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feat(virtual-core): add startItem option #1042

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8366177
feat: add custom start
Dsaquel Jul 3, 2025
65a81e4
docs: add startItem doc
Dsaquel Aug 2, 2025
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8 changes: 8 additions & 0 deletions docs/api/virtualizer.md
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Original file line number Diff line number Diff line change
Expand Up @@ -138,6 +138,14 @@ 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.

### `startItem`

```ts
startItem?: (index: number) => number
```

This optional function is called when you need specials gaps between your items. This measurement should return the left or top of the item depending on `horizontal`. Useful when you know your layouts in advance.

### `rangeExtractor`

```tsx
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10 changes: 7 additions & 3 deletions packages/virtual-core/src/index.ts
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Expand Up @@ -337,6 +337,7 @@ export interface VirtualizerOptions<
scrollPaddingEnd?: number
initialOffset?: number | (() => number)
getItemKey?: (index: number) => Key
startItem?: ((index: number) => number) | null
rangeExtractor?: (range: Range) => Array<number>
scrollMargin?: number
gap?: number
Expand Down Expand Up @@ -431,6 +432,7 @@ export class Virtualizer<
getItemKey: defaultKeyExtractor,
rangeExtractor: defaultRangeExtractor,
onChange: () => {},
startItem: null,
measureElement,
initialRect: { width: 0, height: 0 },
scrollMargin: 0,
Expand Down Expand Up @@ -668,9 +670,11 @@ export class Virtualizer<
? measurements[i - 1]
: this.getFurthestMeasurement(measurements, i)

const start = furthestMeasurement
? furthestMeasurement.end + this.options.gap
: paddingStart + scrollMargin
const start =
this.options.startItem?.(i) ??
(furthestMeasurement
? furthestMeasurement.end + this.options.gap
: paddingStart + scrollMargin)

const measuredSize = itemSizeCache.get(key)
const size =
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