fix(core): `isFallback` type is `true` instead of `boolean` (#3030)

4 files changed, 10 insertions, 6 deletions

diff --git a/.changeset/brown-bottles-stare.md b/.changeset/brown-bottles-stare.md
new file mode 100644
index 00000000..8ad54d1e
--- /dev/null
+++ b/.changeset/brown-bottles-stare.md
@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Updates the type of the `isFallback` field in route data from `true` to `boolean`, keeping it optional but allowing `false` as a possible value.
diff --git a/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts b/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
index 972dbfe4..416eb735 100644
--- a/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
+++ b/packages/starlight/__tests__/basics/starlight-page-route-data.test.ts
@@ -64,11 +64,13 @@ test('adds custom data to route shape', async () => {
 		hasSidebar: false,
 		dir: 'rtl',
 		lang: 'ks',
+		isFallback: true,
 	};
 	const data = await generateStarlightPageRouteData({ props, url: starlightPageUrl });
 	expect(data.hasSidebar).toBe(props.hasSidebar);
 	expect(data.entryMeta.dir).toBe(props.dir);
 	expect(data.entryMeta.lang).toBe(props.lang);
+	expect(data.isFallback).toBe(props.isFallback);
 });
 
 test('adds custom frontmatter data to route shape', async () => {
diff --git a/packages/starlight/utils/routing/types.ts b/packages/starlight/utils/routing/types.ts
index 1f352dd3..13e69188 100644
--- a/packages/starlight/utils/routing/types.ts
+++ b/packages/starlight/utils/routing/types.ts
@@ -67,8 +67,8 @@ export interface Route extends LocaleData {
 	slug: string;
 	/** The slug or unique ID if using the `legacy.collections` flag. */
 	id: string;
-	/** True if this page is untranslated in the current language and using fallback content from the default locale. */
-	isFallback?: true;
+	/** Whether this page is untranslated in the current language and using fallback content from the default locale. */
+	isFallback?: boolean;
 	[key: string]: unknown;
 }
 
diff --git a/packages/starlight/utils/starlight-page.ts b/packages/starlight/utils/starlight-page.ts
index 97a3d7d9..48f9ad33 100644
--- a/packages/starlight/utils/starlight-page.ts
+++ b/packages/starlight/utils/starlight-page.ts
@@ -105,7 +105,7 @@ export async function generateStarlightPageRouteData({
 	props: StarlightPageProps;
 	url: URL;
 }): Promise<StarlightRouteData> {
-	const { isFallback, frontmatter, ...routeProps } = props;
+	const { frontmatter, ...routeProps } = props;
 	const slug = urlToSlug(url);
 	const pageFrontmatter = await getStarlightPageFrontmatter(frontmatter);
 	const id = project.legacyCollections ? `${stripLeadingAndTrailingSlashes(slug)}.md` : slug;
@@ -163,9 +163,6 @@ export async function generateStarlightPageRouteData({
 			slug,
 		}),
 	};
-	if (isFallback) {
-		routeData.isFallback = true;
-	}
 	return routeData;
 }