fix: add error result type to preloadData

.changeset/modern-dodos-hang.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': major
+---
+
+feat: add error result type to `preloadData`

packages/kit/src/runtime/client/client.js

@@ -989,10 +989,14 @@ function diff_search_params(old_url, new_url) {
 }
 
 /**
- * @param {Omit<import('./types.js').NavigationFinished['state'], 'branch'> & { error: App.Error }} opts
+ * @param {Omit<import('./types.js').NavigationFinished['state'], 'branch'> & { error: App.Error; status: number }} opts
  * @returns {import('./types.js').NavigationFinished}
  */
-function preload_error({ error, url, route, params }) {
+function preload_error({ error, status, url, route, params }) {
+	// we skipped loading the error page, so we need to use the current page
+	// store, but we still pass the updated status to the preloadData function
+	const new_page = clone_page(page);
+	new_page.status = status;
 	return {
 		type: 'loaded',
 		state: {
@@ -1003,7 +1007,7 @@ function preload_error({ error, url, route, params }) {
 			branch: []
 		},
 		props: {
-			page: clone_page(page),
+			page: new_page,
 			constructors: []
 		}
 	};
@@ -1068,12 +1072,14 @@ async function load_route({ id, invalidating, url, params, route, preload }) {
 			} catch (error) {
 				const handled_error = await handle_error(error, { url, params, route: { id } });
 
+				const status = get_status(error);
+
 				if (preload_tokens.has(preload)) {
-					return preload_error({ error: handled_error, url, params, route });
+					return preload_error({ error: handled_error, status, url, params, route });
 				}
 
 				return load_root_error_page({
-					status: get_status(error),
+					status,
 					error: handled_error,
 					url,
 					route
@@ -1154,20 +1160,23 @@ async function load_route({ id, invalidating, url, params, route, preload }) {
 				if (err instanceof Redirect) {
 					return {
 						type: 'redirect',
+						status: err.status,
 						location: err.location
 					};
 				}
 
+				let status = get_status(err);
+
 				if (preload_tokens.has(preload)) {
 					return preload_error({
 						error: await handle_error(err, { params, url, route: { id: route.id } }),
+						status,
 						url,
 						params,
 						route
 					});
 				}
 
-				let status = get_status(err);
 				/** @type {App.Error} */
 				let error;
 
@@ -2185,7 +2194,7 @@ export function refreshAll({ includeLoadFunctions = true } = {}) {
  * Returns a Promise that resolves with the result of running the new route's `load` functions once the preload is complete.
  *
  * @param {string} href Page to preload
- * @returns {Promise<{ type: 'loaded'; status: number; data: Record<string, any> } | { type: 'redirect'; location: string }>}
+ * @returns {Promise<({ type: 'loaded'; data: Record<string, any> } | { type: 'redirect'; location: string } | { type: 'error'; error: App.Error }) & { status: number; }>}
  */
 export async function preloadData(href) {
 	if (!BROWSER) {
@@ -2203,11 +2212,21 @@ export async function preloadData(href) {
 	if (result.type === 'redirect') {
 		return {
 			type: result.type,
+			status: result.status,
 			location: result.location
 		};
 	}
 
 	const { status, data } = result.props.page ?? page;
+
+	if (result.type === 'loaded' && result.state.error) {
+		return {
+			type: 'error',
+			status,
+			error: result.state.error
+		};
+	}
+
 	return { type: result.type, status, data };
 }
 

packages/kit/src/runtime/client/types.d.ts

@@ -77,6 +77,7 @@ export type NavigationResult = NavigationRedirect | NavigationFinished;
 
 export type NavigationRedirect = {
 	type: 'redirect';
+	status: number;
 	location: string;
 };
 

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

@@ -182,6 +182,7 @@ export function redirect_json_response(redirect) {
 	return json_response(
 		/** @type {import('types').ServerRedirectNode} */ ({
 			type: 'redirect',
+			status: redirect.status,
 			location: redirect.location
 		})
 	);

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

@@ -258,6 +258,7 @@ export async function render_page(
 						if (state.prerendering && should_prerender_data) {
 							const body = JSON.stringify({
 								type: 'redirect',
+								status: err.status,
 								location: err.location
 							});
 

packages/kit/src/types/internal.d.ts

@@ -283,6 +283,7 @@ export interface RouteData {
 
 export type ServerRedirectNode = {
 	type: 'redirect';
+	status: number;
 	location: string;
 };
 

packages/kit/test/apps/basics/src/routes/routing/preloading/error/+page.svelte

@@ -0,0 +1,21 @@
+<script>
+	import { preloadData } from '$app/navigation';
+
+	let data = {};
+</script>
+
+<button
+	type="button"
+	on:click={async () => {
+		data = await preloadData('/routing/preloading/error/404');
+	}}>404</button
+>
+
+<button
+	type="button"
+	on:click={async () => {
+		data = await preloadData('/routing/preloading/error/500');
+	}}>500</button
+>
+
+<p>{data.type} {data.status} {data.error?.message}</p>

packages/kit/test/apps/basics/src/routes/routing/preloading/error/[id]/+page.js

@@ -0,0 +1,11 @@
+import { error } from '@sveltejs/kit';
+
+export function load({ params }) {
+	if (params.id === '404') {
+		error(404, { message: 'Not found' });
+	}
+
+	if (params.id === '500') {
+		throw new Error('Oopsie');
+	}
+}

packages/kit/test/apps/basics/test/cross-platform/client.test.js

@@ -766,6 +766,18 @@ test.describe('Prefetching', () => {
 		await app.goto('/routing/preloading/preload-error');
 		await expect(page.locator('p')).toHaveText('hello');
 	});
+
+	test('preloads errors', async ({ page }) => {
+		await page.goto('/routing/preloading/error');
+
+		await expect(page.locator('p')).toHaveText('undefined undefined undefined');
+
+		page.locator('button', { hasText: '404' }).click();
+		await expect(page.locator('p')).toHaveText('error 404 Not found');
+
+		page.locator('button', { hasText: '500' }).click();
+		await expect(page.locator('p')).toHaveText('error 500 Oopsie (500 Internal Error)');
+	});
 });
 
 test.describe('Routing', () => {

packages/kit/test/prerendering/basics/test/tests.spec.js

@@ -48,6 +48,7 @@ test('renders a server-side redirect', () => {
 
 	expect(data).toEqual({
 		type: 'redirect',
+		status: 301,
 		location: 'https://example.com/redirected'
 	});
 });

packages/kit/types/index.d.ts

@@ -3056,13 +3056,17 @@ declare module '$app/navigation' {
 	 *
 	 * @param href Page to preload
 	 * */
-	export function preloadData(href: string): Promise<{
+	export function preloadData(href: string): Promise<({
 		type: "loaded";
-		status: number;
 		data: Record<string, any>;
 	} | {
 		type: "redirect";
 		location: string;
+	} | {
+		type: "error";
+		error: App.Error;
+	}) & {
+		status: number;
 	}>;
 	/**
 	 * Programmatically imports the code for routes that haven't yet been fetched.