroller supplies a function that renders the actual list using the information from the state.

Show the list of all state properties

A high-level wrapper should supply either getState/setState functions, or onStateChange function (or both of them), and those functions are gonna be responsible for rendering the actual list using the information from state.

The main state properties are:

• items: any[] — The list of items (can be updated via .setItems()).

• firstShownItemIndex: number — The index of the first item that should be rendered.

• lastShownItemIndex: number — The index of the last item that should be rendered.

• beforeItemsHeight: number — The padding-top which should be applied to the "container" element: it emulates all items before firstShownItemIndex as if they were rendered.

• afterItemsHeight: number — The padding-bottom which should be applied to the "container" element: it emulates all items after lastShownItemIndex as if they were rendered.

The following state properties are only used for saving and restoring VirtualScroller state, and normally shouldn't be accessed:

• itemStates: object?[] — The states of all items. Any change in an item's appearance while it's rendered must be reflected in changing that item's state by calling .onItemStateChange(i, itemState) instance method (described below): this way, the item's state is preserved when it's shown next time after being hidden due to going off screen. For example, if an item is a social media comment, and there's a "Show more"/"Show less" button that shows the full text of the comment, then it must call .onItemStateChange(i, { showMore: true/false }) every time.

• itemHeights: number?[] — The measured heights of all items. If an item's height hasn't been measured yet then it's undefined. By default, items are only measured once: when they're initially rendered. If an item's height changes afterwards, then .onItemHeightChange(i) instance method must be called (described below), otherwise VirtualScroller's calculations will be off. For example, if an item is a social media comment, and there's a "Show more"/"Show less" button that shows the full text of the comment, then it must call .onItemHeightChange(i) every time. And every change in an item's height must come as a result of changing some kind of state, be it the item's state in VirtualScroller via .onItemStateChange(), or some other state managed by the application.

• verticalSpacing: number? — Vertical item spacing. Is undefined until it has been measured. Is only measured once, when at least two rows of items have been rendered.

• columnsCount: number? — The count of items in a row. Is undefined if no getColumnsCount() parameter has been passed to VirtualScroller.

• scrollY: number? — The current page scroll position (page vertical scroll offset). If initial state is passed to VirtualScroller, then the page will be scrolled to state.scrollY on .render().

Example

A general idea of using the low-level VirtualScroller class.

import VirtualScroller from 'virtual-scroller'

const virtualScroller = new VirtualScroller(getContainerElement, items, options)

// Start listening to scroll events.
virtualScroller.listen()

// Stop listening to scroll events.
virtualScroller.stop()

• getContainerElement() function returns the list "element" that is gonna contain all list item "elements".
• items is the list of items.
• options is the list of options.

An example of implementing a high-level virtual-scroller/dom component on top of the low-level VirtualScroller class.

import VirtualScroller from 'virtual-scroller'

const items = [
  { title: 'Apple' },
  { title: 'Banana' },
  { title: 'Cranberry' }
]

function renderItem(item) {
  const div = document.createElement('div')
  div.innerText = item.title
  return div
}

const container = document.getElementById('list')

function onStateChange(newState, prevState) {
  const {
    items,
    beforeItemsHeight,
    afterItemsHeight,
    firstShownItemIndex,
    lastShownItemIndex
  } = newState
  // Set `paddingTop` and `paddingBottom` on the container element:
  // it emulates the non-visible items as if they were rendered.
  container.style.paddingTop = Math.round(beforeItemsHeight) + 'px'
  container.style.paddingBottom = Math.round(afterItemsHeight) + 'px'
  // Perform an intelligent "diff" re-render as the user scrolls the page.
  // This also requires that the list of `items` hasn't been changed.
  // On initial render, `prevState` is `undefined`.
  if (prevState && items === prevState.items) {
    // Remove no longer visible items.
    let i = prevState.lastShownItemIndex
    while (i >= prevState.firstShownItemIndex) {
      if (i >= firstShownItemIndex && i <= lastShownItemIndex) {
        // The item is still visible.
      } else {
        // The item is no longer visible. Remove it.
        container.removeChild(container.childNodes[i - prevState.firstShownItemIndex])
      }
      i--
    }
    // Add newly visible items.
    let prependBefore = container.firstChild
    let i = firstShownItemIndex
    while (i <= lastShownItemIndex) {
      if (i >= prevState.firstShownItemIndex && i <= prevState.lastShownItemIndex) {
        // The item is already being rendered.
        // Next items will be appended rather than prepended.
        prependBefore = undefined
      } else {
        if (prependBefore) {
          container.insertBefore(renderItem(items[i]), prependBefore)
        } else {
          container.appendChild(renderItem(items[i]))
        }
      }
      i++
    }
  } else {
    // Re-render the list from scratch.
    while (container.firstChild) {
      container.removeChild(container.firstChild)
    }
    let i = firstShownItemIndex
    while (i <= lastShownItemIndex) {
      container.appendChild(renderItem(items[i]))
      i++
    }
  }
}

