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SvelteKitCore concepts

Routing

At the heart of SvelteKit is a filesystem-based router. The routes of your app — i.e. the URL paths that users can access — are defined by the directories in your codebase:

  • src/routes is the root route
  • src/routes/about creates an /about route
  • src/routes/blog/[slug] creates a route with a parameter, slug, that can be used to load data dynamically when a user requests a page like /blog/hello-world

You can change src/routes to a different directory by editing the project config.

Each route directory contains one or more route files, which can be identified by their + prefix.

We’ll introduce these files in a moment in more detail, but here are a few simple rules to help you remember how SvelteKit’s routing works:

  • All files can run on the server
  • All files run on the client except +server files
  • +layout and +error files apply to subdirectories as well as the directory they live in

+page

+page.svelte

A +page.svelte component defines a page of your app. By default, pages are rendered both on the server (SSR) for the initial request and in the browser (CSR) for subsequent navigation.

src/routes/+page
<h1>Hello and welcome to my site!</h1>
<a href="/about">About my site</a>
src/routes/about/+page
<h1>About this site</h1>
<p>TODO...</p>
<a href="/">Home</a>

Pages can receive data from load functions via the data prop.

src/routes/blog/[slug]/+page
<script>
	/** @type {{ data: import('./$types').PageData }} */
	let { data } = $props();
</script>

<h1>{data.title}</h1>
<div>{@html data.content}</div>
<script lang="ts">
	import type { PageData } from './$types';

	let { data }: { data: PageData } = $props();
</script>

<h1>{data.title}</h1>
<div>{@html data.content}</div>
Legacy mode

In Svelte 4, you’d use export let data instead

SvelteKit uses <a> elements to navigate between routes, rather than a framework-specific <Link> component.

+page.js

Often, a page will need to load some data before it can be rendered. For this, we add a +page.js module that exports a load function:

src/routes/blog/[slug]/+page
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';

/** @type {import('./$types').PageLoad} */
export function 
function load({ params }: {
    params: any;
}): {
    title: string;
    content: string;
}
@type{import('./$types').PageLoad}
load({ params: anyparams }) {
	if (params: anyparams.slug === 'hello-world') {
		return {
			title: stringtitle: 'Hello world!',
			content: stringcontent: 'Welcome to our blog. Lorem ipsum dolor sit amet...'
		};
	}

	
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(404, 'Not found');
}
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';
import type { 
type PageLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
type PageLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
PageLoad } from './$types';

export const const load: PageLoadload: 
type PageLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
type PageLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
PageLoad = ({ params: Record<string, any>
The parameters of the current page - e.g. for a route like /blog/[slug], a { slug: string } object


params }) => {
	if (params: Record<string, any>
The parameters of the current page - e.g. for a route like /blog/[slug], a { slug: string } object


params.slug === 'hello-world') {
		return {
			title: stringtitle: 'Hello world!',
			content: stringcontent: 'Welcome to our blog. Lorem ipsum dolor sit amet...'
		};
	}

	
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(404, 'Not found');
};

This function runs alongside +page.svelte, which means it runs on the server during server-side rendering and in the browser during client-side navigation. See load for full details of the API.

As well as load, +page.js can export values that configure the page’s behaviour:

  • export const prerender = true or false or 'auto'
  • export const ssr = true or false
  • export const csr = true or false

You can find more information about these in page options.

