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expose firstIndex in virtual scroller #823

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expose firstIndex in virtual scroller #823

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270c238
expose the current indices in virtual scroller
lygaret Dec 1, 2025
d694315
remove lastIndex from return, update tests
lygaret Dec 1, 2025
bfaf8ff
add lastIndex to the returned data, undefined for empty lists
lygaret Dec 1, 2025
16727c1
add changeset
lygaret Dec 3, 2025
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28 changes: 15 additions & 13 deletions packages/virtual/src/index.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -15,6 +15,7 @@ type VirtualListReturn<T extends readonly any[]> = [
containerHeight: number;
viewerTop: number;
visibleItems: T;
firstIndex: number;
}>,
onScroll: (e: Event) => void,
];
Expand All @@ -41,20 +42,21 @@ export function createVirtualList<T extends readonly any[]>({

const [offset, setOffset] = createSignal(0);

const getFirstIdx = () => Math.max(0, Math.floor(offset() / rowHeight) - overscanCount);

const getLastIdx = () =>
Math.min(
items.length,
Math.floor(offset() / rowHeight) + Math.ceil(rootHeight / rowHeight) + overscanCount,
);

return [
() => ({
containerHeight: items.length * rowHeight,
viewerTop: getFirstIdx() * rowHeight,
visibleItems: items.slice(getFirstIdx(), getLastIdx()) as unknown as T,
}),
() => {
const firstIndex = Math.max(0, Math.floor(offset() / rowHeight) - overscanCount);
const lastIndex = Math.min(
items.length,
Math.floor(offset() / rowHeight) + Math.ceil(rootHeight / rowHeight) + overscanCount,
);

return {
containerHeight: items.length * rowHeight,
viewerTop: firstIndex * rowHeight,
visibleItems: items.slice(firstIndex, lastIndex) as unknown as T,
firstIndex,
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@atk atk Dec 1, 2025

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while you're at it, you can also expose lastIndex.

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@lygaret lygaret Dec 1, 2025

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Sure, I can. I did originally, but removed it because its a little weird semantically: when a list is empty, should it return -1, or undefined?

firstIndex being zero makes sense, kinda, but I didn't know what to do with last's typing, so I skipped it.

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@lygaret lygaret Dec 1, 2025

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Right now, it's more like nextIndex than last because it's an exclusive range.

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@atk atk Dec 1, 2025

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That's fine, too.

};
},
e => {
// @ts-expect-error
if (e.target?.scrollTop !== undefined) setOffset(e.target.scrollTop);
Expand Down
24 changes: 24 additions & 0 deletions packages/virtual/test/index.test.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,16 @@ describe("createVirtualList", () => {
expect(virtual().visibleItems).toEqual([0, 1, 2]);
});

test("returns firstIndex representing the index of the visibleList", () => {
const [virtual] = createVirtualList({
items: TEST_LIST,
rootHeight: 20,
rowHeight: 10,
});

expect(virtual().firstIndex).toEqual(0);
});

test("returns onScroll which sets viewerTop and visibleItems based on rootElement's scrolltop", () => {
const el = document.createElement("div");

Expand All @@ -62,35 +72,48 @@ describe("createVirtualList", () => {

expect(virtual().visibleItems).toEqual([0, 1, 2]);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().firstIndex).toEqual(0);

el.scrollTop += 10;

// no change until onScroll is called
expect(virtual().visibleItems).toEqual([0, 1, 2]);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().firstIndex).toEqual(0);

onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([0, 1, 2, 3]);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().firstIndex).toEqual(0);

el.scrollTop += 10;
onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([1, 2, 3, 4]);
expect(virtual().viewerTop).toEqual(10);
expect(virtual().firstIndex).toEqual(1);

el.scrollTop -= 10;
onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([0, 1, 2, 3]);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().firstIndex).toEqual(0);

el.scrollTop -= 10;
onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([0, 1, 2]);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().firstIndex).toEqual(0);

el.scrollTop += 7_000;
onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([699, 700, 701, 702]);
expect(virtual().viewerTop).toEqual(6990);
expect(virtual().firstIndex).toEqual(699);
});

test("onScroll handles reaching the bottom of the list", () => {
Expand Down Expand Up @@ -126,6 +149,7 @@ describe("createVirtualList", () => {
onScroll(TARGETED_SCROLL_EVENT(el));

expect(virtual().visibleItems).toEqual([8, 9, 10, 11, 12, 13]);
expect(virtual().firstIndex).toEqual(8);
});

test("overscanCount defaults to 1 if undefined or zero", () => {
Expand Down