Migrating from Sapper

SvelteKit is the successor to Sapper and shares many elements of its design.

If you have an existing Sapper app that you plan to migrate to SvelteKit, there are a number of changes you will need to make. You may find it helpful to view some examples while migrating.

package.json

type: “module”

Add "type": "module" to your package.json. You can do this step separately from the rest as part of an incremental migration if you are using Sapper 0.29.3 or newer.

dependencies

Remove polka or express, if you’re using one of those, and any middleware such as sirv or compression.

devDependencies

Remove sapper from your devDependencies and replace it with @sveltejs/kit and whichever adapter you plan to use (see next section).

scripts

Any scripts that reference sapper should be updated:

• sapper build should become vite build using the Node adapter
• sapper export should become vite build using the static adapter
• sapper dev should become vite dev
• node __sapper__/build should become node build

Project files

The bulk of your app, in src/routes, can be left where it is, but several project files will need to be moved or updated.

Configuration

Your webpack.config.js or rollup.config.js should be replaced with a svelte.config.js, as documented here. Svelte preprocessor options should be moved to config.preprocess.

You will need to add an adapter. sapper build is roughly equivalent to adapter-node while sapper export is roughly equivalent to adapter-static, though you might prefer to use an adapter designed for the platform you’re deploying to.

If you were using plugins for filetypes that are not automatically handled by Vite, you will need to find Vite equivalents and add them to the Vite config.

src/client.js

This file has no equivalent in SvelteKit. Any custom logic (beyond sapper.start(...)) should be expressed in your +layout.svelte file, inside an onMount callback.

src/server.js

When using adapter-node the equivalent is a custom server. Otherwise, this file has no direct equivalent, since SvelteKit apps can run in serverless environments.

src/service-worker.js

Most imports from @sapper/service-worker have equivalents in $service-worker:

• files is unchanged
• routes has been removed
• shell is now build
• timestamp is now version

src/template.html

The src/template.html file should be renamed src/app.html.

Remove %sapper.base%, %sapper.scripts% and %sapper.styles%. Replace %sapper.head% with %sveltekit.head% and %sapper.html% with %sveltekit.body%. The <div id="sapper"> is no longer necessary.

src/node_modules

A common pattern in Sapper apps is to put your internal library in a directory inside src/node_modules. This doesn’t work with Vite, so we use src/lib instead.

Pages and layouts

Renamed files

Routes now are made up of the folder name exclusively to remove ambiguity, the folder names leading up to a +page.svelte correspond to the route. See the routing docs for an overview. The following shows a old/new comparison:

Your custom error page component should be renamed from _error.svelte to +error.svelte. Any _layout.svelte files should likewise be renamed +layout.svelte. Any other files are ignored.

Imports

The goto, prefetch and prefetchRoutes imports from @sapper/app should be replaced with goto, preloadData and preloadCode imports respectively from $app/navigation.

The stores import from @sapper/app should be replaced — see the Stores section below.

Any files you previously imported from directories in src/node_modules will need to be replaced with $lib imports.

Preload

As before, pages and layouts can export a function that allows data to be loaded before rendering takes place.

This function has been renamed from preload to load, it now lives in a +page.js (or +layout.js) next to its +page.svelte (or +layout.svelte), and its API has changed. Instead of two arguments — page and session — there is a single event argument.

There is no more this object, and consequently no this.fetch, this.error or this.redirect. Instead, you can get fetch from the input methods, and both error and redirect are now thrown.

Stores

In Sapper, you would get references to provided stores like so:

import { module "@sapper/app"stores } from '@sapper/app';
const { const preloading: anypreloading, const page: anypage, const session: anysession } = module "@sapper/app"stores();

The page store still exists; preloading has been replaced with a navigating store that contains from and to properties. page now has url and params properties, but no path or query.

You access them differently in SvelteKit. stores is now getStores, but in most cases it is unnecessary since you can import navigating, and page directly from $app/stores. If you’re on Svelte 5 and SvelteKit 2.12 or higher, consider using $app/state instead.

Routing

Regex routes are no longer supported. Instead, use advanced route matching.

Segments

Previously, layout components received a segment prop indicating the child segment. This has been removed; you should use the more flexible $page.url.pathname (or page.url.pathname) value to derive the segment you’re interested in.

URLs

In Sapper, all relative URLs were resolved against the base URL — usually /, unless the basepath option was used — rather than against the current page.