const options = { onStateChange }

const virtualScroller = new VirtualScroller(() => element, items, options)

// Start VirtualScroller listening for scroll events.
virtualScroller.listen()

// Stop VirtualScroller listening for scroll events
// when the user navigates to another page:
// router.onPageUnload(virtualScroller.stop)

Options

Show the list of all available core VirtualScroller options.

• estimatedItemHeight: number — Is used for the initial render of the list: determines how many list items are rendered initially to cover the screen height plus some extra vertical margin for future scrolling. If not set then the list first renders just the first item, measures it, and then assumes it to be the estimatedItemHeight from which it calculates how many items to show on the second render pass to fill the screen height plus some extra vertical margin for future scrolling. Therefore, this setting is only for the initial render minor optimization and is not required.

• state: object — The initial state for VirtualScroller. Can be used, for example, to quicky restore the list when it's re-rendered on "Back" navigation.

• customState: object — (advanced) The initial "custom" state for VirtualScroller. It can be used to initialize the "custom" part of VirtualScroller state in cases when VirtualScroller state is used to store some "custom" list state.

• getState(): object — (advanced) By default, VirtualScroller manages state internally by storing it in an instance variable. For more control, the state could be managed externally, in which case a developer should supply getState/setState options, in which case getState should return the externally stored VirtualScroller state. React VirtualScroller component implementation uses this option.

• setState(newState: object, { willUpdateState, didUpdateState }) — (advanced) By default, VirtualScroller manages state internally by storing it in an instance variable. For more control, the state could be managed externally, in which case a developer should supply getState/setState options, in which case setState should update the externally stored VirtualScroller state (including setting the initial state). setState must also call two functions: willUpdateState(newState, prevState) right before the state is updated and didUpdateState(prevState) right after the state is updated. The list should be re-rendered as part of either setState or onStateChange. React VirtualScroller component implementation uses this option.

• onStateChange(newState: object, prevState: object?) — Is called whenever VirtualScroller state is updated (including setting the initial state). A developer must either supply getState/setState options or onStateChange option (or both). The list should be re-rendered as part of either setState or onStateChange.

• scrollableContainer: Element — (advanced) If the list is being rendered in a "scrollable container" (for example, if one of the parent elements of the list is styled with max-height and overflow: auto), then passing the "scrollable container" DOM Element is required for correct operation. This feature is considered experimental. The width and height of the scrollableContainer shouldn't change unless window is resized.

• bypass: boolean — Pass true to turn off VirtualScroller behavior and just render the full list of items.

