feat: allow error boundaries to catch errors on the server

.changeset/cyan-walls-leave.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: allow error boundaries to catch errors on the server

documentation/docs/30-advanced/25-errors.md

@@ -100,6 +100,54 @@ By default, unexpected errors are printed to the console (or, in production, you
 
 Unexpected errors will go through the [`handleError`](hooks#Shared-hooks-handleError) hook, where you can add your own error handling — for example, sending errors to a reporting service, or returning a custom error object which becomes `page.error`.
 
+## Rendering errors
+
+Ordinarily, if an error happens during server-side rendering (for example inside a component's `<script>` block or template), SvelteKit will return a 500 error page.
+
+Since SvelteKit 2.54 and Svelte 5.53, you can change this by enabling the experimental `handleRenderingErrors` option in your config:
+
+```js
+/// file: svelte.config.js
+/** @type {import('@sveltejs/kit').Config} */
+const config = {
+	kit: {
+		experimental: {
+			handleRenderingErrors: true
+		}
+	}
+};
+
+export default config;
+```
+
+When this is enabled, SvelteKit will wrap your route components in an error boundary. If an error occurs during rendering, the nearest [`+error.svelte`](routing#error) page will be shown, just as if the error had occurred in a `load` function.
+
+The error is first passed to [`handleError`](hooks#Shared-hooks-handleError), allowing you to report it and transform it, before the resulting object is passed to the `+error.svelte` component.
+
+> [!NOTE]
+> Since rendering errors occur after the page has started rendering, and multiple boundaries could in parallel catch distinct errors, the [`page`]($app-state#page) object (and its `error` property) will not be updated. Instead, the error is passed directly to the `+error.svelte` component as a prop.
+
+```svelte
+<!--- file: +error.svelte --->
+<script>
+	let { error } = $props();
+</script>
+
+<h1>{error.message}</h1>
+```
+
+The same applies for other error boundaries you define in your code:
+
+```svelte
+<svelte:boundary>
+	...
+	{#snippet failed(error: App.Error)}
+		<!-- error went through handleError and is of type App.Error -->
+		{error.message}
+	{/snippet}
+</svelte:boundary>
+```
+
 ## Responses
 
 If an error occurs inside `handle` or inside a [`+server.js`](routing#server) request handler, SvelteKit will respond with either a fallback error page or a JSON representation of the error object, depending on the request's `Accept` headers.

packages/kit/src/core/config/index.spec.js

@@ -81,7 +81,8 @@ const get_defaults = (prefix = '') => ({
 			tracing: { server: false },
 			instrumentation: { server: false },
 			remoteFunctions: false,
-			forkPreloads: false
+			forkPreloads: false,
+			handleRenderingErrors: false
 		},
 		files: {
 			src: join(prefix, 'src'),

packages/kit/src/core/config/options.js

@@ -133,7 +133,8 @@ const options = object(
 					server: boolean(false)
 				}),
 				remoteFunctions: boolean(false),
-				forkPreloads: boolean(false)
+				forkPreloads: boolean(false),
+				handleRenderingErrors: boolean(false)
 			}),
 
 			files: object({

packages/kit/src/core/sync/sync.js

@@ -33,7 +33,7 @@ export function create(config) {
 
 	write_client_manifest(config.kit, manifest_data, `${output}/client`);
 	write_server(config, output);
-	write_root(manifest_data, output);
+	write_root(manifest_data, config, output);
 	write_all_types(config, manifest_data);
 	write_non_ambient(config.kit, manifest_data);
 

packages/kit/src/core/sync/write_root.js

@@ -2,11 +2,14 @@ import { dedent, isSvelte5Plus, write_if_changed } from './utils.js';
 
 /**
  * @param {import('types').ManifestData} manifest_data
+ * @param {import('types').ValidatedConfig} config
  * @param {string} output
  */
-export function write_root(manifest_data, output) {
+export function write_root(manifest_data, config, output) {
 	// TODO remove default layout altogether
 
+	const use_boundaries = config.kit.experimental.handleRenderingErrors && isSvelte5Plus();
+
 	const max_depth = Math.max(
 		...manifest_data.routes.map((route) =>
 			route.page ? route.page.layouts.filter(Boolean).length + 1 : 0
@@ -20,17 +23,47 @@ export function write_root(manifest_data, output) {
 	}
 
 	let l = max_depth;
+	/** @type {string} */
+	let pyramid;
 
