feat: show vite error overlay for unhandled errors during load

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

@@ -465,7 +465,27 @@ export async function dev(vite, vite_config, svelte_config) {
 				const { set_assets } = await vite.ssrLoadModule('__sveltekit/paths');
 				set_assets(assets);
 
-				const server = new Server(manifest);
+				const server = new Server(
+					{
+						manifest
+					},
+					// @ts-expect-error internal second argument
+					/** @type {import('types').ServerDevOptions} */ ({
+						on_error: (route_id, e) => {
+							const error = coalesce_to_error(e);
+							// In a timeout, else might be sent too soon before SvelteKit reloads and therefore "deletes" the dialog
+							setTimeout(() => {
+								vite.ws.send({
+									type: 'error',
+									err: {
+										message: `Error while loading data for route ${route_id}: ${error.message}`,
+										stack: error.stack || ''
+									}
+								});
+							}, 500);
+						}
+					})
+				);
 
 				await server.init({ env });
 

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

@@ -10,11 +10,18 @@ export class Server {
 	/** @type {import('types').SSRManifest} */
 	#manifest;
 
-	/** @param {import('types').SSRManifest} manifest */
-	constructor(manifest) {
+	/** @type {import('types').ServerDevOptions | undefined} */
+	#dev;
+
+	/**
+	 * @param {import('types').SSRManifest} manifest
+	 * @param {import('types').ServerDevOptions} [dev]
+	 */
+	constructor(manifest, dev) {
 		/** @type {import('types').SSROptions} */
 		this.#options = options;
 		this.#manifest = manifest;
+		this.#dev = dev;
 	}
 
 	/**
@@ -45,6 +52,14 @@ export class Server {
 					handleError: module.handleError || (({ error }) => console.error(error?.stack)),
 					handleFetch: module.handleFetch || (({ request, fetch }) => fetch(request))
 				};
+				if (DEV && this.#dev) {
+					const handle_error = this.#options.hooks.handleError;
+					const on_error = this.#dev.on_error;
+					this.#options.hooks.handleError = (event) => {
+						on_error(event.event.route.id, event.error);
+						return handle_error(event);
+					};
+				}
 			} catch (error) {
 				if (DEV) {
 					this.#options.hooks = {

packages/kit/types/internal.d.ts

@@ -349,6 +349,10 @@ export interface SSROptions {
 	version_hash: string;
 }
 
+export interface ServerDevOptions {
+	on_error: (route_id: string | null, error: unknown) => void;
+}
+
 export interface PageNodeIndexes {
 	errors: Array<number | undefined>;
 	layouts: Array<number | undefined>;