+page.server.js

If your load function can only run on the server — for example, if it needs to fetch data from a database or you need to access private environment variables like API keys — then you can rename +page.js to +page.server.js and change the PageLoad type to PageServerLoad.

src/routes/blog/[slug]/+page.server
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';

/** @type {import('./$types').PageServerLoad} */
export async function function load(event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>): MaybePromise<void | Record<string, any>>
@type{import('./$types').PageServerLoad}
load({ params: Record<string, any>
The parameters of the current route - e.g. for a route like /blog/[slug], a { slug: string } object


params }) {
	const 
const post: {
    title: string;
    content: string;
}
post = await 
const getPostFromDatabase: (slug: string) => {
    title: string;
    content: string;
}
getPostFromDatabase(params: Record<string, any>
The parameters of the current route - e.g. for a route like /blog/[slug], a { slug: string } object


params.slug);

	if (
const post: {
    title: string;
    content: string;
}
post) {
		return 
const post: {
    title: string;
    content: string;
}
post;
	}

	
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(404, 'Not found');
}
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';
import type { type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad } from './$types';

export const const load: PageServerLoadload: type PageServerLoad = (event: ServerLoadEvent<Record<string, any>, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>PageServerLoad = async ({ params: Record<string, any>
The parameters of the current route - e.g. for a route like /blog/[slug], a { slug: string } object


params }) => {
	const 
const post: {
    title: string;
    content: string;
}
post = await 
const getPostFromDatabase: (slug: string) => {
    title: string;
    content: string;
}
getPostFromDatabase(params: Record<string, any>
The parameters of the current route - e.g. for a route like /blog/[slug], a { slug: string } object


params.slug);

	if (
const post: {
    title: string;
    content: string;
}
post) {
		return 
const post: {
    title: string;
    content: string;
}
post;
	}

	
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(404, 'Not found');
};

During client-side navigation, SvelteKit will load this data from the server, which means that the returned value must be serializable using devalue. See load for full details of the API.

Like +page.js, +page.server.js can export page options — prerender, ssr and csr.

A +page.server.js file can also export actions. If load lets you read data from the server, actions let you write data to the server using the <form> element. To learn how to use them, see the form actions section.

+error

If an error occurs during load, SvelteKit will render a default error page. You can customise this error page on a per-route basis by adding an +error.svelte file:

src/routes/blog/[slug]/+error
<script>
	import { page } from '$app/state';
</script>

<h1>{page.status}: {page.error.message}</h1>
<script lang="ts">
	import { page } from '$app/state';
</script>

<h1>{page.status}: {page.error.message}</h1>
Legacy mode

$app/state was added in SvelteKit 2.12. If you’re using an earlier version or are using Svelte 4, use $app/stores instead.

SvelteKit will ‘walk up the tree’ looking for the closest error boundary — if the file above didn’t exist it would try src/routes/blog/+error.svelte and then src/routes/+error.svelte before rendering the default error page. If that fails (or if the error was thrown from the load function of the root +layout, which sits ‘above’ the root +error), SvelteKit will bail out and render a static fallback error page, which you can customise by creating a src/error.html file.

If the error occurs inside a load function in +layout(.server).js, the closest error boundary in the tree is an +error.svelte file above that layout (not next to it).

If no route can be found (404), src/routes/+error.svelte (or the default error page, if that file does not exist) will be used.

+error.svelte is not used when an error occurs inside handle or a +server.js request handler.

You can read more about error handling here.

+layout

So far, we’ve treated pages as entirely standalone components — upon navigation, the existing +page.svelte component will be destroyed, and a new one will take its place.

But in many apps, there are elements that should be visible on every page, such as top-level navigation or a footer. Instead of repeating them in every +page.svelte, we can put them in layouts.

+layout.svelte

To create a layout that applies to every page, make a file called src/routes/+layout.svelte. The default layout (the one that SvelteKit uses if you don’t bring your own) looks like this...

<script>
	let { children } = $props();
</script>

{@render children()}

...but we can add whatever markup, styles and behaviour we want. The only requirement is that the component includes a @render tag for the page content. For example, let’s add a nav bar:

src/routes/+layout
<script>
	let { children } = $props();
</script>

<nav>
	<a href="/">Home</a>
	<a href="/about">About</a>
	<a href="/settings">Settings</a>
</nav>

{@render children()}
<script lang="ts">
	let { children } = $props();
</script>

<nav>
	<a href="/">Home</a>
	<a href="/about">About</a>
	<a href="/settings">Settings</a>
</nav>

{@render children()}

If we create pages for /, /about and /settings...

src/routes/+page
<h1>Home</h1>
src/routes/about/+page
<h1>About</h1>
src/routes/settings/+page
<h1>Settings</h1>

...the nav will always be visible, and clicking between the three pages will only result in the <h1> being replaced.

