Routing

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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.

+pagepermalink

+page.sveltepermalink

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.

<h1>Hello and welcome to my site!</h1>
<a href="/about">About my site</a>

<h1>About this site</h1>
<p>TODO...</p>
<a href="/">Home</a>

<script>
  /** @type {import('./$types').PageData} */  export let data;
</script>

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

<script lang="ts">
  import type { PageData } from './$types';

  export let data: PageData;
</script>

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

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

+page.jspermalink

Often, a page will need to load some data before it can be rendered. For this, we add a +page.js (or +page.ts, if you're TypeScript-inclined) module that exports a load function:

ts
import { error } from '@sveltejs/kit';
 
/** @type {import('./$types').PageLoad} */
export function load({ params }) {
  if (params.slug === 'hello-world') {
    return {
      title: 'Hello world!',
      content: 'Welcome to our blog. Lorem ipsum dolor sit amet...'
    };
  }
 
  throw error(404, 'Not found');
}

ts
import { error } from '@sveltejs/kit';
import type { PageLoad } from './$types';
 
export const load: PageLoad = ({ params }) => {
  if (params.slug === 'hello-world') {
    return {
      title: 'Hello world!',
      content: 'Welcome to our blog. Lorem ipsum dolor sit amet...'
    };
  }
 
  throw 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.jspermalink

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.

ts
import { error } from '@sveltejs/kit';
 
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
  const post = await getPostFromDatabase(params.slug);
 
  if (post) {
    return post;
  }
 
  throw error(404, 'Not found');
}

ts
import { error } from '@sveltejs/kit';
import type { PageServerLoad } from './$types';
 
export const load: PageServerLoad = async ({ params }) => {
  const post = await getPostFromDatabase(params.slug);
 
  if (post) {
    return post;
  }
 
  throw 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.

+errorpermalink

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:

<script>
  import { page } from '$app/stores';
</script>

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

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 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.

+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.

+layoutpermalink

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.sveltepermalink

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...

<slot></slot>

...but we can add whatever markup, styles and behaviour we want. The only requirement is that the component includes a <slot> for the page content. For example, let's add a nav bar:

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

<slot></slot>

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

<h1>Home</h1>

<h1>About</h1>

<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):

<script>
  /** @type {import('./$types').LayoutData} */  export let data;
</script>

<h1>Settings</h1>

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

<slot></slot>

<script lang="ts">
  import type { LayoutData } from './$types';

  export let data: LayoutData;
</script>

<h1>Settings</h1>

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

<slot></slot>

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

+layout.jspermalink

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

ts
/** @type {import('./$types').LayoutLoad} */
export function load() {
  return {
    sections: [
      { slug: 'profile', title: 'Profile' },
      { slug: 'notifications', title: 'Notifications' }
    ]
  };
}

ts
import type { LayoutLoad } from './$types';
 
export const load: LayoutLoad = () => {
  return {
    sections: [
      { slug: 'profile', title: 'Profile' },
      { slug: 'notifications', title: '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:

<script>
  /** @type {import('./$types').PageData} */  export let data;

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

<script lang="ts">
  import type { PageData } from './$types';

  export let data: PageData;

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

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

+layout.server.jspermalink

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.

+serverpermalink

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 (or +server.ts) exports functions corresponding to HTTP verbs like GET, POST, PATCH, PUT and DELETE that take a RequestEvent argument and return a Response object.

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

ts
import { error } from '@sveltejs/kit';
 
/** @type {import('./$types').RequestHandler} */
export function GET({ url }) {
  const min = Number(url.searchParams.get('min') ?? '0');
  const max = Number(url.searchParams.get('max') ?? '1');
 
  const d = max - min;
 
  if (isNaN(d) || d < 0) {
    throw error(400, 'min and max must be numbers, and min must be less than max');
  }
 
  const random = min + Math.random() * d;
 
  return new Response(String(random));
}

ts
import { error } from '@sveltejs/kit';
import type { RequestHandler } from './$types';
 
export const GET: RequestHandler = ({ url }) => {
  const min = Number(url.searchParams.get('min') ?? '0');
  const max = Number(url.searchParams.get('max') ?? '1');
 
  const d = max - min;
 
  if (isNaN(d) || d < 0) {
    throw error(400, 'min and max must be numbers, and min must be less than max');
  }
 
  const random = min + Math.random() * d;
 
  return new Response(String(random));
}

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 throw 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.

Receiving datapermalink

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

<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 on:click={add}>Calculate</button>

ts
import { json } from '@sveltejs/kit';
 
/** @type {import('./$types').RequestHandler} */
export async function POST({ request }) {
  const { a, b } = await request.json();
  return json(a + b);
}

ts
import { json } from '@sveltejs/kit';
import type { RequestHandler } from './$types';
 
export const POST: RequestHandler = async ({ request }) => {
  const { a, b } = await request.json();
  return json(a + b);
}

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

Content negotiationpermalink

+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 requests are always handled by +server.js since they do not apply to pages
GET/POST 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

$typespermalink

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 export let data with PageData (or LayoutData, for a +layout.svelte file) tells TypeScript that the type of data is whatever was returned from load:

<script>
  /** @type {import('./$types').PageData} */  export let data;
</script>

<script lang="ts">
  import type { PageData } from './$types';

  export let data: PageData;
</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.

Other filespermalink

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.

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