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SvelteKitAppendix

Frequently asked questions

Other resources

Please see the Svelte FAQ and vite-plugin-svelte FAQ as well for the answers to questions deriving from those libraries.

What can I make with SvelteKit?

See the documentation regarding project types for more details.

How do I include details from package.json in my application?

If you’d like to include your application’s version number or other information from package.json in your application, you can load JSON like so:

svelte.config
import pkg from './package.json' with { type: 'json' };

How do I fix the error I’m getting trying to include a package?

Most issues related to including a library are due to incorrect packaging. You can check if a library’s packaging is compatible with Node.js by entering it into the publint website.

Here are a few things to keep in mind when checking if a library is packaged correctly:

  • exports takes precedence over the other entry point fields such as main and module. Adding an exports field may not be backwards-compatible as it prevents deep imports.
  • ESM files should end with .mjs unless "type": "module" is set in which any case CommonJS files should end with .cjs.
  • main should be defined if exports is not. It should be either a CommonJS or ESM file and adhere to the previous bullet. If a module field is defined, it should refer to an ESM file.
  • Svelte components should be distributed as uncompiled .svelte files with any JS in the package written as ESM only. Custom script and style languages, like TypeScript and SCSS, should be preprocessed as vanilla JS and CSS respectively. We recommend using svelte-package for packaging Svelte libraries, which will do this for you.

Libraries work best in the browser with Vite when they distribute an ESM version, especially if they are dependencies of a Svelte component library. You may wish to suggest to library authors that they provide an ESM version. However, CommonJS (CJS) dependencies should work as well since, by default, vite-plugin-svelte will ask Vite to pre-bundle them using esbuild to convert them to ESM.

If you are still encountering issues we recommend searching both the Vite issue tracker and the issue tracker of the library in question. Sometimes issues can be worked around by fiddling with the optimizeDeps or ssr config values though we recommend this as only a short-term workaround in favor of fixing the library in question.

How do I use the view transitions API?

While SvelteKit does not have any specific integration with view transitions, you can call document.startViewTransition in onNavigate to trigger a view transition on every client-side navigation.

import { onNavigate } from '$app/navigation';

onNavigate((navigation) => {
	if (!document.startViewTransition) return;

	return new Promise((resolve) => {
		document.startViewTransition(async () => {
			resolve();
			await navigation.complete;
		});
	});
});

For more, see “Unlocking view transitions” on the Svelte blog.

How do I set up a database?

Put the code to query your database in a server route - don’t query the database in .svelte files. You can create a db.js or similar that sets up a connection immediately and makes the client accessible throughout the app as a singleton. You can execute any one-time setup code in hooks.server.js and import your database helpers into any endpoint that needs them.

You can use the Svelte CLI to automatically set up database integrations.

How do I use a client-side library accessing document or window?

If you need access to the document or window variables or otherwise need code to run only on the client-side you can wrap it in a browser check:

import { browser } from '$app/environment';

if (browser) {
	// client-only code here}

You can also run code in onMount if you’d like to run it after the component has been first rendered to the DOM:

import { onMount } from 'svelte';

onMount(async () => {
	const { method } = await import('some-browser-only-library');
	method('hello world');
});

If the library you’d like to use is side-effect free you can also statically import it and it will be tree-shaken out in the server-side build where onMount will be automatically replaced with a no-op:

import { onMount } from 'svelte';
import { method } from 'some-browser-only-library';

onMount(() => {
	method('hello world');
});

Finally, you may also consider using an {#await} block:

index
<script>
	import { browser } from '$app/environment';

	const promise = browser
		? import('./BrowserComponent.svelte')
		: import('./ServerComponent.svelte');
</script>

{#await promise}
	<p>Loading...</p>
{:then module}
	<module.default />
{:catch error}
	<p>Something went wrong: {error.message}</p>
{/await}
<script lang="ts">
	import { browser } from '$app/environment';

	const promise = browser
		? import('./BrowserComponent.svelte')
		: import('./ServerComponent.svelte');
</script>

{#await promise}
	<p>Loading...</p>
{:then module}
	<module.default />
{:catch error}
	<p>Something went wrong: {error.message}</p>
{/await}

How do I use a different backend API server?

You can use event.fetch to request data from an external API server, but be aware that you would need to deal with CORS, which will result in complications such as generally requiring requests to be preflighted resulting in higher latency. Requests to a separate subdomain may also increase latency due to an additional DNS lookup, TLS setup, etc. If you wish to use this method, you may find handleFetch helpful.

Another approach is to set up a proxy to bypass CORS headaches. In production, you would rewrite a path like /api to the API server; for local development, use Vite’s server.proxy option.

How to set up rewrites in production will depend on your deployment platform. If rewrites aren’t an option, you could alternatively add an API route:

src/routes/api/[...path]/+server
/** @type {import('./$types').RequestHandler} */
export function GET({ params, url }) {
	return fetch(`https://example.com/${params.path + url.search}`);
}
import type { RequestHandler } from './$types';

export const GET: RequestHandler = ({ params, url }) => {
	return fetch(`https://example.com/${params.path + url.search}`);
};

(Note that you may also need to proxy POST / PATCH etc requests, and forward request.headers, depending on your needs.)

How do I use middleware?

adapter-node builds a middleware that you can use with your own server for production mode. In dev, you can add middleware to Vite by using a Vite plugin. For example:

vite.config
import { sveltekit } from '@sveltejs/kit/vite';
/** @type {import('vite').Plugin} */const myPlugin = {
	name: 'log-request-middleware',
	configureServer(server) {
		server.middlewares.use((req, res, next) => {
			console.log(`Got request ${req.url}`);
			next();
		});
	}
};
/** @type {import('vite').UserConfig} */const config = {
	plugins: [myPlugin, sveltekit()]
};

export default config;
import { sveltekit } from '@sveltejs/kit/vite';
import type { Plugin, UserConfig } from 'vite';

const myPlugin: Plugin = {
	name: 'log-request-middleware',
	configureServer(server) {
		server.middlewares.use((req, res, next) => {
			console.log(`Got request ${req.url}`);
			next();
		});
	}
};

const config: UserConfig = {
	plugins: [myPlugin, sveltekit()]
};

export default config;

See Vite’s configureServer docs for more details including how to control ordering.

How do I use Yarn?

Does it work with Yarn 2?

Sort of. The Plug’n’Play feature, aka ‘pnp’, is broken (it deviates from the Node module resolution algorithm, and doesn’t yet work with native JavaScript modules which SvelteKit — along with an increasing number of packages — uses). You can use nodeLinker: 'node-modules' in your .yarnrc.yml file to disable pnp, but it’s probably easier to just use npm or pnpm, which is similarly fast and efficient but without the compatibility headaches.

How do I use with Yarn 3?

Currently ESM Support within the latest Yarn (version 3) is considered experimental.

The below seems to work although your results may vary. First create a new application:

yarn create svelte myapp
cd myapp

And enable Yarn Berry:

yarn set version berry
yarn install

One of the more interesting features of Yarn Berry is the ability to have a single global cache for packages, instead of having multiple copies for each project on the disk. However, setting enableGlobalCache to true causes building to fail, so it is recommended to add the following to the .yarnrc.yml file:

nodeLinker: node-modules

This will cause packages to be downloaded into a local node_modules directory but avoids the above problem and is your best bet for using version 3 of Yarn at this point in time.

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