fix: prerender data when there's no server load but trailingSlash o…

…ption is set from the server (#13262)

fixes #13255

trailingSlash is evaluated on the client by making a server call, so we need to prerender it

.changeset/gold-tips-cover.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: prerender data when there is no server load but the `trailingSlash` option is set from the server

packages/kit/src/runtime/server/page/index.js

@@ -70,7 +70,10 @@ export async function render_page(event, page, options, manifest, state, resolve
 			}
 		}
 
-		const should_prerender_data = nodes.some((node) => node?.server?.load);
+		const should_prerender_data = nodes.some(
+			// prerender in case of trailingSlash because the client retrieves that value from the server
+			(node) => node?.server?.load || node?.server?.trailingSlash !== undefined
+		);
 		const data_pathname = add_data_suffix(event.url.pathname);
 
 		// it's crucial that we do this before returning the non-SSR response, otherwise

packages/kit/test/apps/options-2/src/routes/trailing-slash-server/+page.svelte

@@ -0,0 +1,5 @@
+<script>
+	import { base } from '$app/paths';
+</script>
+
+<a href="{base}/trailing-slash-server/prerender">{base}/trailing-slash-server/prerender</a>

packages/kit/test/apps/options-2/src/routes/trailing-slash-server/prerender/+page.server.js

@@ -0,0 +1,2 @@
+export const trailingSlash = 'always';
+export const prerender = true;

packages/kit/test/apps/options-2/src/routes/trailing-slash-server/prerender/+page.svelte

@@ -0,0 +1,5 @@
+<script>
+	import { page } from '$app/state';
+</script>
+
+<h2>{page.url.pathname}</h2>

packages/kit/test/apps/options-2/test/test.js

@@ -55,6 +55,23 @@ test.describe('paths', () => {
 	});
 });
 
+test.describe('trailing slash', () => {
+	if (!process.env.DEV) {
+		test('trailing slash server prerendered without server load', async ({
+			page,
+			clicknav,
+			javaScriptEnabled
+		}) => {
+			if (!javaScriptEnabled) return;
+
+			await page.goto('/basepath/trailing-slash-server');
+
+			await clicknav('a[href="/basepath/trailing-slash-server/prerender"]');
+			expect(await page.textContent('h2')).toBe('/basepath/trailing-slash-server/prerender/');
+		});
+	}
+});
+
 test.describe('Service worker', () => {
 	if (process.env.DEV) return;