Layouts can be nested. Suppose we don’t just have a single /settings page, but instead have nested pages like /settings/profile and /settings/notifications with a shared submenu (for a real-life example, see github.com/settings).

We can create a layout that only applies to pages below /settings (while inheriting the root layout with the top-level nav):

src/routes/settings/+layout
<script>
	/** @type {{ data: import('./$types').LayoutData, children: import('svelte').Snippet }} */
	let { data, children } = $props();
</script>

<h1>Settings</h1>

<div class="submenu">
	{#each data.sections as section}
		<a href="/settings/{section.slug}">{section.title}</a>
	{/each}
</div>

{@render children()}
<script lang="ts">
	import type { LayoutData } from './$types';
	import type { Snippet } from 'svelte';

	let { data, children }: { data: LayoutData, children: Snippet } = $props();
</script>

<h1>Settings</h1>

<div class="submenu">
	{#each data.sections as section}
		<a href="/settings/{section.slug}">{section.title}</a>
	{/each}
</div>

{@render children()}

You can see how data is populated by looking at the +layout.js example in the next section just below.

By default, each layout inherits the layout above it. Sometimes that isn’t what you want - in this case, advanced layouts can help you.

+layout.js

Just like +page.svelte loading data from +page.js, your +layout.svelte component can get data from a load function in +layout.js.

src/routes/settings/+layout
/** @type {import('./$types').LayoutLoad} */
export function 
function load(): {
    sections: {
        slug: string;
        title: string;
    }[];
}
@type{import('./$types').LayoutLoad}
load() {
	return {
		
sections: {
    slug: string;
    title: string;
}[]
sections: [
			{ slug: stringslug: 'profile', title: stringtitle: 'Profile' },
			{ slug: stringslug: 'notifications', title: stringtitle: 'Notifications' }
		]
	};
}
import type { 
type LayoutLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
type LayoutLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
LayoutLoad } from './$types';

export const const load: LayoutLoadload: 
type LayoutLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
type LayoutLoad = (event: Kit.LoadEvent<Record<string, any>, Record<string, any> | null, Record<string, any>, string | null>) => MaybePromise<void | Record<string, any>>
LayoutLoad = () => {
	return {
		
sections: {
    slug: string;
    title: string;
}[]
sections: [
			{ slug: stringslug: 'profile', title: stringtitle: 'Profile' },
			{ slug: stringslug: 'notifications', title: stringtitle: 'Notifications' }
		]
	};
};

If a +layout.js exports page options — prerender, ssr and csr — they will be used as defaults for child pages.

Data returned from a layout’s load function is also available to all its child pages:

src/routes/settings/profile/+page
<script>
	/** @type {{ data: import('./$types').PageData }} */
	let { data } = $props();

	console.log(data.sections); // [{ slug: 'profile', title: 'Profile' }, ...]
</script>
<script lang="ts">
	import type { PageData } from './$types';

	let { data }: { data: PageData } = $props();

	console.log(data.sections); // [{ slug: 'profile', title: 'Profile' }, ...]
</script>

Often, layout data is unchanged when navigating between pages. SvelteKit will intelligently rerun load functions when necessary.

+layout.server.js

To run your layout’s load function on the server, move it to +layout.server.js, and change the LayoutLoad type to LayoutServerLoad.

Like +layout.js, +layout.server.js can export page options — prerender, ssr and csr.

