Appendix

Migrating to SvelteKit v2

Upgrading from SvelteKit version 1 to version 2 should be mostly seamless. There are a few breaking changes to note, which are listed here. You can use npx svelte-migrate sveltekit-2 to migrate some of these changes automatically.

We highly recommend upgrading to the most recent 1.x version before upgrading to 2.0, so that you can take advantage of targeted deprecation warnings. We also recommend updating to Svelte 4 first: Later versions of SvelteKit 1.x support it, and SvelteKit 2.0 requires it.

redirect and error are no longer thrown by youpermalink

Previously, you had to throw the values returned from error(...) and redirect(...) yourself. In SvelteKit 2 this is no longer the case — calling the functions is sufficient.

import { error } from '@sveltejs/kit'

...
 throw error(500, 'something went wrong');
 error(500, 'something went wrong');

svelte-migrate will do these changes automatically for you.

If the error or redirect is thrown inside a try {...} block (hint: don't do this!), you can distinguish them from unexpected errors using isHttpError and isRedirect imported from @sveltejs/kit.

path is required when setting cookiespermalink

When receiving a Set-Cookie header that doesn't specify a path, browsers will set the cookie path to the parent of the resource in question. This behaviour isn't particularly helpful or intuitive, and frequently results in bugs because the developer expected the cookie to apply to the domain as a whole.

As of SvelteKit 2.0, you need to set a path when calling cookies.set(...), cookies.delete(...) or cookies.serialize(...) so that there's no ambiguity. Most of the time, you probably want to use path: '/', but you can set it to whatever you like, including relative paths — '' means 'the current path', '.' means 'the current directory'.

export function load({ cookies }) {
	cookies.set(name, value);
	cookies.set(name, value, { path: '/' });
	return { response }
}

svelte-migrate will add comments highlighting the locations that need to be adjusted.

Top-level promises are no longer awaitedpermalink

In SvelteKit version 1, if the top-level properties of the object returned from a load function were promises, they were automatically awaited. With the introduction of streaming this behavior became a bit awkward as it forces you to nest your streamed data one level deep.

As of version 2, SvelteKit no longer differentiates between top-level and non-top-level promises. To get back the blocking behavior, use await (with Promise.all to prevent waterfalls, where appropriate):

// If you have a single promise
export function load({ fetch }) {
	const response = fetch(...).then(r => r.json());
	const response = await fetch(...).then(r => r.json());
	return { response }
}

// If you have multiple promises
export function load({ fetch }) {
	const a = fetch(...).then(r => r.json());
	const b = fetch(...).then(r => r.json());
	const [a, b] = await Promise.all([
	  fetch(...).then(r => r.json()),
	  fetch(...).then(r => r.json()),
	]);
	return { a, b };
}

goto(...) no longer accepts external URLspermalink

To navigate to an external URL, use window.location = url.

paths are now relative by defaultpermalink

In SvelteKit 1, %sveltekit.assets% in your app.html was replaced with a relative path by default (i.e. . or .. or ../.. etc, depending on the path being rendered) during server-side rendering unless the paths.relative config option was explicitly set to false. The same was true for base and assets imported from $app/paths, but only if the paths.relative option was explicitly set to true.

This inconsistency is fixed in version 2. Paths are either always relative or always absolute, depending on the value of paths.relative. It defaults to true as this results in more portable apps: if the base is something other than the app expected (as is the case when viewed on the Internet Archive, for example) or unknown at build time (as is the case when deploying to IPFS and so on), fewer things are likely to break.

Server fetches are not trackable anymorepermalink

Previously it was possible to track URLs from fetches on the server in order to rerun load functions. This poses a possible security risk (private URLs leaking), and as such it was behind the dangerZone.trackServerFetches setting, which is now removed.

preloadCode arguments must be prefixed with basepermalink

SvelteKit exposes two functions, preloadCode and preloadData, for programmatically loading the code and data associated with a particular path. In version 1, there was a subtle inconsistency — the path passed to preloadCode did not need to be prefixed with the base path (if set), while the path passed to preloadData did.

This is fixed in SvelteKit 2 — in both cases, the path should be prefixed with base if it is set.

Additionally, preloadCode now takes a single argument rather than n arguments.

resolvePath has been removedpermalink

SvelteKit 1 included a function called resolvePath which allows you to resolve a route ID (like /blog/[slug]) and a set of parameters (like { slug: 'hello' }) to a pathname. Unfortunately the return value didn't include the base path, limiting its usefulness in cases where base was set.