• getItemId(item) — (advanced) When items are dynamically updated via .setItems(), VirtualScroller detects an "incremental" update by comparing "new" and "old" item "references": this way, VirtualScroller can understand that the "new" items are (mostly) the same as the "old" items when some items get prepended or appended to the list, in which case it doesn't re-render the whole list from scratch, but rather just renders the "new" items that got prepended or appended. Sometimes though, some of the "old" items might get updated: for example, if items is a list of comments, then some of those comments might get edited in-between the refreshes. In that case, the edited comment object reference should change in order to indicate that the comment's content has changed and that the comment should be re-rendered (at least that's how it has to be done in React world). At the same time, changing the edited comment object reference would break VirtualScroller's "incremental" update detection, and it would re-render the whole list of comments from scratch, which is not what it should be doing in such cases. So, in cases like this, VirtualScroller should have some way to understand that the updated item, even if its object reference has changed, is still the same as the old one, so that it doesn't break "incremental" update detection. For that, getItemId(item) parameter could be passed, which VirtualScroller would use to compare "old" and "new" items (instead of the default "reference equality" check), and that would fix the "re-rendering the whole list from scratch" issue. It can also be used when items are fetched from an external API, in which case all item object references change on every such fetch.

• onItemInitialRender(item) — Is called for each item when it's about to be rendered for the first time. Is guaranteed to be called at least once for each item rendered, though, in "asynchronous" rendering systems like React, it could be called multiple times for a given item, because "an item is calculated to be rendered" doesn't necessarily mean that the actual rendering will take place before a later calculation supercedes the former one. This function can be used to somehow "initialize" items before they're rendered for the first time. For example, consider a list of items that must be somehow "preprocessed" (parsed, enhanced, etc) before being rendered, and such "preprocessing" puts some load on the CPU (and therefore takes some time). In such case, instead of "preprocessing" the whole list of items up front, a developer could "preprocess" the items as they're being rendered, thereby eliminating any associated lag or freezing that would be inevitable have all the items been "preprocessed" up front. If a user only wants to see a few of the items, "preprocessing" all the items up front would simply be a waste.

• preserveScrollPositionOfTheBottomOfTheListOnMount: boolean — (advanced) Set to true to preserve scroll position relative to the bottom of the list when it's first mounted. A possible use case: consider a forum thread only showing unread posts by default. If a user navigates to such thread, it could show "No new posts" message with a "Show previous" button above it. When the user clicks the "Show previous" button, a <VirtualScroller/> is mounted with a list of posts. If preserveScrollPositionOfTheBottomOfTheListOnMount: true is set, then, after the list of posts is shown, page scroll will be restored so that the bottom of the list remains on screen so that the user could start scrolling up to read the "previous" posts starting from the most recent ones to the oldest ones.

• shouldUpdateLayoutOnScreenResize(event: Event): boolean — By default, VirtualScroller always performs a re-layout on window resize event. The resize event is not only triggered when a user resizes the window itself: it's also triggered when the user switches into (and out of) fullscreen mode. By default, VirtualScroller performs a re-layout on all window resize events, except for ones that don't result in actual window width or height change, and except for cases when, for example, a video somewhere in a list is maximized into fullscreen. There still can be other "custom" cases: for example, when an application uses a custom "slideshow" component (rendered outside of the list DOM element) that goes into fullscreen when a user clicks a picture or a video in the list. For such "custom" cases shouldUpdateLayoutOnScreenResize(event) option / property can be specified.

• measureItemsBatchSize: number — (advanced) (experimental) Imagine a situation when a user doesn't gradually scroll through a huge list but instead hits an End key to scroll right to the end of such huge list: this will result in the whole list rendering at once (because an item needs to know the height of all previous items in order to render at correct scroll position) which could be CPU-intensive in some cases (for example, when using React due to its slow performance when initially rendering components on a page). To prevent freezing the UI in the process, a measureItemsBatchSize could be configured, that would limit the maximum count of items that're being rendered in a single pass for measuring their height: if measureItemsBatchSize is configured, then such items will be rendered and measured in batches. By default it's set to 100. This is an experimental feature and could be removed in future non-major versions of this library. For example, the future React 17 will come with Fiber rendering engine that is said to resolve such freezing issues internally. In that case, introducing this option may be reconsidered.

• getColumnsCount(container: ScrollableContainer): number — (advanced) Provides support for "grid" layout. The container argument provides a .getWidth() method.

Show the list of all available core VirtualScroller instance methods.

• listen() — Starts VirtualScroller listening for scroll events. Should be called immediately after the list has been rendered on a page.