+server

As well as pages, you can define routes with a +server.js file (sometimes referred to as an ‘API route’ or an ‘endpoint’), which gives you full control over the response. Your +server.js file exports functions corresponding to HTTP verbs like GET, POST, PATCH, PUT, DELETE, OPTIONS, and HEAD that take a RequestEvent argument and return a Response object.

For example we could create an /api/random-number route with a GET handler:

src/routes/api/random-number/+server
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';

/** @type {import('./$types').RequestHandler} */
export function 
function GET({ url }: {
    url: any;
}): Response
@type{import('./$types').RequestHandler}
GET({ url: anyurl }) {
	const const min: numbermin = 
var Number: NumberConstructor
(value?: any) => number
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.


Number(url: anyurl.searchParams.get('min') ?? '0');
	const const max: numbermax = 
var Number: NumberConstructor
(value?: any) => number
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.


Number(url: anyurl.searchParams.get('max') ?? '1');

	const const d: numberd = const max: numbermax - const min: numbermin;

	if (function isNaN(number: number): boolean
Returns a Boolean value that indicates whether a value is the reserved value NaN (not a number).


@paramnumber A numeric value.
isNaN(const d: numberd) || const d: numberd < 0) {
		
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(400, 'min and max must be numbers, and min must be less than max');
	}

	const const random: numberrandom = const min: numbermin + var Math: Math
An intrinsic object that provides basic mathematics functionality and constants.


Math.Math.random(): number
Returns a pseudorandom number between 0 and 1.


random() * const d: numberd;

	return new var Response: new (body?: BodyInit | null, init?: ResponseInit) => Response
This Fetch API interface represents the response to a request.


MDN Reference


Response(
var String: StringConstructor
(value?: any) => string
Allows manipulation and formatting of text strings and determination and location of substrings within strings.


String(const random: numberrandom));
}
import { function error(status: number, body: App.Error): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error } from '@sveltejs/kit';
import type { 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler } from './$types';

export const const GET: RequestHandlerGET: 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler = ({ url: URL
The requested URL.


url }) => {
	const const min: numbermin = 
var Number: NumberConstructor
(value?: any) => number
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.


Number(url: URL
The requested URL.


url.URL.searchParams: URLSearchParams
MDN Reference


searchParams.URLSearchParams.get(name: string): string | null
Returns the first value associated to the given search parameter.


MDN Reference


get('min') ?? '0');
	const const max: numbermax = 
var Number: NumberConstructor
(value?: any) => number
An object that represents a number of any kind. All JavaScript numbers are 64-bit floating-point numbers.


Number(url: URL
The requested URL.


url.URL.searchParams: URLSearchParams
MDN Reference


searchParams.URLSearchParams.get(name: string): string | null
Returns the first value associated to the given search parameter.


MDN Reference


get('max') ?? '1');

	const const d: numberd = const max: numbermax - const min: numbermin;

	if (function isNaN(number: number): boolean
Returns a Boolean value that indicates whether a value is the reserved value NaN (not a number).


@paramnumber A numeric value.
isNaN(const d: numberd) || const d: numberd < 0) {
		
function error(status: number, body?: {
    message: string;
} extends App.Error ? App.Error | string | undefined : never): never (+1 overload)
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking handleError.
Make sure you’re not catching the thrown error, which would prevent SvelteKit from handling it.


@paramstatus The HTTP status code. Must be in the range 400-599.
@parambody An object that conforms to the App.Error type. If a string is passed, it will be used as the message property.
@throwsHttpError This error instructs SvelteKit to initiate HTTP error handling.
@throwsError If the provided status is invalid (not between 400 and 599).
error(400, 'min and max must be numbers, and min must be less than max');
	}

	const const random: numberrandom = const min: numbermin + var Math: Math
An intrinsic object that provides basic mathematics functionality and constants.


Math.Math.random(): number
Returns a pseudorandom number between 0 and 1.


random() * const d: numberd;

	return new var Response: new (body?: BodyInit | null, init?: ResponseInit) => Response
This Fetch API interface represents the response to a request.