This caused problems and is no longer the case in SvelteKit. Instead, relative URLs are resolved against the current page (or the destination page, for fetch URLs in load functions) instead. In most cases, it’s easier to use root-relative (i.e. starts with /) URLs, since their meaning is not context-dependent.

<a> attributes

• sapper:prefetch is now data-sveltekit-preload-data
• sapper:noscroll is now data-sveltekit-noscroll

Endpoints

In Sapper, server routes received the req and res objects exposed by Node’s http module (or the augmented versions provided by frameworks like Polka and Express).

SvelteKit is designed to be agnostic as to where the app is running — it could be running on a Node server, but could equally be running on a serverless platform or in a Cloudflare Worker. For that reason, you no longer interact directly with req and res. Your endpoints will need to be updated to match the new signature.

To support this environment-agnostic behavior, fetch is now available in the global context, so you don’t need to import node-fetch, cross-fetch, or similar server-side fetch implementations in order to use it.

Integrations

See integrations for detailed information about integrations.

HTML minifier

Sapper includes html-minifier by default. SvelteKit does not include this, but you can add it as a prod dependency and then use it through a hook:

import { module "html-minifier"minify } from 'html-minifier';
import { const building: boolean
SvelteKit analyses your app during the build step by running it. During this process, building is true. This also applies during prerendering.
building } from '$app/environment';

const 
const minification_options: {
    collapseBooleanAttributes: boolean;
    collapseWhitespace: boolean;
    conservativeCollapse: boolean;
    decodeEntities: boolean;
    html5: boolean;
    ignoreCustomComments: RegExp[];
    minifyCSS: boolean;
    ... 8 more ...;
    sortClassName: boolean;
}minification_options = {
	collapseBooleanAttributes: booleancollapseBooleanAttributes: true,
	collapseWhitespace: booleancollapseWhitespace: true,
	conservativeCollapse: booleanconservativeCollapse: true,
	decodeEntities: booleandecodeEntities: true,
	html5: booleanhtml5: true,
	ignoreCustomComments: RegExp[]ignoreCustomComments: [/^#/],
	minifyCSS: booleanminifyCSS: true,
	minifyJS: booleanminifyJS: false,
	removeAttributeQuotes: booleanremoveAttributeQuotes: true,
	removeComments: booleanremoveComments: false, // some hydration code needs comments, so leave them in
	removeOptionalTags: booleanremoveOptionalTags: true,
	removeRedundantAttributes: booleanremoveRedundantAttributes: true,
	removeScriptTypeAttributes: booleanremoveScriptTypeAttributes: true,
	removeStyleLinkTypeAttributes: booleanremoveStyleLinkTypeAttributes: true,
	sortAttributes: booleansortAttributes: true,
	sortClassName: booleansortClassName: true
};

/** @type {import('@sveltejs/kit').Handle} */
export async function 
function handle(input: {
    event: RequestEvent;
    resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>;
}): MaybePromise<...>
@type{import('@sveltejs/kit').Handle}handle({ event: RequestEvent<Record<string, string>, string | null>event, resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve }) {
	let let page: stringpage = '';

	return resolve: (event: RequestEvent, opts?: ResolveOptions) => MaybePromise<Response>resolve(event: RequestEvent<Record<string, string>, string | null>event, {
		
ResolveOptions.transformPageChunk?: ((input: {
    html: string;
    done: boolean;
}) => MaybePromise<string | undefined>) | undefined
Applies custom transforms to HTML. If done is true, it’s the final chunk. Chunks are not guaranteed to be well-formed HTML
(they could include an element’s opening tag but not its closing tag, for example)
but they will always be split at sensible boundaries such as %sveltekit.head% or layout/page components.
@paraminput the html chunk and the info if this is the last chunktransformPageChunk: ({ html: stringhtml, done: booleandone }) => {
			let page: stringpage += html: stringhtml;
			if (done: booleandone) {
				return const building: boolean
SvelteKit analyses your app during the build step by running it. During this process, building is true. This also applies during prerendering.
building ? module "html-minifier"minify(let page: stringpage, 
const minification_options: {
    collapseBooleanAttributes: boolean;
    collapseWhitespace: boolean;
    conservativeCollapse: boolean;
    decodeEntities: boolean;
    html5: boolean;
    ignoreCustomComments: RegExp[];
    minifyCSS: boolean;
    ... 8 more ...;
    sortClassName: boolean;
}minification_options) : let page: stringpage;
			}
		}
	});
}

Note that prerendering is false when using vite preview to test the production build of the site, so to verify the results of minifying, you’ll need to inspect the built HTML files directly.