📄 tanstack/virtual/v3/docs/api/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.

tsx

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


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

Required Options
---------------- ### count

tsx

count: number


count: number

The total number of items to virtualize.

### getScrollElement

tsx

getScrollElement: () => TScrollElement


getScrollElement: () => TScrollElement

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

### estimateSize

tsx

estimateSize: (index: number) => number


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
---------------- ### enabled

tsx

enabled?: boolean


enabled?: boolean

Set to false to disable scrollElement observers and reset the virtualizer's state

### debug

tsx

debug?: boolean


debug?: boolean

Set to true to enable debug logs

### initialRect

tsx

initialRect?: Rect


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

tsx

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


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

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

The sync parameter indicates whether scrolling is currently in progress. It is true when scrolling is ongoing, and false when scrolling has stopped or other actions (such as resizing) are being performed.

### overscan

tsx

overscan?: number


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. The default value is 1.

### horizontal

tsx

horizontal?: boolean


horizontal?: boolean

Set this to true if your virtualizer is oriented horizontally.

### paddingStart

tsx

paddingStart?: number


paddingStart?: number

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

### paddingEnd

tsx

paddingEnd?: number


paddingEnd?: number

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

### scrollPaddingStart

tsx

scrollPaddingStart?: number


scrollPaddingStart?: number

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

### scrollPaddingEnd

tsx

scrollPaddingEnd?: number


scrollPaddingEnd?: number

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

### initialOffset

tsx

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


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

The position where the list is scrolled to on render. This is useful if you are rendering the virtualizer in a SSR environment or are conditionally rendering the virtualizer.

### getItemKey

tsx

getItemKey?: (index: number) => Key


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

tsx

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


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

tsx

scrollToFn?: (
  offset: number,
  options: { adjustments?: number; behavior?: 'auto' | 'smooth' },
  instance: Virtualizer<TScrollElement, TItemElement>,
) => void


scrollToFn?: (
  offset: number,
  options: { adjustments?: number; behavior?: 'auto' | 'smooth' },
  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 following arguments:

An offset (in pixels) to scroll towards.
An object indicating whether there was a difference between the estimated size and actual size (adjustments) and/or whether scrolling was called with a smooth animation (behaviour).
The virtualizer instance itself.

Note that built-in scroll implementations are exported as elementScroll and windowScroll, which are automatically configured by the framework adapter functions like useVirtualizer or useWindowVirtualizer.

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

### observeElementRect

tsx

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


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

tsx

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


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

tsx

measureElement?: (
  element: TItemElement,
  entry: ResizeObserverEntry | undefined,
  instance: Virtualizer<TScrollElement, TItemElement>,
) => number


measureElement?: (
  element: TItemElement,
  entry: ResizeObserverEntry | undefined,
  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.

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

### scrollMargin

tsx

scrollMargin?: number


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. If you are using absolute positioning of elements, you should take into account the scrollMargin in your CSS transform:

tsx

transform: `translateY(${
   virtualRow.start - rowVirtualizer.options.scrollMargin
}px)` 


transform: `translateY(${
   virtualRow.start - rowVirtualizer.options.scrollMargin
}px)`

To dynamically measure value for scrollMargin you can use getBoundingClientRect() or ResizeObserver. This is helpful in scenarios when items above your virtual list might change their height.

### gap

tsx

gap?: number


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

tsx

lanes: number


lanes: number

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

### isScrollingResetDelay

tsx

isScrollingResetDelay: number


isScrollingResetDelay: number

This option allows you to specify the duration to wait after the last scroll event before resetting the isScrolling instance property. The default value is 150 milliseconds.

The implementation of this option is driven by the need for a reliable mechanism to handle scrolling behavior across different browsers. Until all browsers uniformly support the scrollEnd event.

### useScrollendEvent

tsx

useScrollendEvent: boolean


useScrollendEvent: boolean

Determines whether to use the native scrollend event to detect when scrolling has stopped. If set to false, a debounced fallback is used to reset the isScrolling instance property after isScrollingResetDelay milliseconds. The default value is false.

The implementation of this option is driven by the need for a reliable mechanism to handle scrolling behavior across different browsers. Until all browsers uniformly support the scrollEnd event.

### isRtl

tsx

isRtl: boolean


isRtl: boolean

Whether to invert horizontal scrolling to support right-to-left language locales.

### useAnimationFrameWithResizeObserver

tsx

useAnimationFrameWithResizeObserver: boolean


useAnimationFrameWithResizeObserver: boolean

This option enables wrapping ResizeObserver measurements in requestAnimationFrame for smoother updates and reduced layout thrashing. The default value is false.

It helps prevent the "ResizeObserver loop completed with undelivered notifications" error by ensuring that measurements align with the rendering cycle. This can improve performance and reduce UI jitter, especially when resizing elements dynamically. However, since ResizeObserver already runs asynchronously, adding requestAnimationFrame may introduce a slight delay in measurements, which could be noticeable in some cases. If resizing operations are lightweight and do not cause reflows, enabling this option may not provide significant benefits.

Virtualizer Instance
--------------------

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

### options

tsx

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


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

tsx

scrollElement: readonly TScrollElement | null


scrollElement: readonly TScrollElement | null

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

### getVirtualItems

tsx

type getVirtualItems = () => VirtualItem[]


type getVirtualItems = () => VirtualItem[]

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

### getVirtualIndexes

tsx

type getVirtualIndexes = () => number[]


type getVirtualIndexes = () => number[]

Returns the virtual row indexes for the current state of the virtualizer.

### scrollToOffset

tsx

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


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

tsx

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


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

tsx

getTotalSize: () => number


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

tsx

measure: () => void


measure: () => void

Resets any prev item measurements.

### measureElement

tsx

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


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

tsx

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


 <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

tsx

resizeItem: (index: number, size: number) => void


resizeItem: (index: number, size: number) => void

Change the virtualized item's size manually. Use this function to manually set the size calculated for this index. 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

tsx

scrollRect: Rect


scrollRect: Rect

Current Rect of the scroll element.

### shouldAdjustScrollPositionOnItemSizeChange

tsx

shouldAdjustScrollPositionOnItemSizeChange: undefined | ((item: VirtualItem, delta: number, instance: Virtualizer<TScrollElement, TItemElement>) => boolean)


shouldAdjustScrollPositionOnItemSizeChange: undefined | ((item: VirtualItem, delta: number, instance: Virtualizer<TScrollElement, TItemElement>) => boolean)

The shouldAdjustScrollPositionOnItemSizeChange method enables fine-grained control over the adjustment of scroll position when the size of dynamically rendered items differs from the estimated size. When jumping in the middle of the list and scrolling backward new elements may have a different size than the initially estimated size. This discrepancy can cause subsequent items to shift, potentially disrupting the user's scrolling experience, particularly when navigating backward through the list.

### isScrolling

tsx

isScrolling: boolean


isScrolling: boolean

Boolean flag indicating if list is currently being scrolled.

### scrollDirection

tsx

scrollDirection: 'forward' | 'backward' | null


scrollDirection: 'forward' | 'backward' | null

This option indicates the direction of scrolling, with possible values being 'forward' for scrolling downwards and 'backward' for scrolling upwards. The value is set to null when there is no active scrolling.

### scrollOffset

tsx

scrollOffset: number


scrollOffset: number

This option represents the current scroll position along the scrolling axis. It is measured in pixels from the starting point of the scrollable area.

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