State management

If you're used to building client-only apps, state management in an app that spans server and client might seem intimidating. This section provides tips for avoiding some common gotchas.

Avoid shared state on the serverpermalink

Browsers are stateful — state is stored in memory as the user interacts with the application. Servers, on the other hand, are stateless — the content of the response is determined entirely by the content of the request.

Conceptually, that is. In reality, servers are often long-lived and shared by multiple users. For that reason it's important not to store data in shared variables. For example, consider this code:

ts
let user;

/** @type {import('./$types').PageServerLoad} */
export function load() {
	return { user };
}

/** @type {import('./$types').Actions} */
export const actions = {
	default: async ({ request }) => {
		const data = await request.formData();

		// NEVER DO THIS!
		user = {
			name: data.get('name'),
			embarrassingSecret: data.get('secret')
		};
	}
}

ts
import type { PageServerLoad, Actions } from './$types';
let user;

export const load: PageServerLoad = () => {
	return { user };
};

export const actions = {
	default: async ({ request }) => {
		const data = await request.formData();

		// NEVER DO THIS!
		user = {
			name: data.get('name'),
			embarrassingSecret: data.get('secret'),
		};
	},
} satisfies Actions;

The user variable is shared by everyone who connects to this server. If Alice submitted an embarrassing secret, and Bob visited the page after her, Bob would know Alice's secret. In addition, when Alice returns to the site later in the day, the server may have restarted, losing her data.

Instead, you should authenticate the user using cookies and persist the data to a database.

No side-effects in loadpermalink

For the same reason, your load functions should be pure — no side-effects (except maybe the occasional console.log(...)). For example, you might be tempted to write to a store inside a load function so that you can use the store value in your components:

ts
import { user } from '$lib/user';

/** @type {import('./$types').PageLoad} */
export async function load({ fetch }) {
	const response = await fetch('/api/user');

	// NEVER DO THIS!
	user.set(await response.json());
}

ts
import { user } from '$lib/user';
import type { PageLoad } from './$types';

export const load: PageLoad = async ({ fetch }) => {
	const response = await fetch('/api/user');

	// NEVER DO THIS!
	user.set(await response.json());
};

As with the previous example, this puts one user's information in a place that is shared by all users. Instead, just return the data...

export async function load({ fetch }) {
	const response = await fetch('/api/user');

	return {
		user: await response.json()
	};
}

...and pass it around to the components that need it, or use $page.data.

If you're not using SSR, then there's no risk of accidentally exposing one user's data to another. But you should still avoid side-effects in your load functions — your application will be much easier to reason about without them.

Using stores with contextpermalink

You might wonder how we're able to use $page.data and other app stores if we can't use our own stores. The answer is that app stores on the server use Svelte's context API — the store is attached to the component tree with setContext, and when you subscribe you retrieve it with getContext. We can do the same thing with our own stores:

<script>
	import { setContext } from 'svelte';
	import { writable } from 'svelte/store';

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

	// Create a store and update it when necessary...
	const user = writable();
	$: user.set(data.user);

	// ...and add it to the context for child components to access
	setContext('user', user);
</script>

<script lang="ts">
	import { setContext } from 'svelte';
	import { writable } from 'svelte/store';
	
	import type { LayoutData } from './$types';
	
	export let data: LayoutData;
	
	// Create a store and update it when necessary...
	const user = writable();
	$: user.set(data.user);
	
	// ...and add it to the context for child components to access
	setContext('user', user);
</script>

<script>
	import { getContext } from 'svelte';

	// Retrieve user store from context
	const user = getContext('user');
</script>

<p>Welcome {$user.name}</p>

<script lang="ts">
	import { getContext } from 'svelte';
	
	// Retrieve user store from context
	const user = getContext('user');
</script>

<p>Welcome {$user.name}</p>

Updating the value of a context-based store in deeper-level pages or components while the page is being rendered via SSR will not affect the value in the parent component because it has already been rendered by the time the store value is updated. In contrast, on the client (when CSR is enabled, which is the default) the value will be propagated and components, pages, and layouts higher in the hierarchy will react to the new value. Therefore, to avoid values 'flashing' during state updates during hydration, it is generally recommended to pass state down into components rather than up.

If you're not using SSR (and can guarantee that you won't need to use SSR in future) then you can safely keep state in a shared module, without using the context API.

Component and page state is preservedpermalink

When you navigate around your application, SvelteKit reuses existing layout and page components. For example, if you have a route like this...

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

	// THIS CODE IS BUGGY!
	const wordCount = data.content.split(' ').length;
	const estimatedReadingTime = wordCount / 250;
</script>

<header>
	<h1>{data.title}</h1>
	<p>Reading time: {Math.round(estimatedReadingTime)} minutes</p>
</header>

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

<script lang="ts">
	import type { PageData } from './$types';
	
	export let data: PageData;
	
	// THIS CODE IS BUGGY!
	const wordCount = data.content.split(' ').length;
	const estimatedReadingTime = wordCount / 250;
</script>

<header>
	<h1>{data.title}</h1>
	<p>Reading time: {Math.round(estimatedReadingTime)} minutes</p>
</header>

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

...then navigating from /blog/my-short-post to /blog/my-long-post won't cause the layout, page and any other components within to be destroyed and recreated. Instead the data prop (and by extension data.title and data.content) will update (as it would with any other Svelte component) and, because the code isn't rerunning, lifecycle methods like onMount and onDestroy won't rerun and estimatedReadingTime won't be recalculated.

Instead, we need to make the value reactive:

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

	$: wordCount = data.content.split(' ').length;
	$: estimatedReadingTime = wordCount / 250;
</script>

If your code in onMount and onDestroy has to run again after navigation you can use afterNavigate and beforeNavigate respectively.

Reusing components like this means that things like sidebar scroll state are preserved, and you can easily animate between changing values. In the case that you do need to completely destroy and remount a component on navigation, you can use this pattern:

{#key $page.url.pathname}
	<BlogPost title={data.title} content={data.title} />
{/key}

Storing state in the URLpermalink

If you have state that should survive a reload and/or affect SSR, such as filters or sorting rules on a table, URL search parameters (like ?sort=price&order=ascending) are a good place to put them. You can put them in <a href="..."> or <form action="..."> attributes, or set them programmatically via goto('?key=value'). They can be accessed inside load functions via the url parameter, and inside components via $page.url.searchParams.

Storing ephemeral state in snapshotspermalink

Some UI state, such as 'is the accordion open?', is disposable — if the user navigates away or refreshes the page, it doesn't matter if the state is lost. In some cases, you do want the data to persist if the user navigates to a different page and comes back, but storing the state in the URL or in a database would be overkill. For this, SvelteKit provides snapshots, which let you associate component state with a history entry.