[fix] prevent caching of __data.js files (#6904)

Closes #6458

.changeset/cyan-years-live.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Prevent caching of `__data.js` files

packages/kit/src/runtime/server/utils.js

@@ -70,21 +70,18 @@ export function allowed_methods(mod) {
 
 /** @param {any} data */
 export function data_response(data) {
+	const headers = {
+		'content-type': 'application/javascript',
+		'cache-control': 'private, no-store'
+	};
+
 	try {
-		return new Response(`window.__sveltekit_data = ${devalue(data)}`, {
-			headers: {
-				'content-type': 'application/javascript'
-			}
-		});
+		return new Response(`window.__sveltekit_data = ${devalue(data)}`, { headers });
 	} catch (e) {
 		const error = /** @type {any} */ (e);
 		const match = /\[(\d+)\]\.data\.(.+)/.exec(error.path);
 		const message = match ? `${error.message} (data.${match[2]})` : error.message;
-		return new Response(`throw new Error(${JSON.stringify(message)})`, {
-			headers: {
-				'content-type': 'application/javascript'
-			}
-		});
+		return new Response(`throw new Error(${JSON.stringify(message)})`, { headers });
 	}
 }
 

packages/kit/test/apps/basics/src/routes/load/server-data-nostore/+page.server.js

@@ -0,0 +1,6 @@
+/** @type {import('./$types').PageServerLoad} */
+export function load({ url }) {
+	return {
+		x: url.searchParams.get('x')
+	};
+}

packages/kit/test/apps/basics/src/routes/load/server-data-nostore/+page.svelte

@@ -0,0 +1,7 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<h1>x: {data.x}</h1>
+<a href="/load/server-data-nostore?x=2">2</a>

packages/kit/test/apps/basics/test/client.test.js

@@ -431,6 +431,17 @@ test.describe('Load', () => {
 		await expect(page.locator('p')).toHaveText('Count is 2');
 	});
 
+	test('__data.js has cache-control: private, no-store', async ({ page, clicknav }) => {
+		await page.goto('/load/server-data-nostore?x=1');
+
+		const [response] = await Promise.all([
+			page.waitForResponse((response) => /__data\.js/.test(response.url())),
+			clicknav('[href="/load/server-data-nostore?x=2"]')
+		]);
+
+		expect(response.headers()['cache-control']).toBe('private, no-store');
+	});
+
 	if (process.env.DEV) {
 		test('using window.fetch causes a warning', async ({ page }) => {
 			const port = 5173;