Frequently Asked Questions

SvelteKit is not 1.0 yet. Should I use it? What about Sapper? permalink

SvelteKit is currently in beta while we finalize the APIs. No new features will be added to Sapper and all development will be focused on SvelteKit.

How do I use HMR with SvelteKit? permalink

SvelteKit has HMR enabled by default powered by svelte-hmr. If you saw Rich's presentation at the 2020 Svelte Summit, you may have seen a more powerful-looking version of HMR presented. This demo had svelte-hmr's preserveLocalState flag on. This flag is now off by default because it may lead to unexpected behaviour and edge cases. But don't worry, you are still getting HMR with SvelteKit! If you'd like to preserve local state you can use the @hmr:keep or @hmr:keep-all directives as documented on the svelte-hmr page.

Why am I getting a 404? permalink

Please make sure you're returning something from your page's load function. See the section about fallthrough routes for more details.

I'm having trouble using an adapter. permalink

Please make sure the version of the adapter specified in your package.json is "next".

How do I hash asset file names for caching? permalink

You can have Vite process your assets by importing them as shown below:

<script>
	import imageSrc from '$lib/assets/image.png';
</script>

<img src="{imageSrc}" />

If you prefer to directly import in the markup, try svelte-preprocess-import-assets and you can write this instead:

<img src="$lib/assets/image.png" />

How do I setup a path alias? permalink

First, you need to add it to the Vite configuration. In svelte.config.js add vite.resolve.alias:

// svelte.config.js
import path from 'path';

export default {
	kit: {
		vite: {
			resolve: {
				alias: {
					$utils: path.resolve('./src/utils')
				}
			}
		}
	}
};

Then, to make TypeScript aware of the alias, add it to tsconfig.json (for TypeScript users) or jsconfig.json:

{
  "compilerOptions": {
    "paths": {
      "$utils/*": ["src/utils/*"]
    }
  }
}

How do I use environment variables? permalink

Vite uses dotenv to load environment variables from a file named .env or similar. Only environment variables prefixed with VITE_ are exposed (you can set envPrefix to change this). For now, you would need to instantiate dotenv yourself if you want all environment variables exposed in process.env['YOUR_ENV_VAR']. Vite 2.7 will expose all environment variables on the server-side.

Please see the Vite documentation for more info about environment variables.

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

You cannot directly require JSON files, since SvelteKit expects svelte.config.js to be an ES module. 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:

const pkg = JSON.parse(fs.readFileSync(new URL('package.json', import.meta.url), 'utf8'));

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

SSR in Vite is not yet stable. Libraries work best with Vite when they distribute both CJS and ESM in their package and you may wish to work with library authors to make this happen.

Svelte components must be written entirely in ESM. It is encouraged to make sure the dependencies of external Svelte components provide an ESM version. However, in order to handle CJS dependencies vite-plugin-svelte will look for any CJS dependencies of external Svelte components and ask Vite to pre-bundle them by automatically adding them to Vite's optimizeDeps.include which will use esbuild to convert them to ESM.

A side effect of the approach above is that it takes longer to load the initial page. If this becomes noticable, try setting experimental.prebundleSvelteLibraries: true in svelte.config.js. Note that this option is experimental.

If you are still encountering issues we recommend checking the list of known Vite issues most commonly affecting SvelteKit users and 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.

How do I use X with SvelteKit? permalink

Make sure you've read the documentation section on integrations. If you're still having trouble, solutions to common issues are listed below.

How do I setup a database?

Put the code to query your database in endpoints - 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.js and import your database helpers into any endpoint that needs them.

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:

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

/** @type {import('@sveltejs/kit').Config} */
const config = {
	kit: {
		target: '#svelte',
		vite: {
			plugins: [ myPlugin ]
		}
	}
};

export default config;

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

How do I use a client-side only library that depends on document or window?

Vite will attempt to process all imported libraries and may fail when encountering a library that is not compatible with SSR. This currently occurs even when SSR is disabled.

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

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

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');
});

Otherwise, if the library has side effects and you'd still prefer to use static imports, check out vite-plugin-iso-import to support the ?client import suffix. The import will be stripped out in SSR builds. However, note that you will lose the ability to use VS Code Intellisense if you use this method.

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

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

How do I use Firebase?

Please use Firebase SDK v9. You will also need to add Firebase dependencies to ssr.external (example) or set kit.ssr: false until Vite 2.7 is released.

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.

See also the Svelte FAQ for questions relating to Svelte directly.