Advanced

Errors

Errors are an inevitable fact of software development. SvelteKit handles errors differently depending on where they occur, what kind of errors they are, and the nature of the incoming request.

Error objectspermalink

SvelteKit distinguishes between expected and unexpected errors, both of which are represented as simple { message: string } objects by default.

You can add additional properties, like a code or a tracking id, as shown in the examples below. (When using TypeScript this requires you to redefine the Error type as described in type safety).

Expected errorspermalink

An expected error is one created with the error helper imported from @sveltejs/kit:

src/routes/blog/[slug]/+page.server.js

ts
import { error } from '@sveltejs/kit';
import * as db from '$lib/server/database';

/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
	const post = await db.getPost(params.slug);

	if (!post) {
		error(404, {
			message: 'Not found'
		});
	}

	return { post };
}

src/routes/blog/[slug]/+page.server.ts

ts
import { error } from '@sveltejs/kit';
import * as db from '$lib/server/database';
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ params }) => {
	const post = await db.getPost(params.slug);

	if (!post) {
		error(404, {
			message: 'Not found',
		});
	}

	return { post };
};

This tells SvelteKit to set the response status code to 404 and render an +error.svelte component, where $page.error is the object provided as the second argument to error(...).

src/routes/+error.svelte

<script>
	import { page } from '$app/stores';
</script>

<h1>{$page.error.message}</h1>

src/routes/+error.svelte

<script lang="ts">
	import { page } from '$app/stores';
</script>

<h1>{$page.error.message}</h1>

You can add extra properties to the error object if needed...

error(404, {
	message: 'Not found',
	code: 'NOT_FOUND'
});

...otherwise, for convenience, you can pass a string as the second argument:

error(404, { message: 'Not found' });
error(404, 'Not found');

Unexpected errorspermalink

An unexpected error is any other exception that occurs while handling a request. Since these can contain sensitive information, unexpected error messages and stack traces are not exposed to users.

By default, unexpected errors are printed to the console (or, in production, your server logs), while the error that is exposed to the user has a generic shape:

ts
{ "message": "Internal Error" }

Unexpected errors will go through the handleError hook, where you can add your own error handling — for example, sending errors to a reporting service, or returning a custom error object.

src/hooks.server.js

ts
import * as Sentry from '@sentry/sveltekit';

Sentry.init({/*...*/})

/** @type {import('@sveltejs/kit').HandleServerError} */
Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.
Property 'code' does not exist on type '{}'.2353
2339Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.
Property 'code' does not exist on type '{}'.export function handleError({ error, event }) {
	// example integration with https://sentry.io/
	Sentry.captureException(error, { extra: { event } });

	return {
		message: 'Whoops!',
		code: error?.code ?? 'UNKNOWN'
	};
}

src/hooks.server.ts

ts
import * as Sentry from '@sentry/sveltekit';
import type { HandleServerError } from '@sveltejs/kit';

Sentry.init({
	/*...*/
});

Property 'code' does not exist on type '{}'.2339Property 'code' does not exist on type '{}'.export const handleError: HandleServerError = ({ error, event }) => {
	// example integration with https://sentry.io/
	Sentry.captureException(error, { extra: { event } });

	return {
		message: 'Whoops!',
		code: error?.code ?? 'UNKNOWN',
	};
};

SvelteKit may handle certain errors as non-fatal. In these cases, it throws a NonFatalError which contains a non-sensitive message and the status code. Examples for this are 404s (page not found) or 415s for actions (wrong content-type). These go through handleError as well, and you can distinguish them from other unexpected errors via an instanceof check.

src/hooks.server.js

ts
import * as Sentry from '@sentry/sveltekit';
import { NonFatalError } from '@sveltejs/kit';

Sentry.init({/*...*/})
Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.2353Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.
/** @type {import('@sveltejs/kit').HandleServerError} */
export function handleError({ error, event }) {
	if (error instanceof NonFatalError) {
		return {
			message: error.message, // safe to forward
			code: 'UNKNOWN'
		};
Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.
Property 'code' does not exist on type '{}'.2353
2339Object literal may only specify known properties, and 'code' does not exist in type 'Error | Promise<void | Error>'.
Property 'code' does not exist on type '{}'.	} else {
		// example integration with https://sentry.io/
		Sentry.captureException(error, { extra: { event } });

		return {
			message: 'Whoops!',
			code: error?.code ?? 'UNKNOWN'
		};
	}
}

src/hooks.server.ts

ts
import * as Sentry from '@sentry/sveltekit';
import { NonFatalError } from '@sveltejs/kit';
import type { HandleServerError } from '@sveltejs/kit';

Sentry.init({
	/*...*/
});

export const handleError: HandleServerError = ({ error, event }) => {
	if (error instanceof NonFatalError) {
		return {
			message: error.message, // safe to forward
			code: 'UNKNOWN',
		};
Property 'code' does not exist on type '{}'.2339Property 'code' does not exist on type '{}'.	} else {
		// example integration with https://sentry.io/
		Sentry.captureException(error, { extra: { event } });

		return {
			message: 'Whoops!',
			code: error?.code ?? 'UNKNOWN',
		};
	}
};

Make sure that handleError never throws an error

Responsespermalink

If an error occurs inside handle or inside a +server.js 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.

You can customise the fallback error page by adding a src/error.html file:

<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="utf-8" />
		<title>%sveltekit.error.message%</title>
	</head>
	<body>
		<h1>My custom error page</h1>
		<p>Status: %sveltekit.status%</p>
		<p>Message: %sveltekit.error.message%</p>
	</body>
</html>

SvelteKit will replace %sveltekit.status% and %sveltekit.error.message% with their corresponding values.

If the error instead occurs inside a load function while rendering a page, SvelteKit will render the +error.svelte component nearest to where the error occurred. If the error occurs inside a load function in +layout(.server).js, the closest error boundary in the tree is an +error.svelte file above that layout (not next to it).

The exception is when the error occurs inside the root +layout.js or +layout.server.js, since the root layout would ordinarily contain the +error.svelte component. In this case, SvelteKit uses the fallback error page.

Type safetypermalink

If you're using TypeScript and need to customize the shape of errors, you can do so by declaring an App.Error interface in your app (by convention, in src/app.d.ts, though it can live anywhere that TypeScript can 'see'):

src/app.d.ts

declare global {
	namespace App {
		interface Error {
			code: string;
			id: string;
		}
	}
}

export {};

This interface always includes a message: string property.