There are two types of errors in SvelteKit — expected errors and unexpected errors.

An expected error is one that was thrown via the error helper from @sveltejs/kit, as in src/routes/expected/+page.server.js:

import { error } from '@sveltejs/kit';

export function load() {
	error(420, 'Enhance your calm');
}

Any other error — such as the one in src/routes/unexpected/+page.server.js — is treated as unexpected:

export function load() {
	throw new Error('Kaboom!');
}

When you throw an expected error, you’re telling SvelteKit ‘don’t worry, I know what I’m doing here’. An unexpected error, by contrast, is assumed to be a bug in your app. When an unexpected error is thrown, its message and stack trace will be logged to the console.

In a later chapter we’ll learn about how to add custom error handling using the handleError hook.

If you click the links in this app, you’ll notice an important difference: the expected error message is shown to the user, whereas the unexpected error message is redacted and replaced with a generic ‘Internal Error’ message and a 500 status code. That’s because error messages can contain sensitive data.