• stop() — Stops VirtualScroller listening for scroll events. Should be called when the list is about to be removed from the page. Once stopped, a VirtualScroller can't be restarted.

• getState(): object — Returns VirtualScroller state. Is used for React VirtualScroller component implementation.

• onItemHeightChange(i: number) — Must be called whenever a list item's height changes (for example, when a user clicks an "Expand"/"Collapse" button of a list item): it re-measures the item's height and updates VirtualScroller layout. Every change in an item's height must come as a result of changing some kind of state, be it the item's state in VirtualScroller via .onItemStateChange(), or some other state managed by the application. Implementation-wise, calling onItemHeightChange() manually could be replaced with detecting item height changes automatically via Resize Observer.

• onItemStateChange(i: number, itemState: object) — Updates a list item's state inside VirtualScroller state. Must be called whenever an item's "state" changes: this way, the item's state is preserved when the item is unmounted due to going off screen, and then restored when the item is on screen again. Calling onItemStateChange() doesn't trigger a re-layout of VirtualScroller because changing a list item's state doesn't necessarily mean a change of its height, so a re-layout might not be required. If an item's height did change as a result of changing its state, then VirtualScroller layout must be updated, and to do that, call onItemHeightChange(i) after calling onItemStateChange(). For example, consider a social network feed, each post optionally having an attachment. Suppose there's a post in the feed having a YouTube video attachment. The attachment is initially shown as a small thumbnail that expands into a full-sized embedded YouTube video player when a user clicks on it. If the expanded/collapsed state of such attachment isn't been managed in VirtualScroller, then, when the user expands the video, then scrolls down so that the post with the video is no longer visible and is unmounted as a result, then scrolls back up so that the post with the video is visible again, the video's expanded state would be lost, and it would be rendered as a small thumbnail as if the user didn't click on it. And don't forget about calling onItemHeightChange(i) in such cases: if onItemHeightChange(i) isn't called after expanding the thumbnail into a video player, then the scroll position would "jump" when such item goes off screen, because VirtualScroller would have based its calculations on the initially measured item height, not the "expanded" one.

• setItems(newItems: any[], options: object?) — Updates VirtualScroller items. For example, it can be used to prepend or append new items to the list. See Dynamically Loaded Lists section for more details. Available options: preserveScrollPositionOnPrependItems: boolean — Set to true to enable "restore scroll position after prepending new items" feature (should be used when implementing a "Show previous items" button).

• updateLayout() — (advanced) Triggers a re-layout of VirtualScroller. It's what's called every time on page scroll or window resize. You most likely won't ever need to call this method manually. Still, it can be called manually when the list's top position changes not as a result of scrolling the page or resizing the window. For example, if some DOM elements above the list are removed (like a closeable "info" panel) or collapsed (like an "accordion" panel), then the list's top position changes, which means that now some of the previoulsy shown items might go off screen, and the user might be seeing a blank area where items haven't been rendered yet because they were off-screen during the previous VirtualScroller layout. VirtualScroller automatically performs a layout only on page scroll or window resize; in all other cases, when layout needs to be re-run, call it manually via this instance method.

DOM

virtual-scroller/dom component implements a VirtualScroller in a standard Document Object Model environment (a web browser).

Here's an example of using virtual-scroller/dom component (it's basically the source code for the DOM demo).

import VirtualScroller from 'virtual-scroller/dom'

const messages = [
  {
    username: ...,
    date: ...,
    text: ...
  },
  ...
]

function renderMessage(message) {
  // Message element.
  const root = document.createElement('article')
  // Message author.
  const author = document.createElement('a')
  author.setAttribute('href', `/users/${message.username}`)
  author.textContent = `@${message.username}`
  root.appendChild(author)
  // Message date.
  const time = document.createElement('time')
  time.setAttribute('datetime', message.date.toISOString())
  time.textContent = message.date.toString()
  root.appendChild(time)
  // Message text.
  const text = document.createElement('p')
  text.textContent = message.text
  root.appendChild(text)
  // Return message element.
  return root
}

const virtualScroller = new VirtualScroller(
  document.getElementById('messages'),
  messages,
  renderMessage
)

// For "Single Page Apps":
// router.onPageUnload(virtualScroller.stop)

Show the list of DOM VirtualScroller arguments and options.