MDN Reference


Response(
var String: StringConstructor
(value?: any) => string
Allows manipulation and formatting of text strings and determination and location of substrings within strings.


String(const random: numberrandom));
};

The first argument to Response can be a ReadableStream, making it possible to stream large amounts of data or create server-sent events (unless deploying to platforms that buffer responses, like AWS Lambda).

You can use the error, redirect and json methods from @sveltejs/kit for convenience (but you don’t have to).

If an error is thrown (either error(...) or an unexpected error), the response will be a JSON representation of the error or a fallback error page — which can be customised via src/error.html — depending on the Accept header. The +error.svelte component will not be rendered in this case. You can read more about error handling here.

When creating an OPTIONS handler, note that Vite will inject Access-Control-Allow-Origin and Access-Control-Allow-Methods headers — these will not be present in production unless you add them.

+layout files have no effect on +server.js files. If you want to run some logic before each request, add it to the server handle hook.

Receiving data

By exporting POST / PUT/PATCH/DELETE/OPTIONS/HEAD handlers, +server.js files can be used to create a complete API:

src/routes/add/+page
<script>
	let a = 0;
	let b = 0;
	let total = 0;

	async function add() {
		const response = await fetch('/api/add', {
			method: 'POST',
			body: JSON.stringify({ a, b }),
			headers: {
				'content-type': 'application/json'
			}
		});

		total = await response.json();
	}
</script>

<input type="number" bind:value={a}> +
<input type="number" bind:value={b}> =
{total}

<button onclick={add}>Calculate</button>
<script lang="ts">
	let a = 0;
	let b = 0;
	let total = 0;

	async function add() {
		const response = await fetch('/api/add', {
			method: 'POST',
			body: JSON.stringify({ a, b }),
			headers: {
				'content-type': 'application/json'
			}
		});

		total = await response.json();
	}
</script>

<input type="number" bind:value={a}> +
<input type="number" bind:value={b}> =
{total}

<button onclick={add}>Calculate</button>
src/routes/api/add/+server
import { function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json } from '@sveltejs/kit';

/** @type {import('./$types').RequestHandler} */
export async function 
function POST({ request }: {
    request: any;
}): Promise<Response>
@type{import('./$types').RequestHandler}
POST({ request: anyrequest }) {
	const { const a: anya, const b: anyb } = await request: anyrequest.json();
	return function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json(const a: anya + const b: anyb);
}
import { function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json } from '@sveltejs/kit';
import type { 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler } from './$types';

export const const POST: RequestHandlerPOST: 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler = async ({ request: Request
The original request object


request }) => {
	const { const a: anya, const b: anyb } = await request: Request
The original request object


request.Body.json(): Promise<any>
MDN Reference


json();
	return function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json(const a: anya + const b: anyb);
};

In general, form actions are a better way to submit data from the browser to the server.

If a GET handler is exported, a HEAD request will return the content-length of the GET handler’s response body.

Fallback method handler

Exporting the fallback handler will match any unhandled request methods, including methods like MOVE which have no dedicated export from +server.js.

src/routes/api/add/+server
import { function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json, function text(body: string, init?: ResponseInit | undefined): Response
Create a Response object from the supplied body.


@parambody The value that will be used as-is.
@paraminit Options such as status and headers that will be added to the response. A Content-Length header will be added automatically.
text } from '@sveltejs/kit';

export async function 
function POST({ request }: {
    request: any;
}): Promise<Response>
POST({ request: anyrequest }) {
	const { const a: anya, const b: anyb } = await request: anyrequest.json();
	return function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json(const a: anya + const b: anyb);
}

// This handler will respond to PUT, PATCH, DELETE, etc.
/** @type {import('./$types').RequestHandler} */
export async function 
function fallback({ request }: {
    request: any;
}): Promise<Response>
@type{import('./$types').RequestHandler}
fallback({ request: anyrequest }) {
	return function text(body: string, init?: ResponseInit | undefined): Response
Create a Response object from the supplied body.


