fix: fetch doesn't respect headers from 1st arg if its type is Request (#10136)

Co-authored-by: Willow (GHOST) <[email protected]>

Author: codercms

.changeset/tricky-bikes-tan.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: Fetch doesn't respect headers from 1st arg if its type is Request

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

@@ -203,6 +203,12 @@ export function create_universal_fetch(event, state, fetched, csr, resolve_opts)
 	 */
 	return async (input, init) => {
 		const cloned_body = input instanceof Request && input.body ? input.clone().body : null;
+
+		const cloned_headers =
+			input instanceof Request && [...input.headers].length
+				? new Headers(input.headers)
+				: init?.headers;
+
 		let response = await event.fetch(input, init);
 
 		const url = new URL(input instanceof Request ? input.url : input, event.url);
@@ -260,7 +266,7 @@ export function create_universal_fetch(event, state, fetched, csr, resolve_opts)
 									? await stream_to_string(cloned_body)
 									: init?.body
 							),
-							request_headers: init?.headers,
+							request_headers: cloned_headers,
 							response_body: body,
 							response
 						});

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

@@ -264,10 +264,10 @@ test.describe('Load', () => {
 			const payload_b = '{"status":200,"statusText":"","headers":{},"body":"Y"}';
 			// by the time JS has run, hydration will have nuked these scripts
 			const script_contents_a = await page.innerHTML(
-				'script[data-sveltekit-fetched][data-url="/load/serialization-post.json"][data-hash="3t25"]'
+				'script[data-sveltekit-fetched][data-url="/load/serialization-post.json"][data-hash="1vn6nlx"]'
 			);
 			const script_contents_b = await page.innerHTML(
-				'script[data-sveltekit-fetched][data-url="/load/serialization-post.json"][data-hash="3t24"]'
+				'script[data-sveltekit-fetched][data-url="/load/serialization-post.json"][data-hash="1vn6nlw"]'
 			);
 
 			expect(script_contents_a).toBe(payload_a);