virtualizer.md

title	Virtualizer

The Virtualizer class is the core of TanStack Virtual. Virtualizer instances are usually created for you by your framework adapter, but you do receive the virtualizer directly.

export class Virtualizer<TScrollElement = unknown, TItemElement = unknown> {
  constructor(options: VirtualizerOptions<TScrollElement, TItemElement>)
}

Required Options

count

count: number

The total number of items to virtualize.

getScrollElement

getScrollElement: () => TScrollElement

A function that returns the scrollable element for the virtualizer. It may return null if the element is not available yet.

estimateSize

estimateSize: (index: number) => number

🧠 If you are dynamically measuring your elements, it's recommended to estimate the largest possible size (width/height, within comfort) of your items. This will ensure features like smooth-scrolling will have a better chance at working correctly.

This function is passed the index of each item and should return the actual size (or estimated size if you will be dynamically measuring items with virtualItem.measureElement) for each item. This measurement should return either the width or height depending on the orientation of your virtualizer.

Optional Options

debug

debug?: boolean

Set to true to enable debug logs

initialRect

initialRect?: Rect

The initial Rect of the scrollElement. This is mostly useful if you need to run the virtualizer in an SSR environment, otherwise the initialRect will be calculated on mount by the observeElementRect implementation.

onChange

onChange?: (instance: Virtualizer<TScrollElement, TItemElement>) => void

A callback function that fires when the virtualizer's internal state changes. It's passed the virtualizer instance.

overscan

overscan?: number

The number of items to render above and below the visible area. Increasing this number will increase the amount of time it takes to render the virtualizer, but might decrease the likelihood of seeing slow-rendering blank items at the top and bottom of the virtualizer when scrolling.

horizontal

horizontal?: boolean

Set this to true if your virtualizer is oriented horizontally.

paddingStart

paddingStart?: number

The padding to apply to the start of the virtualizer in pixels.

paddingEnd

paddingEnd?: number

The padding to apply to the end of the virtualizer in pixels.

scrollPaddingStart

scrollPaddingStart?: number

The padding to apply to the start of the virtualizer in pixels when scrolling to an element.

scrollPaddingEnd

scrollPaddingEnd?: number

The padding to apply to the end of the virtualizer in pixels when scrolling to an element.

initialOffset

initialOffset?: number | (() => number)

The initial offset to apply to the virtualizer. This is usually only useful if you are rendering the virtualizer in a SSR environment.

getItemKey

getItemKey?: (index: number) => Key

This function is passed the index of each item and should return a unique key for that item. The default functionality of this function is to return the index of the item, but you should override this when possible to return a unique identifier for each item across the entire set. This function should be memoized to prevent unnecessary re-renders.

rangeExtractor

rangeExtractor?: (range: Range) => number[]

This function receives visible range indexes and should return array of indexes to render. This is useful if you need to add or remove items from the virtualizer manually regardless of the visible range, eg. rendering sticky items, headers, footers, etc. The default range extractor implementation will return the visible range indexes and is exported as defaultRangeExtractor.

scrollToFn

scrollToFn?: (
  offset: number,
  canSmooth: boolean,
  instance: Virtualizer<TScrollElement, TItemElement>,
) => void

An optional function that if provided should implement the scrolling behavior for your scrollElement. It will be called with the offset to scroll to, a boolean indicating if the scrolling is allowed to be smoothed, and the virtualizer instance. Built-in scroll implementations are exported as elementScroll and windowScroll which are automatically configured for you by your framework adapter's exported functions like useVirtualizer or useWindowVirtualizer.

⚠️ Attempting to use smoothScroll with dynamically measured elements will not work.

observeElementRect

observeElementRect: (
  instance: Virtualizer<TScrollElement, TItemElement>,
  cb: (rect: Rect) => void,
) => void | (() => void)

An optional function that if provided is called when the scrollElement changes and should implement the initial measurement and continuous monitoring of the scrollElement's Rect (an object with width and height). It's called with the instance (which also gives you access to the scrollElement via instance.scrollElement. Built-in implementations are exported as observeElementRect and observeWindowRect which are automatically configured for you by your framework adapter's exported functions like useVirtualizer or useWindowVirtualizer.

