fix: clone server data before running universal load on server

.changeset/quiet-lobsters-agree.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: clone server data before passing it to universal load

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

@@ -192,7 +192,7 @@ export async function load_data({
 	const result = await node.universal.load.call(null, {
 		url: event.url,
 		params: event.params,
-		data: server_data_node?.data ?? null,
+		data: server_data_node?.data ? Object.assign({}, server_data_node.data) : null,
 		route: event.route,
 		fetch: create_universal_fetch(event, state, fetched, csr, resolve_opts),
 		setHeaders: event.setHeaders,

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

@@ -6,18 +6,6 @@ import { HttpError } from '../control.js';
 import { fix_stack_trace } from '../shared-server.js';
 import { ENDPOINT_METHODS } from '../../constants.js';
 
-/** @param {any} body */
-export function is_pojo(body) {
-	if (typeof body !== 'object') return false;
-
-	if (body) {
-		if (body instanceof Uint8Array) return false;
-		if (body instanceof ReadableStream) return false;
-	}
-
-	return true;
-}
-
 /**
  * @param {Partial<Record<import('types').HttpMethod, any>>} mod
  * @param {import('types').HttpMethod} method

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

@@ -0,0 +1,11 @@
+class CustomClass {}
+
+export function load({ data }) {
+	// @ts-ignore we want to mutate the server load data
+	data.b = new CustomClass();
+	// @ts-ignore we want to mutate the server load data
+	data.d = {
+		a: new CustomClass()
+	};
+	return data;
+}

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

@@ -0,0 +1,6 @@
+export function load() {
+	return {
+		a: 1,
+		c: new Promise((resolve) => resolve())
+	};
+}

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

@@ -0,0 +1 @@
+<p>hello world</p>

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

@@ -575,6 +575,11 @@ test.describe('Load', () => {
 
 		expect(await page.textContent('h1')).toBe('404');
 	});
+
+	test('data is not mutated by universal load', async ({ page }) => {
+		await page.goto('/load/mutated-server-data');
+		expect(await page.textContent('p')).toBe('hello world');
+	});
 });
 
 test.describe('Nested layouts', () => {