-	let pyramid = dedent`
+	if (isSvelte5Plus() && use_boundaries) {
+		// with the @const we force the data[depth] access to be derived, which is important to not fire updates needlessly
+		// TODO in Svelte 5 we should rethink the client.js side, we can likely make data a $state and only update indexes that changed there, simplifying this a lot
+		pyramid = dedent`
+			{#snippet pyramid(depth)}
+				{@const Pyramid = constructors[depth]}
+				{#snippet failed(error)}
+					{@const ErrorPage = errors[depth]}
+					<ErrorPage {error} />
+				{/snippet}
+				<svelte:boundary failed={errors[depth] ? failed : undefined}>
+					{#if constructors[depth + 1]}
+						{@const d = data[depth]}
+						<!-- svelte-ignore binding_property_non_reactive -->
+						<Pyramid bind:this={components[depth]} data={d} {form} params={page.params}>
+							{@render pyramid(depth + 1)}
+						</Pyramid>
+					{:else}
+						{@const d = data[depth]}
+						<!-- svelte-ignore binding_property_non_reactive -->
+						<Pyramid bind:this={components[depth]} data={d} {form} params={page.params} {error} />
+					{/if}
+				</svelte:boundary>
+			{/snippet}
+
+			{@render pyramid(0)}
+		`;
+	} else {
+		pyramid = dedent`
 	${
 		isSvelte5Plus()
 			? `<!-- svelte-ignore binding_property_non_reactive -->
 		<Pyramid_${l} bind:this={components[${l}]} data={data_${l}} {form} params={page.params} />`
 			: `<svelte:component this={constructors[${l}]} bind:this={components[${l}]} data={data_${l}} {form} params={page.params} />`
 	}`;
 
-	while (l--) {
-		pyramid = dedent`
+		while (l--) {
+			pyramid = dedent`
 			{#if constructors[${l + 1}]}
 				${
 					isSvelte5Plus()
@@ -57,6 +90,7 @@ export function write_root(manifest_data, output) {
 
 			{/if}
 		`;
+		}
 	}
 
 	write_if_changed(
@@ -72,9 +106,10 @@ export function write_root(manifest_data, output) {
 				${
 					isSvelte5Plus()
 						? dedent`
-							let { stores, page, constructors, components = [], form, ${levels
+							let { stores, page, constructors, components = [], form, ${use_boundaries ? 'errors = [], error, ' : ''}${levels
 								.map((l) => `data_${l} = null`)
 								.join(', ')} } = $props();
+							${use_boundaries ? `let data = $derived({${levels.map((l) => `'${l}': data_${l}`).join(', ')}})` : ''}
 						`
 						: dedent`
 							export let stores;
@@ -108,7 +143,7 @@ export function write_root(manifest_data, output) {
 					isSvelte5Plus()
 						? dedent`
 							$effect(() => {
-								stores;page;constructors;components;form;${levels.map((l) => `data_${l}`).join(';')};
+								stores;page;constructors;components;form;${use_boundaries ? 'errors;error;' : ''}${levels.map((l) => `data_${l}`).join(';')};
 								stores.page.notify();
 							});
 						`

packages/kit/src/core/sync/write_server.js

@@ -50,6 +50,7 @@ export const options = {
 	root,
 	service_worker: ${has_service_worker},
 	service_worker_options: ${config.kit.serviceWorker.register ? s(config.kit.serviceWorker.options) : 'null'},
+	server_error_boundaries: ${s(!!config.kit.experimental.handleRenderingErrors)},
 	templates: {
 		app: ({ head, body, assets, nonce, env }) => ${s(template)
 			.replace('%sveltekit.head%', '" + head + "')

packages/kit/src/exports/public.d.ts

@@ -513,6 +513,15 @@ export interface KitConfig {
 		 * @default false
 		 */
 		forkPreloads?: boolean;
+
+		/**
+		 * Whether to enable the experimental handling of rendering errors.
+		 * When enabled, `<svelte:boundary>` is used to wrap components at each level
+		 * where there's an `+error.svelte`, rendering the error page if the component fails.
+		 * In addition, error boundaries also work on the server and the error object goes through `handleError`.
+		 * @default false
+		 */
+		handleRenderingErrors?: boolean;
 	};
 	/**
 	 * Where to find various files within your project.

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

@@ -367,7 +367,8 @@ async function kit({ svelte_config }) {
 				__SVELTEKIT_PATHS_RELATIVE__: s(kit.paths.relative),
 				__SVELTEKIT_CLIENT_ROUTING__: s(kit.router.resolution === 'client'),
 				__SVELTEKIT_HASH_ROUTING__: s(kit.router.type === 'hash'),
-				__SVELTEKIT_SERVER_TRACING_ENABLED__: s(kit.experimental.tracing.server)
+				__SVELTEKIT_SERVER_TRACING_ENABLED__: s(kit.experimental.tracing.server),
+				__SVELTEKIT_EXPERIMENTAL_USE_TRANSFORM_ERROR__: s(kit.experimental.handleRenderingErrors)
 			};
 
 			if (is_build) {