observeElementOffset

observeElementOffset: (
    instance: Virtualizer<TScrollElement, TItemElement>,
    cb: (offset: number) => void,
  ) => void | (() => void)

An optional function that if provided is called when the scrollElement changes and should implement the initial measurement and continuous monitoring of the scrollElement's scroll offset (a number). It's called with the instance (which also gives you access to the scrollElement via instance.scrollElement. Built-in implementations are exported as observeElementOffset and observeWindowOffset which are automatically configured for you by your framework adapter's exported functions like useVirtualizer or useWindowVirtualizer.

measureElement

measureElement?: (
  el: TItemElement,
  instance: Virtualizer<TScrollElement, TItemElement>
) => number

This optional function is called when the virtualizer needs to dynamically measure the size (width or height) of an item when virtualItem.measureElement is called. It's passed the element given when you call virtualItem.measureElement(TItemElement) and the virtualizer instance. It should return the size of the element as a number.

🧠 You can use instance.options.horizontal to determine if the width or height of the item should be measured.

scrollMargin

scrollMargin?: number

With this option, you can specify where the scroll offset should originate. Typically, this value represents the space between the beginning of the scrolling element and the start of the list. This is especially useful in common scenarios such as when you have a header preceding a window virtualizer or when multiple virtualizers are utilized within a single scrolling element.

gap

gap?: number

This option allows you to set the spacing between items in the virtualized list. It's particularly useful for maintaining a consistent visual separation between items without having to manually adjust each item's margin or padding. The value is specified in pixels.

lanes

lanes: number

The number of lanes the list is divided into (aka columns for vertical lists and rows for horizontal lists).

Virtualizer Instance

The following properties and methods are available on the virtualizer instance:

options

options: readonly Required<VirtualizerOptions<TScrollElement, TItemElement>>

The current options for the virtualizer. This property is updated via your framework adapter and is read-only.

scrollElement

scrollElement: readonly TScrollElement | null

The current scrollElement for the virtualizer. This property is updated via your framework adapter and is read-only.

getVirtualItems

type getVirtualItems = () => VirtualItem[]

Returns the virtual items for the current state of the virtualizer.

scrollToOffset

scrollToOffset: (
  toOffset: number,
  options?: {
    align?: 'start' | 'center' | 'end' | 'auto',
    smoothScroll?: boolean
  }
) => void

Scrolls the virtualizer to the pixel offset provided. You can optionally pass an alignment mode to anchor the scroll to a specific part of the scrollElement.

scrollToIndex

scrollToIndex: (
  index: number,
  options?: {
    align?: 'start' | 'center' | 'end' | 'auto',
    smoothScroll?: boolean
  }
) => void

Scrolls the virtualizer to the items of the index provided. You can optionally pass an alignment mode to anchor the scroll to a specific part of the scrollElement.

getTotalSize

getTotalSize: () => number

Returns the total size in pixels for the virtualized items. This measurement will incrementally change if you choose to dynamically measure your elements as they are rendered.

measure

measure: () => void

Resets any prev item measurements.

measureElement

measureElement: (el: TItemElement | null) => void

Measures the element using your configured measureElement virtualizer option. You are responsible for calling this in your virtualizer markup when the component is rendered (eg. using something like React's ref callback prop) also adding data-index

 <div
  key={virtualRow.key}
  data-index={virtualRow.index}
  ref={virtualizer.measureElement}
  style={...}
>...</div>

By default the measureElement virtualizer option is configured to measure elements with getBoundingClientRect().

resizeItem

resizeItem: (item: VirtualItem, size: number) => void

Change the virtualized item's size manually. Use this function to manually set the size calculated for this item. Useful in occations when using some custom morphing transition and you know the morphed item's size beforehand.

You can also use this method with a throttled ResizeObserver instead of Virtualizer.measureElement to reduce re-rendering.

⚠️ Please be aware that manually changing the size of an item when using Virtualizer.measureElement to monitor that item, will result in unpredictable behaviour as the Virtualizer.measureElement is also changing the size. However you can use one of resizeItem or measureElement in the same virtualizer instance but on different item indexes.

scrollRect

scrollRect: Rect

Current Rect of the scroll element.