Skip to main content
SvelteKitAdvanced

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 objects

Every error passes through the handleError hook — which can log it and customise it — before it is rendered. The hook’s kind property identifies where the error came from: your app ('app'), SvelteKit ('framework'), validation of a remote function argument ('validation') or an unknown source ('unknown'). By default, all are represented as simple { status: number, message: string } objects.

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 below).

App errors

An app error is one thrown from your app code using the error function imported from @sveltejs/kit:

src/routes/blog/[slug]/+page.server
import { error } from '@sveltejs/kit';
import * as db from '#lib/server/database.js';
/** @type {import('./$types').PageServerLoad} */export async function load({ params }) {
	const post = await db.getPost(params.slug);

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

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

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

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

	return { post };
};

This throws an exception that SvelteKit catches, causing it to set the response status code to 404 and render an +error.svelte component, where the error is an App.Error object with the provided status and message.

On its way there, the error passes through the handleError hook with kind: 'app'. Since the shape of the error is determined by your app, it is considered safe to expose, and the hook can pass it through unchanged.

src/routes/+error
<script>
	let { error } = $props();
</script>

<h1>{error.message}</h1>
<script lang="ts">
	let { error } = $props();
</script>

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

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

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

In SvelteKit 1.x you had to throw the error yourself

Framework errors

Some errors are generated by SvelteKit itself rather than by your code — a request that doesn’t match any route (404), a POST request to a page without actions (405), a request body that exceeds the size limit (413), and so on.

These also go through handleError, with kind: 'framework'. The error you receive is a { status, message } object whose message is a terse but safe description of what went wrong, such as 'Not Found', so it can be exposed to users as-is.

If you log errors inside handleError, remember that framework errors such as 404s are routine — you will generally want to avoid logging them.

Validation errors

Validation errors occur when a remote function is called with invalid data. When these are passed to handleError, they are accompanied by an array of issues. See Handling validation errors for more details.

Unknown errors

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

By default, unknown 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:

{ "status": 500, "message": "Internal Error" }

Unknown errors go through the handleError hook with kind: 'unknown', because SvelteKit does not know what went wrong. There you can add your own error handling, for example sending errors to a reporting service, or returning a custom error object which becomes the error prop passed to +error.svelte. The value you receive is the raw thrown value, and nothing about it is exposed unless you choose to expose it.

Anything you return overrides the defaults, so you can — for example — use the type of the thrown error to determine the HTTP status code used in the response:

src/hooks.server
// Assuming you have this ...
class NotFound extends Error {}
/** @type {import('@sveltejs/kit/hooks').HandleServerError} */export function handleError({ kind, error, event }) {
	if (kind === 'unknown') {
		// ... you can do this		if (error instanceof NotFound) {
			return {
				status: 404,
				message: 'Not found'
			};
		}

		return { message: 'Something went wrong' };
	}
	// app and framework errors are already safe to expose	return error;
}
import type { HandleServerError } from '@sveltejs/kit/hooks';
// Assuming you have this ...
class NotFound extends Error {}

export const handleError: HandleServerError = ({ kind, error, event }) => {
	if (kind === 'unknown') {
		// ... you can do this		if (error instanceof NotFound) {
			return {
				status: 404,
				message: 'Not found'
			};
		}

		return { message: 'Something went wrong' };
	}
	// app and framework errors are already safe to expose	return error;
};

Error boundaries

Errors that occur during load or rendering (for example inside a component’s <script> block or template) bubble up to the nearest +error.svelte component. To handle errors at a more granular level, you can use a <svelte:boundary>:

<svelte:boundary>
	...
	{#snippet failed(error: App.Error)}
		<!-- error went through the `handleError` hook and is of type `App.Error` -->
		{error.message}
	{/snippet}
</svelte:boundary>

Responses

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 safety

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

export {};

This interface always includes status: number and message: string properties.

Further reading

Edit this page on GitHub llms.txt

previous next
Hooks