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Configuration

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Your project's configuration lives in a svelte.config.js file. All values are optional. The complete list of options, with defaults, is shown here:

svelte.config.js
ts
/** @type {import('@sveltejs/kit').Config} */
const config = {
  // options passed to svelte.compile (https://svelte.dev/docs#compile-time-svelte-compile)
  compilerOptions: {},
 
  // an array of file extensions that should be treated as Svelte components
  extensions: ['.svelte'],
 
  kit: {
    adapter: undefined,
    amp: false,
    appDir: '_app',
    browser: {
      hydrate: true,
      router: true
    },
    csp: {
      mode: 'auto',
      directives: {
        'default-src': undefined
        // ...
      }
    },
    files: {
      assets: 'static',
      hooks: 'src/hooks',
      lib: 'src/lib',
      routes: 'src/routes',
      serviceWorker: 'src/service-worker',
      template: 'src/app.html'
    },
    floc: false,
    inlineStyleThreshold: 0,
    methodOverride: {
      parameter: '_method',
      allowed: []
    },
    package: {
      dir: 'package',
      emitTypes: true,
      // excludes all .d.ts and files starting with _ as the name
      exports: (filepath) => !/^_|\/_|\.d\.ts$/.test(filepath),
      files: () => true
    },
    paths: {
      assets: '',
      base: ''
    },
    prerender: {
      concurrency: 1,
      crawl: true,
      enabled: true,
      entries: ['*'],
      onError: 'fail'
    },
    routes: (filepath) => !/(?:(?:^_|\/_)|(?:^\.|\/\.)(?!well-known))/.test(filepath),
    serviceWorker: {
      register: true,
      files: (filepath) => !/\.DS_Store/.test(filepath)
    },
    trailingSlash: 'never',
    version: {
      name: Date.now().toString(),
      pollInterval: 0
    },
    vite: () => ({})
  },
 
  // SvelteKit uses vite-plugin-svelte. Its options can be provided directly here.
  // See the available options at https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/config.md
 
  // options passed to svelte.preprocess (https://svelte.dev/docs#compile-time-svelte-preprocess)
  preprocess: null
};
 
export default config;

adapter

Required when running svelte-kit build and determines how the output is converted for different platforms. See Adapters.

amp

Enable AMP mode.

appDir

The directory relative to paths.assets where the built JS and CSS (and imported assets) are served from. (The filenames therein contain content-based hashes, meaning they can be cached indefinitely). Must not start or end with /.

browser

An object containing zero or more of the following boolean values:

  • hydrate — whether to hydrate the server-rendered HTML with a client-side app. (It's rare that you would set this to false on an app-wide basis.)
  • router — enables or disables the client-side router app-wide.

csp

An object containing zero or more of the following values:

  • mode — 'hash', 'nonce' or 'auto'
  • directives — an object of [directive]: value[] pairs.

Content Security Policy configuration. CSP helps to protect your users against cross-site scripting (XSS) attacks, by limiting the places resources can be loaded from. For example, a configuration like this...

svelte.config.js
ts
/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    csp: {
      directives: {
        'script-src': ['self']
      }
    }
  }
};
 
export default config;

...would prevent scripts loading from external sites. SvelteKit will augment the specified directives with nonces or hashes (depending on mode) for any inline styles and scripts it generates.

When pages are prerendered, the CSP header is added via a <meta http-equiv> tag (note that in this case, frame-ancestors, report-uri and sandbox directives will be ignored).

When mode is 'auto', SvelteKit will use nonces for dynamically rendered pages and hashes for prerendered pages. Using nonces with prerendered pages is insecure and therefore forbiddem.

files

An object containing zero or more of the following string values:

  • assets — a place to put static files that should have stable URLs and undergo no processing, such as favicon.ico or manifest.json
  • hooks — the location of your hooks module (see Hooks)
  • lib — your app's internal library, accessible throughout the codebase as $lib
  • routes — the files that define the structure of your app (see Routing)
  • serviceWorker — the location of your service worker's entry point (see Service workers)
  • template — the location of the template for HTML responses

floc

Google's FLoC is a technology for targeted advertising that the Electronic Frontier Foundation has deemed harmful to user privacy. Browsers other than Chrome have declined to implement it.

In common with services like GitHub Pages, SvelteKit protects your users by automatically opting out of FLoC. It adds the following header to responses unless floc is true:

Permissions-Policy: interest-cohort=()

This only applies to server-rendered responses — headers for prerendered pages (e.g. created with adapter-static) are determined by the hosting platform.

inlineStyleThreshold

Inline CSS inside a <style> block at the head of the HTML. This option is a number that specifies the maximum length of a CSS file to be inlined. All CSS files needed for the page and smaller than this value are merged and inlined in a <style> block.