DOM VirtualScroller constructor takes arguments:

• container — Items list container DOM Element.
• items — The items list.
• renderItem(item) — Renders an item as a DOM Element.
• options — VirtualScroller options.

Additional options:

• onItemUnmount(itemElement) — Is called after a VirtualScroller item DOM Element is unmounted. Can be used to add DOM Element "pooling".

Show the list of DOM VirtualScroller instance methods.

• setItems(items, options) — A proxy for the corresponding VirtualScroller method.

• onItemHeightChange(i) — A proxy for the corresponding VirtualScroller method.

• onItemStateChange(i, itemState) — A proxy for the corresponding VirtualScroller method.

• stop() — A proxy for the corresponding VirtualScroller method.

React

virtual-scroller/react component implements a VirtualScroller in a React environment.

Here's an example of using virtual-scroller/react component (it's basically the source code for the React demo).

import React from 'react'
import PropTypes from 'prop-types'
import VirtualScroller from 'virtual-scroller/react'

function Messages({ messages }) {
  return (
    <VirtualScroller
      items={messages}
      itemComponent={Message}
  )
}

const message = PropTypes.shape({
  username: PropTypes.string.isRequired,
  date: PropTypes.instanceOf(Date).isRequired,
  text: PropTypes.string.isRequired
})

Messages.propTypes = {
  messages: PropTypes.arrayOf(message).isRequired
}

function Message({ children: message }) {
  const {
    username,
    date,
    text
  } = message
  return (
    <article>
      <a href={`/users/${username}`}>
        @{username}
      </a>
      <time dateTime={date.toISOString()}>
        {date.toString()}
      </time>
      <p>
        {text}
      </p>
    </article>
  )
}

Message.propTypes = {
  children: message.isRequired
}

Show the list of React <VirtualScroller/> properties.

• items — The items list.

• itemComponent — List item component. Receives the list item as the children property. Can optionally receive state and onSaveState() properties for saving list item state before unmounting it and then restoring that state after the item is re-mounted (for example, this supports "Show more" buttons, "Expand YouTube video" buttons, etc). For best performance, make sure it's a React.PureComponent or a React.memo(), otherwise it'll be re-rendering as the user scrolls.

• itemComponentProps: object — (optional) The props passed to itemComponent.

• as — A component used as a container for the list items. Is "div" by default.

• estimatedItemHeight: number — (optional) The estimatedItemHeight option of VirtualScroller class.

• bypass: boolean — (optional) The bypass option of VirtualScroller class.

• preserveScrollPositionOnPrependItems: boolean — (optional) The preserveScrollPositionOnPrependItems option of VirtualScroller.setItems() method.

• preserveScrollPositionOfTheBottomOfTheListOnMount: boolean — (optional) The preserveScrollPositionOfTheBottomOfTheListOnMount option of VirtualScroller.

• measureItemsBatchSize: number — (optional) The measureItemsBatchSize option of VirtualScroller.

• getColumnsCount(): number — (optional) The getColumnsCount() option of VirtualScroller.

• getItemId(item): any — (optional) The getItemId option of VirtualScroller class. The React component also uses it as a source for a React key for rendering an item. If getItemId() is not supplied, then item keys are autogenerated from a random-generated prefix (that changes every time items are updated) and an item index. Can be used to prevent <VirtualScroller/> from re-rendering all visible items every time items property is updated.

• onItemInitialRender(item) — (optional) The onItemInitialRender option of VirtualScroller class.

• shouldUpdateLayoutOnScreenResize(event) — (optional) The shouldUpdateLayoutOnScreenResize option of VirtualScroller class.

• initialState: object — (optional) The initial state for VirtualScroller: the state option of VirtualScroller. For example, can be used to quicky restore the list on "Back" navigation.

• initialCustomState: object — (advanced) (optional) The initial "custom" state for VirtualScroller: the customState option of VirtualScroller. It can be used to initialize the "custom" part of VirtualScroller state in cases when VirtualScroller state is used to store some "custom" list state.

