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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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5 changes: 5 additions & 0 deletions .changeset/shaky-chicken-marry.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
---
"@solid-primitives/virtual": patch
---

expose `firstIndex` and `lastIndex` in virtual lists, to support counting
31 changes: 18 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 +15,8 @@ type VirtualListReturn<T extends readonly any[]> = [
containerHeight: number;
viewerTop: number;
visibleItems: T;
firstIndex: number;
lastIndex: number | undefined;
}>,
onScroll: (e: Event) => void,
];
Expand All @@ -41,20 +43,23 @@ 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.

lastIndex: lastIndex > 0 ? lastIndex - 1 : undefined,
// -1 because slice is an exclusive range
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@atk atk Dec 2, 2025

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Shouldn't it be items.length if it is -1?

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@lygaret lygaret Dec 2, 2025

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I'm sorry, I'm not sure what you're asking.

The -1 is an explanation of the reason we're not just passing it in straight, and it's already based on items.length, hence the whole thing ?

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@atk atk Dec 2, 2025

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Ah, I read the whole thing in a different order. My bad, I'm currently down with a flu.

};
},
e => {
// @ts-expect-error
if (e.target?.scrollTop !== undefined) setOffset(e.target.scrollTop);
Expand Down
79 changes: 79 additions & 0 deletions packages/virtual/test/index.test.tsx
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Expand Up @@ -51,6 +51,26 @@ describe("createVirtualList", () => {
expect(virtual().visibleItems).toEqual([0, 1, 2]);
});

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

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

test("returns lastIndex representing the last item in the visibleList's index", () => {
const [virtual] = createVirtualList({
items: TEST_LIST,
rootHeight: 20,
rowHeight: 10,
});

expect(virtual().lastIndex).toEqual(2);
});

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

Expand All @@ -62,35 +82,55 @@ describe("createVirtualList", () => {

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

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);
expect(virtual().lastIndex).toEqual(2);

onScroll(TARGETED_SCROLL_EVENT(el));

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

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);
expect(virtual().lastIndex).toEqual(4);

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);
expect(virtual().lastIndex).toEqual(3);

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);
expect(virtual().lastIndex).toEqual(2);

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);
expect(virtual().lastIndex).toEqual(702);
});

test("onScroll handles reaching the bottom of the list", () => {
Expand Down Expand Up @@ -126,6 +166,8 @@ describe("createVirtualList", () => {
onScroll(TARGETED_SCROLL_EVENT(el));

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

test("overscanCount defaults to 1 if undefined or zero", () => {
Expand All @@ -147,6 +189,41 @@ describe("createVirtualList", () => {
expect(virtualZero().visibleItems).toEqual([0, 1, 2]);
});

test("lastIndex is undefined in an empty list", () => {
const [virtual] = createVirtualList({
items: [],
rootHeight: 20,
rowHeight: 10,
});

expect(virtual().lastIndex).toEqual(undefined);
});

test("lastIndex is 0 in a singleton list", () => {
const [virtual] = createVirtualList({
items: [10],
rootHeight: 20,
rowHeight: 10,
});

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

test("handles singleton list", () => {
const [virtual] = createVirtualList({
items: [10],
rootHeight: 20,
rowHeight: 10,
overscanCount: 0,
});

expect(virtual().containerHeight).toEqual(10);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().visibleItems).toEqual([10]);
expect(virtual().firstIndex).toEqual(0);
expect(virtual().lastIndex).toEqual(0);
});

test("handles empty list", () => {
const [virtual] = createVirtualList({
items: [],
Expand All @@ -158,6 +235,8 @@ describe("createVirtualList", () => {
expect(virtual().containerHeight).toEqual(0);
expect(virtual().viewerTop).toEqual(0);
expect(virtual().visibleItems).toEqual([]);
expect(virtual().firstIndex).toEqual(0);
expect(virtual().lastIndex).toEqual(undefined);
});
});

Expand Down
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