check if data is serializable (#5987)

* check data is serializable

* ok we do need a separate pass after all, much simpler that way

* update test

* only test serializability in dev

* Update packages/kit/src/runtime/server/index.js

.changeset/shiny-scissors-listen.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Check that data is serializable

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

@@ -267,6 +267,7 @@ export async function respond(request, options, state) {
 									return {
 										// TODO return `uses`, so we can reuse server data effectively
 										data: await load_server_data({
+											dev: options.dev,
 											event,
 											node,
 											parent: async () => {

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

@@ -161,6 +161,7 @@ export async function render_page(event, route, options, state, resolve_opts) {
 					}
 
 					return await load_server_data({
+						dev: options.dev,
 						event,
 						node,
 						parent: async () => {

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

@@ -3,12 +3,13 @@ import { LoadURL, PrerenderingURL } from '../../../utils/url.js';
 /**
  * Calls the user's `load` function.
  * @param {{
+ *   dev: boolean;
  *   event: import('types').RequestEvent;
  *   node: import('types').SSRNode | undefined;
  *   parent: () => Promise<Record<string, any>>;
  * }} opts
  */
-export async function load_server_data({ event, node, parent }) {
+export async function load_server_data({ dev, event, node, parent }) {
 	if (!node?.server) return null;
 
 	const server_data = await node.server.load?.call(null, {
@@ -27,7 +28,13 @@ export async function load_server_data({ event, node, parent }) {
 		url: event.url
 	});
 
-	return server_data ? unwrap_promises(server_data) : null;
+	const result = server_data ? await unwrap_promises(server_data) : null;
+
+	if (dev) {
+		check_serializability(result, /** @type {string} */ (node.server_id), 'data');
+	}
+
+	return result;
 }
 
 /**
@@ -79,3 +86,42 @@ async function unwrap_promises(object) {
 
 	return unwrapped;
 }
+
+/**
+ * Check that the data can safely be serialized to JSON
+ * @param {any} value
+ * @param {string} id
+ * @param {string} path
+ */
+function check_serializability(value, id, path) {
+	const type = typeof value;
+
+	if (type === 'string' || type === 'boolean' || type === 'number' || type === 'undefined') {
+		// primitives are fine
+		return;
+	}
+
+	if (type === 'object') {
+		// nulls are fine...
+		if (!value) return;
+
+		// ...so are plain arrays...
+		if (Array.isArray(value)) {
+			value.forEach((child, i) => {
+				check_serializability(child, id, `${path}[${i}]`);
+			});
+			return;
+		}
+
+		// ...and objects
+		const tag = Object.prototype.toString.call(value);
+		if (tag === '[object Object]') {
+			for (const key in value) {
+				check_serializability(value[key], id, `${path}.${key}`);
+			}
+			return;
+		}
+	}
+
+	throw new Error(`${path} returned from 'load' in ${id} cannot be serialized as JSON`);
+}

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

@@ -35,6 +35,7 @@ export async function respond_with_error({ event, options, state, status, error,
 			const default_layout = await options.manifest._.nodes[0](); // 0 is always the root layout
 
 			const server_data_promise = load_server_data({
+				dev: options.dev,
 				event,
 				node: default_layout,
 				parent: async () => ({})

packages/kit/src/vite/dev/index.js

@@ -119,6 +119,7 @@ export async function dev(vite, vite_config, svelte_config, illegal_imports) {
 						if (node.server) {
 							const { module } = await resolve(node.server);
 							result.server = module;
+							result.server_id = node.server;
 						}
 
 						// in dev we inline all styles to avoid FOUC. this gets populated lazily so that

packages/kit/test/apps/basics/src/routes/shadowed/+page.svelte

@@ -5,6 +5,7 @@
 <a href="/shadowed/no-get">no-get</a>
 <a href="/shadowed/dynamic/foo">dynamic/foo</a>
 <a href="/shadowed/missing-get">missing-get</a>
+<a href="/shadowed/serialization">serialization</a>
 
 <form action="/shadowed/redirect-post" method="post">
 	<button type="submit" id="redirect-post">redirect</button>

packages/kit/test/apps/basics/src/routes/shadowed/serialization/+page.server.js

@@ -0,0 +1,5 @@
+export function load() {
+	return {
+		regex: /nope/
+	};
+}

packages/kit/test/apps/basics/src/routes/shadowed/serialization/+page.svelte

@@ -0,0 +1,6 @@
+<script>
+	/** @type {import('./$types').PageData} */
+	export let data;
+</script>
+
+<h1>{data.regex.test('nope')}</h1>

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

@@ -277,6 +277,18 @@ test.describe('Shadowed pages', () => {
 			'Page data: {"sub":"sub","data":{"rootlayout":"rootlayout","layout":"layout"}}'
 		);
 	});
+
+	if (process.env.DEV) {
+		test('Data must be serializable', async ({ page, clicknav }) => {
+			await page.goto('/shadowed');
+			await clicknav('[href="/shadowed/serialization"]');
+
+			expect(await page.textContent('h1')).toBe('500');
+			expect(await page.textContent('#message')).toBe(
+				'This is your custom error page saying: "data.regex returned from \'load\' in src/routes/shadowed/serialization/+page.server.js cannot be serialized as JSON"'
+			);
+		});
+	}
 });
 
 test.describe('Encoded paths', () => {

packages/kit/types/internal.d.ts

@@ -221,6 +221,9 @@ export interface SSRNode {
 		PUT?: Action;
 		DELETE?: Action;
 	};
+
+	// store this in dev so we can print serialization errors
+	server_id?: string;
 }
 
 export type SSRNodeLoader = () => Promise<SSRNode>;