• onStateChange(newState: object, prevState: object) — (optional) Is called whenever VirtualScroller state is updated (including setting the initial state). For example, can be used to keep VirtualScroller state copy in an instance variable and later in componentWillUnmount() persist it somewhere in global application state for quickly restoring it later on "Back" navigation:

import {
  getVirtualScrollerState,
  setVirtualScrollerState
} from './globalState'

class Example extends React.Component {
  componentWillUnmount() {
    saveVirtualScrollerState(this.virtualScrollerState)
  }
  render() {
    return (
      <VirtualScroller
        items={...}
        itemComponent={...}
        state={hasUserNavigatedBack ? getVirtualScrollerState() : undefined}
        onStateChange={state => this.virtualScrollerState = state}/>
    )
  }
}

Show the list of properties passed to itemComponent.

• children — The item.

• state — Item's state. Can be used for storing and then restoring the state of components which are about to be shown again after being previously hidden. For example, consider a post with an "Expand YouTube Video" button. If a user clicks the button the post is expanded and its height changes. Then the user scrolls down until the post is no longer visible and so it's no longer rendered. Then the user scrolls back up and the post is visible again and so it's rendered again. If the "Expand YouTube Video" state wasn't preserved then the list items would "jump" for the expanded minus non-expanded height difference. To avoid that, some { videoExpanded: true } state could be passed to the post component which would make it auto-expand the video on mount and so the list items wouldn't "jump". For that, onStateChange({ videoExpanded: true }) would have to be called when the user clicks the "Expand YouTube Video" button.

• onStateChange(itemState) — A proxy for VirtualScroller.onItemStateChange(i, itemState). Call this function to update item's state.

• onHeightChange() — A proxy for VirtualScroller.onItemHeightChange(i). Call this function to re-measure the item if it shrinks in height.

Show the list of React <VirtualScroller/> instance methods.

• renderItem(i) — Calls .forceUpdate() on the itemComponent instance for the item with index i. Does nothing if the item isn't currently rendered. Is only supported for itemComponents that are React.Components. The i item index argument could be replaced with the item object itself, in which case <VirtualScroller/> will find the index of the item by itself.

• updateLayout() — A proxy for the corresponding VirtualScroller method.

Rendering Engine

VirtualScroller is written in such a way that it supports any type of a rendering engine, not just DOM. For example, it could support something like React Native or <canvas/>: for that, someone would have to write custom versions of Screen.js and ScrollableContainer.js, and then instruct VirtualScroller to use those instead of the default ones by passing custom renderingEngine configuration when constructing a VirtualScroller instance:

import VirtualScroller from 'virtual-scroller'

import Screen from './Screen'
import ScrollableContainer from './ScrollableContainer'

new VirtualScroller(getContainerElement, {
  scrollableContainer,
  renderingEngine: {
    name: 'Non-DOM Rendering Engine',
    createScreen() {
      return new Screen()
    },
    createScrollableContainer(scrollableContainer) {
      return new ScrollableContainer(scrollableContainer)
    }
  },
  ...
})

getContainerElement() function would simply return a list "element", whatever that could mean. The concept of an "element" is "something, that can be rendered", so it could be anything, not just a DOM Element. Any operations with "elements" are done either in Screen.js or in ScrollableContainer.js: Screen.js defines the operations that could be applied to an "element", such as getting its height or getting its child elements' heights, and ScrollableContainer.js defines the operations that could be applied to a "scrollable container", such as getting its dimensions, listening for "resize" and "scroll" events, controlling scroll position, etc.

Dynamically Loaded Lists

All previous examples described cases with a static items list. When there's a need to update the items list dynamically, one can use virtualScroller.setItems(newItems) instance method. For example, when the user clicks "Show previous items" button, the newItems should be previousItems.concat(currentlyShownItems), and when the user clicks "Show next items" button, the newItems should be currentlyShownItems.concat(nextItems).

Find out what are "incremental" and "non-incremental" items updates, and why "incremental" updates are better.

