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Allow For to accept readonly arrays #950

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Merged
JoviDeCroock merged 1 commit into from
Jul 6, 2026
Merged

Allow For to accept readonly arrays #950

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6 changes: 6 additions & 0 deletions .changeset/readonly-for-inputs.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
---
"@preact/signals": patch
"@preact/signals-react": patch
---

Allow `<For>` to accept readonly arrays and signals containing readonly arrays.
14 changes: 8 additions & 6 deletions packages/preact/utils/src/index.tsx
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Expand Up @@ -28,11 +28,13 @@ export function Show<T = boolean>(

Show.displayName = "Show";

type ForEach<T> =
| ReadonlyArray<T>

@joeyTedeschi joeyTedeschi Jul 6, 2026

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This means that we'd now accept normal arrays as a valid type as well. The const list + listValue logic already supported, but is there a concern with supporting it explicitly?

| Signal<ReadonlyArray<T>>
| ReadonlySignal<ReadonlyArray<T>>;

interface ForProps<T> {
each:
| Signal<Array<T>>
| ReadonlySignal<Array<T>>
| (() => Array<T> | Signal<Array<T>> | ReadonlySignal<Array<T>>);
each: ForEach<T> | (() => ForEach<T>);
fallback?: ComponentChildren;
getKey?: (item: T, index: number) => string | number;
children: (value: T, index: number) => ComponentChildren;
Expand All @@ -41,8 +43,8 @@ interface ForProps<T> {
export function For<T>(props: ForProps<T>): ComponentChildren | null {
const cache = useMemo(() => new Map(), []);
const list = (typeof props.each === "function" ? props.each() : props.each) as
| Signal<Array<T>>
| Array<T>;
| Signal<ReadonlyArray<T>>
| ReadonlyArray<T>;

const listValue = list instanceof Signal ? list.value : list;

Expand Down
24 changes: 24 additions & 0 deletions packages/preact/utils/test/browser/index.test.tsx
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Expand Up @@ -164,6 +164,30 @@ describe("@preact/signals-utils", () => {
);
});

it("Should accept readonly list types", () => {
const plain: readonly string[] = ["foo", "bar"];
const list = signal<readonly string[]>(["baz", "qux"]);
const computedList = computed((): readonly string[] => list.value);
const Paragraph = (p: any) => <p>{p.children}</p>;

act(() => {
render(
<div>
<For each={plain}>{item => <Paragraph>{item}</Paragraph>}</For>
<For each={list}>{item => <Paragraph>{item}</Paragraph>}</For>
<For each={() => computedList}>
{item => <Paragraph>{item}</Paragraph>}
</For>
</div>,
scratch
);
});

expect(scratch.innerHTML).to.eq(
"<div><p>foo</p><p>bar</p><p>baz</p><p>qux</p><p>baz</p><p>qux</p></div>"
);
});

it("Should pass updated indexes to reused items after removal", () => {
const alice = { id: "a", label: "Alice" };
const bob = { id: "b", label: "Bob" };
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14 changes: 8 additions & 6 deletions packages/react/utils/src/index.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -35,11 +35,13 @@ export function Show<T = boolean>(props: ShowProps<T>): JSX.Element | null {

Show.displayName = "Show";

type ForEach<T> =
| ReadonlyArray<T>
| Signal<ReadonlyArray<T>>
| ReadonlySignal<ReadonlyArray<T>>;

interface ForProps<T> {
each:
| Signal<Array<T>>
| ReadonlySignal<Array<T>>
| (() => Array<T> | Signal<Array<T>> | ReadonlySignal<Array<T>>);
each: ForEach<T> | (() => ForEach<T>);
fallback?: ReactNode;
getKey?: (item: T, index: number) => string | number;
children: (value: T, index: number) => ReactNode;
Expand All @@ -49,8 +51,8 @@ export function For<T>(props: ForProps<T>): JSX.Element | null {
useSignals();
const cache = useMemo(() => new Map(), []);
const list = (typeof props.each === "function" ? props.each() : props.each) as
| Signal<Array<T>>
| Array<T>;
| Signal<ReadonlyArray<T>>
| ReadonlyArray<T>;

const listValue = list instanceof Signal ? list.value : list;

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23 changes: 23 additions & 0 deletions packages/react/utils/test/browser/index.test.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -197,6 +197,29 @@ describe("@preact/signals-react-utils", () => {
);
});

it("Should accept readonly list types", async () => {
const plain: readonly string[] = ["foo", "bar"];
const list = signal<readonly string[]>(["baz", "qux"]);
const computedList = computed((): readonly string[] => list.value);
const Paragraph = (p: any) => <p>{p.children}</p>;

await act(() => {
render(
<div>
<For each={plain}>{item => <Paragraph>{item}</Paragraph>}</For>
<For each={list}>{item => <Paragraph>{item}</Paragraph>}</For>
<For each={() => computedList}>
{item => <Paragraph>{item}</Paragraph>}
</For>
</div>
);
});

expect(scratch.innerHTML).to.eq(
"<div><p>foo</p><p>bar</p><p>baz</p><p>qux</p><p>baz</p><p>qux</p></div>"
);
});

it("Should pass updated indexes to reused items after removal", async () => {
const alice = { id: "a", label: "Alice" };
const bob = { id: "b", label: "Bob" };
Expand Down