@parambody The value that will be used as-is.
@paraminit Options such as status and headers that will be added to the response. A Content-Length header will be added automatically.
text(`I caught your ${request: anyrequest.method} request!`);
}
import { function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json, function text(body: string, init?: ResponseInit | undefined): Response
Create a Response object from the supplied body.


@parambody The value that will be used as-is.
@paraminit Options such as status and headers that will be added to the response. A Content-Length header will be added automatically.
text } from '@sveltejs/kit';
import type { 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler } from './$types';

export async function 
function POST({ request }: {
    request: any;
}): Promise<Response>
POST({ request: anyrequest }) {
	const { const a: anya, const b: anyb } = await request: anyrequest.json();
	return function json(data: any, init?: ResponseInit | undefined): Response
Create a JSON Response object from the supplied data.


@paramdata The value that will be serialized as JSON.
@paraminit Options such as status and headers that will be added to the response. Content-Type: application/json and Content-Length headers will be added automatically.
json(const a: anya + const b: anyb);
}

// This handler will respond to PUT, PATCH, DELETE, etc.
export const const fallback: RequestHandlerfallback: 
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
type RequestHandler = (event: Kit.RequestEvent<Record<string, any>, string | null>) => MaybePromise<Response>
RequestHandler = async ({ request: Request
The original request object


request }) => {
	return function text(body: string, init?: ResponseInit | undefined): Response
Create a Response object from the supplied body.


@parambody The value that will be used as-is.
@paraminit Options such as status and headers that will be added to the response. A Content-Length header will be added automatically.
text(`I caught your ${request: Request
The original request object


request.Request.method: string
Returns request’s HTTP method, which is “GET” by default.


MDN Reference


method} request!`);
};

For HEAD requests, the GET handler takes precedence over the fallback handler.

Content negotiation

+server.js files can be placed in the same directory as +page files, allowing the same route to be either a page or an API endpoint. To determine which, SvelteKit applies the following rules:

  • PUT / PATCH/DELETE/OPTIONS requests are always handled by +server.js since they do not apply to pages
  • GET / POST/HEAD requests are treated as page requests if the accept header prioritises text/html (in other words, it’s a browser page request), else they are handled by +server.js.
  • Responses to GET requests will include a Vary: Accept header, so that proxies and browsers cache HTML and JSON responses separately.

$types

Throughout the examples above, we’ve been importing types from a $types.d.ts file. This is a file SvelteKit creates for you in a hidden directory if you’re using TypeScript (or JavaScript with JSDoc type annotations) to give you type safety when working with your root files.

For example, annotating let { data } = $props() with PageData (or LayoutData, for a +layout.svelte file) tells TypeScript that the type of data is whatever was returned from load:

src/routes/blog/[slug]/+page
<script>
	/** @type {{ data: import('./$types').PageData }} */
	let { data } = $props();
</script>
<script lang="ts">
	import type { PageData } from './$types';

	let { data }: { data: PageData } = $props();
</script>

In turn, annotating the load function with PageLoad, PageServerLoad, LayoutLoad or LayoutServerLoad (for +page.js, +page.server.js, +layout.js and +layout.server.js respectively) ensures that params and the return value are correctly typed.

If you’re using VS Code or any IDE that supports the language server protocol and TypeScript plugins then you can omit these types entirely! Svelte’s IDE tooling will insert the correct types for you, so you’ll get type checking without writing them yourself. It also works with our command line tool svelte-check.

You can read more about omitting $types in our blog post about it.

Other files

Any other files inside a route directory are ignored by SvelteKit. This means you can colocate components and utility modules with the routes that need them.

If components and modules are needed by multiple routes, it’s a good idea to put them in $lib.

Further reading

Edit this page on GitHub

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