As such, SvelteKit 2 replaces resolvePath with a (slightly better named) function called resolveRoute, which is imported from $app/paths and which takes base into account.

import { resolvePath } from '@sveltejs/kit';
import { base } from '$app/paths';
import { resolveRoute } from '$app/paths';

const path = base + resolvePath('/blog/[slug]', { slug });
const path = resolveRoute('/blog/[slug]', { slug });

svelte-migrate will do the method replacement for you, though if you later prepend the result with base, you need to remove that yourself.

Improved error handlingpermalink

Errors are handled inconsistently in SvelteKit 1. Some errors trigger the handleError hook but there is no good way to discern their status (for example, the only way to tell a 404 from a 500 is by seeing if event.route.id is null), while others (such as 405 errors for POST requests to pages without actions) don't trigger handleError at all, but should. In the latter case, the resulting $page.error will deviate from the App.Error type, if it is specified.

SvelteKit 2 cleans this up by calling handleError hooks with two new properties: status and message. For errors thrown from your code (or library code called by your code) the status will be 500 and the message will be Internal Error. While error.message may contain sensitive information that should not be exposed to users, message is safe.

Dynamic environment variables cannot be used during prerenderingpermalink

The $env/dynamic/public and $env/dynamic/private modules provide access to run time environment variables, as opposed to the build time environment variables exposed by $env/static/public and $env/static/private.

During prerendering in SvelteKit 1, they are one and the same. As such, prerendered pages that make use of 'dynamic' environment variables are really 'baking in' build time values, which is incorrect. Worse, $env/dynamic/public is populated in the browser with these stale values if the user happens to land on a prerendered page before navigating to dynamically-rendered pages.

Because of this, dynamic environment variables can no longer be read during prerendering in SvelteKit 2 — you should use the static modules instead. If the user lands on a prerendered page, SvelteKit will request up-to-date values for $env/dynamic/public from the server (by default from a module called _env.js — this can be configured with config.kit.env.publicModule) instead of reading them from the server-rendered HTML.

form and data have been removed from use:enhance callbackspermalink

If you provide a callback to use:enhance, it will be called with an object containing various useful properties.

In SvelteKit 1, those properties included form and data. These were deprecated some time ago in favour of formElement and formData, and have been removed altogether in SvelteKit 2.

Forms containing file inputs must use multipart/form-datapermalink

If a form contains an <input type="file"> but does not have an enctype="multipart/form-data" attribute, non-JS submissions will omit the file. SvelteKit 2 will throw an error if it encounters a form like this during a use:enhance submission to ensure that your forms work correctly when JavaScript is not present.

Generated tsconfig.json is more strictpermalink

Previously, the generated tsconfig.json was trying its best to still produce a somewhat valid config when your tsconfig.json included paths or baseUrl. In SvelteKit 2, the validation is more strict and will warn when you use either paths or baseUrl in your tsconfig.json. These settings are used to generate path aliases and you should use the alias config option in your svelte.config.js instead, to also create a corresponding alias for the bundler.

getRequest no longer throws errorspermalink

The @sveltejs/kit/node module exports helper functions for use in Node environments, including getRequest which turns a Node ClientRequest into a standard Request object.

In SvelteKit 1, getRequest could throw if the Content-Length header exceeded the specified size limit. In SvelteKit 2, the error will not be thrown until later, when the request body (if any) is being read. This enables better diagnostics and simpler code.

vitePreprocess is no longer exported from @sveltejs/kit/vitepermalink

Since @sveltejs/vite-plugin-svelte is now a peer dependency, SvelteKit 2 no longer re-exports vitePreprocess. You should import it directly from @sveltejs/vite-plugin-svelte.

Updated dependency requirementspermalink

SvelteKit 2 requires Node 18.13 or higher, and the following minimum dependency versions:

svelte@4
vite@5
typescript@5
@sveltejs/adapter-static@3 (if you're using it)
@sveltejs/vite-plugin-svelte@3 (this is now required as a peerDependency of SvelteKit — previously it was directly depended upon)

svelte-migrate will update your package.json for you.

As part of the TypeScript upgrade, the generated tsconfig.json (the one your tsconfig.json extends from) now uses "moduleResolution": "bundler" (which is recommended by the TypeScript team, as it properly resolves types from packages with an exports map in package.json) and verbatimModuleSyntax (which replaces the existing importsNotUsedAsValues and preserveValueImports flags — if you have those in your tsconfig.json, remove them. svelte-migrate will do this for you).

SvelteKit 2 uses ES2022 features, which are supported in all modern browsers.

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