When using virtual-scroller/dom component, a developer should call .setItems(newItems) instance method in order to update items.

When using virtual-scroller/react React component, it calls .setItems(newItems) method automatically when new items property is passed.

The basic equality check (===) is used to intelligently compare newItems to the existing items. If getItemId() parameter is passed, then items are compared by their ids rather than by themselves. If a simple append and/or prepend operation is detected, then the update is an "incremental" one, and the list seamlessly transitions from the current state to the new state, preserving its state and scroll position. If, however, the items have been updated in such a way that it's not a simple append and/or prepend operation, then such update is a "non-incremental" one, and the entire list is rerendered from scratch, losing its state and resetting the scroll position. There're valid use cases for both situations.

For example, suppose a user navigates to a page where a list of items: object[] is shown using a VirtualScroller. When a user scrolls down to the last item in the list, a developer might want to query the database for the newly added items, and then show those new items to the user. In that case, the developer could send a query to the API with afterId: number parameter being the id: number of the last item in the list, and the API would then return a list of the newItems: object[] whose id: number is greater than the afterId: number parameter. Then, the developer would append the newItems: object[] to the items: object[], and then call VirtualScroller.setItems() with the updated items: object[], resulting in a "seamless" update of the list, preserving its state and scroll position.

Another example. Suppose a user navigates to a page where they can filter a huge list by a query entered in a search bar. In that case, when the user edits the query in the search bar, VirtualScroller.setItems() method is called with a list of filtered items, and the entire list is rerendered from scratch. In this case, it's ok to reset the VirtualScroller state and the scroll position.

When new items are appended to the list, the page scroll position remains unchanged. Same's for prepending new items to the list: the scroll position of the page stays the same, resulting in the list "jumping" down when new items get prepended. To fix that, pass preserveScrollPositionOnPrependItems: true option to the VirtualScroller. When using virtual-scroller/dom component, pass that option when creating a new instance, and when using virtual-scroller/react React component, pass preserveScrollPositionOnPrependItems property.

For implementing "infinite scroll" lists, a developer could also use on-scroll-to component.

Grid Layout

To display items using a "grid" layout (i.e. multiple columns in a row), supply a getColumnsCount(container: ScrollableContainer): number parameter to VirtualScroller.

For example, to show a three-column layout on screens wider than 1280px:

function getColumnsCount(container) {
  // The `container` argument provides a `.getWidth()` method.
  if (container.getWidth() > 1280) {
    return 3
  }
  return 1
}

<VirtualScroller getColumnsCount={getColumnsCount} .../>

.container {
  display: flex;
  flex-wrap: wrap;
}

.item {
  flex-basis: 33.333333%;
  box-sizing: border-box;
}

@media screen and (max-width: 1280px) {
  .item {
    flex-basis: 100%;
  }
}

Gotchas

Images

VirtualScroller measures item heights as soon as they've rendered and later uses those measurements to determine which items should be rendered when the user scrolls. This means that things like <img/>s require special handling to prevent them from changing their size. For example, when rendering a simple <img src="..."/> first it renders an element with zero width and height and only after the image file header has been parsed does it resize itself to the actual image's width and height. When used inside VirtualScroller items such images would result in scroll position "jumping" as the user scrolls. To avoid that, any <img/>s rendered inside VirtualScroller items must either have their width and height set explicitly or have their aspect ratio set explicitly by making them position: absolute and wrapping them in a parent <div/> having position: relative and padding-bottom: ${100/aspectRatio}%.