This results in fewer initial requests and can improve your First Contentful Paint score. However, it generates larger HTML output and reduces the effectiveness of browser caches. Use it advisedly.

methodOverride

See HTTP Method Overrides. An object containing zero or more of the following:

  • parameter — query parameter name to use for passing the intended method value
  • allowed - array of HTTP methods that can be used when overriding the original request method

package

Options related to creating a package.

  • dir - output directory
  • emitTypes - by default, svelte-kit package will automatically generate types for your package in the form of .d.ts files. While generating types is configurable, we believe it is best for the ecosystem quality to generate types, always. Please make sure you have a good reason when setting it to false (for example when you want to provide handwritten type definitions instead)
  • exports - a function with the type of (filepath: string) => boolean. When true, the filepath will be included in the exports field of the package.json. Any existing values in the package.json source will be merged with values from the original exports field taking precedence
  • files - a function with the type of (filepath: string) => boolean. When true, the file will be processed and copied over to the final output folder, specified in dir

For advanced filepath matching, you can use exports and files options in conjunction with a globbing library:

svelte.config.js
ts
import mm from 'micromatch';
 
/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    package: {
      exports: (filepath) => {
        if (filepath.endsWith('.d.ts')) return false;
        return mm.isMatch(filepath, ['!**/_*', '!**/internal/**']);
      },
      files: mm.matcher('!**/build.*')
    }
  }
};
 
export default config;

paths

An object containing zero or more of the following string values:

  • assets — an absolute path that your app's files are served from. This is useful if your files are served from a storage bucket of some kind
  • base — a root-relative path that must start, but not end with / (e.g. /base-path). This specifies where your app is served from and allows the app to live on a non-root path

prerender

See Prerendering. An object containing zero or more of the following:

  • concurrency — how many pages can be prerendered simultaneously. JS is single-threaded, but in cases where prerendering performance is network-bound (for example loading content from a remote CMS) this can speed things up by processing other tasks while waiting on the network response

  • crawl — determines whether SvelteKit should find pages to prerender by following links from the seed page(s)

  • enabled — set to false to disable prerendering altogether

  • entries — an array of pages to prerender, or start crawling from (if crawl: true). The * string includes all non-dynamic routes (i.e. pages with no [parameters] )

  • onError

    • 'fail' — (default) fails the build when a routing error is encountered when following a link

    • 'continue' — allows the build to continue, despite routing errors

    • function — custom error handler allowing you to log, throw and fail the build, or take other action of your choosing based on the details of the crawl

      ts
      import adapter from '@sveltejs/adapter-static';
       
      /** @type {import('@sveltejs/kit').Config} */
      const config = {
        kit: {
          adapter: adapter(),
          prerender: {
            onError: ({ status, path, referrer, referenceType }) => {
              if (path.startsWith('/blog')) throw new Error('Missing a blog page!');
              console.warn(
                `${status} ${path}${referrer ? ` (${referenceType} from ${referrer})` : ''}`
              );
            }
          }
        }
      };
       
      export default config;

routes

A (filepath: string) => boolean function that determines which files create routes and which are treated as private modules.

serviceWorker

An object containing zero or more of the following values:

  • register - if set to false, will disable automatic service worker registration
  • files - a function with the type of (filepath: string) => boolean. When true, the given file will be available in $service-worker.files, otherwise it will be excluded.

trailingSlash

Whether to remove, append, or ignore trailing slashes when resolving URLs to routes.

  • "never" — redirect /x/ to /x
  • "always" — redirect /x to /x/
  • "ignore" — don't automatically add or remove trailing slashes. /x and /x/ will be treated equivalently

This option also affects prerendering. If trailingSlash is always, a route like /about will result in an about/index.html file, otherwise it will create about.html, mirroring static webserver conventions.

Ignoring trailing slashes is not recommended — the semantics of relative paths differ between the two cases (./y from /x is /y, but from /x/ is /x/y), and /x and /x/ are treated as separate URLs which is harmful to SEO. If you use this option, ensure that you implement logic for conditionally adding or removing trailing slashes from request.path inside your handle function.

version

An object containing zero or more of the following values:

  • name - current app version string
  • pollInterval - interval in milliseconds to poll for version changes

Client-side navigation can be buggy if you deploy a new version of your app while people are using it. If the code for the new page is already loaded, it may have stale content; if it isn't, the app's route manifest may point to a JavaScript file that no longer exists. SvelteKit solves this problem by falling back to traditional full-page navigation if it detects that a new version has been deployed, using the name specified here (which defaults to a timestamp of the build).

If you set pollInterval to a non-zero value, SvelteKit will poll for new versions in the background and set the value of the updated store to true when it detects one.

vite

A Vite config object, or a function that returns one. You can pass Vite and Rollup plugins via the plugins option to customize your build in advanced ways such as supporting image optimization, Tauri, WASM, Workbox, and more. SvelteKit will prevent you from setting certain build-related options since it depends on certain configuration values.

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