Margin collapse

If any vertical margin is set on the list items, then this may lead to page content "jumping" by the value of that margin while scrolling. The reason is that when the top of the list is visible on screen, no padding-top gets applied to the list element, and the CSS spec states that having padding on an element disables its "margin collapse", so, while there's no padding-top on the list element, its margins do "collapse" with outer margins, but when the first item is no longer visible (and no longer rendered), padding-top gets applied to the list element to compensate for the non-rendered items, and that padding-top prevents the list's margins from "collapsing" with outer margins. So that results in the page content "jumping" when the first item in the list becomes invisible or becomes visible again. To fix that, don't set any margin-top on the first item of the list, and don't set any margin-bottom on the last item of the list. An example of fixing margin for the first and the last items of the list:

/* This margin is supposed to "collapse" with the outer ones
   but requires a fix below to work correctly with `VirtualScroller`. */
.list-item {
  margin: 10px;
}
/* Fixes margin "collapse" for the first item. */
.list-item:first-child {
  margin-top: 0;
}
/* Fixes margin "collapse" for the last item. */
.list-item:last-child {
  margin-top: 0;
}

Styling :first-child and :last-child

When styling the first and the last items of the list via :first-child and :last-child one should also check that such styles don't change the item's height which means that one should not add any border or padding styles to :first-child and :last-child otherwise the list items will jump by that extra height while scrolling.

An example of a :first-child/:last-child style that will not work correctly with VirtualScroller:

.list-item {
  border-bottom: 1px solid black;
}
.list-item:first-child {
  border-top: 1px solid black;
}

<tbody/>

Due to the inherent limitations of the <tbody/> HTML tag, when used as a container for the list items, a workaround involving CSS variables has to be used, and CSS variables aren't supported in Internet Explorer, so using a <tbody/> as a list items container won't work in Internet Explorer: in such case, VirtualScroller renders in "bypass" mode (render all items).

Search, focus management.

Due to offscreen list items not being rendered native browser features like "Find on page", moving focus through items via Tab key, screen reader announcement and such won't work. A workaround for "search on page" is adding a custom "🔍 Search" input field that would filter items by their content and then call VirtualScroller.setItems().

If only the first item is rendered on page load in dev mode.

See the description of this very rare dev mode bug.

VirtualScroller calculates the shown item indexes when its .onMount() method is called, but if the page styles are applied after VirtualScroller is mounted (for example, if styles are applied via javascript, like Webpack does it in dev mode with its style-loader) then the list might not render correctly and will only show the first item. The reason for that is because calling .getBoundingClientRect() on the list container DOM element on mount returns "incorrect" top position because the styles haven't been applied yet, and so VirtualScroller thinks it's offscreen.

For example, consider a page:

<div class="page">
  <nav class="sidebar">...</nav>
  <main>...</main>
</div>

The sidebar is styled as position: fixed, but until the page styles have been applied it's gonna be a regular <div/> meaning that <main/> will be rendered below the sidebar causing it to be offscreen and so the list will only render the first item. Then, the page styles are loaded and applied and the sidebar is now position: fixed so <main/> is now rendered at the top of the page but VirtualScroller's .render() has already been called and it won't re-render until the user scrolls or the window is resized.

This type of a bug doesn't occur in production, but it can appear in development mode when using Webpack. The workaround VirtualScroller implements for such cases is calling .getBoundingClientRect() on the list container DOM element periodically (every second) to check if the top coordinate has changed as a result of CSS being applied: if it has then it recalculates the shown item indexes and re-renders.

Debug

Set window.VirtualScrollerDebug to true to output debug messages to console.

CDN

One can use any npm CDN service, e.g. unpkg.com or jsdelivr.net

<!-- Core. -->
<script src="https://unpkg.com/virtual-scroller@1.x/bundle/virtual-scroller.js"></script>
<script>
  new VirtualScroller(...)
</script>

<!-- DOM component. -->
<script src="https://unpkg.com/virtual-scroller@1.x/bundle/virtual-scroller-dom.js"></script>
<script>
  new VirtualScroller(...)
</script>

<!-- React component. -->
<script src="https://unpkg.com/virtual-scroller@1.x/bundle/virtual-scroller-react.js"></script>
<script>
  <VirtualScroller .../>
</script>

GitHub

On March 9th, 2020, GitHub, Inc. silently banned my account (erasing all my repos, issues and comments) without any notice or explanation. Because of that, all source codes had to be promptly moved to GitLab. The GitHub repo is now only used as a backup (you can star the repo there too), and the primary repo is now the GitLab one. Issues can be reported in any repo.

License

MIT