svelte/documentation/docs/04-runtime/02-context.md

title
Context

• get/set/hasContext
• how to use, best practises (like encapsulating them)

Most state is component-level state that lives as long as its component lives. There's also section-wide or app-wide state however, which also needs to be handled somehow.

The easiest way to do that is to create global state and just import that.

/// file: state.svelte.ts

export const myGlobalState = $state({
	user: {
		/* ... */
	}
	/* ... */
});

<!--- file: App.svelte --->
<script>
	import { myGlobalState } from './state.svelte';
	// ...
</script>

This has a few drawbacks though:

• it only safely works when your global state is only used client-side - for example, when you're building a single page application that does not render any of your components on the server. If your state ends up being managed and updated on the server, it could end up being shared between sessions and/or users, causing bugs
• it may give the false impression that certain state is global when in reality it should only used in a certain part of your app

To solve these drawbacks, Svelte provides a few context primitives which alleviate these problems.

Setting and getting context

To associate an arbitrary object with the current component, use setContext.

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

	setContext('key', value);
</script>

The context is then available to children of the component (including slotted content) with getContext.

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

	const value = getContext('key');
</script>

setContext and getContext solve the above problems:

• the state is not global, it's scoped to the component. That way it's safe to render your components on the server and not leak state
• it's clear that the state is not global but rather scoped to a specific component tree and therefore can't be used in other parts of your app

setContext/getContext must be called during component initialisation.

Context is not inherently reactive. If you need reactive values in context then you can pass a $state object into context, whos properties will be reactive.

<!--- file: Parent.svelte --->
<script>
	import { setContext } from 'svelte';

	let value = $state({ count: 0 });
	setContext('counter', value);
</script>

<button onclick={() => value.count++}>increment</button>

<!--- file: Child.svelte --->
<script>
	import { setContext } from 'svelte';

	const value = setContext('counter');
</script>

<p>Count is {value.count}</p>

To check whether a given key has been set in the context of a parent component, use hasContext.

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

	if (hasContext('key')) {
		// do something
	}
</script>

You can also retrieve the whole context map that belongs to the closest parent component using getAllContexts. This is useful, for example, if you programmatically create a component and want to pass the existing context to it.

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

	const contexts = getAllContexts();
</script>

Encapsulating context interactions

The above methods are very unopionated about how to use them. When your app grows in scale, it's worthwhile to encapsulate setting and getting the context into functions and properly type them.

// @errors: 2304
import { getContext, setContext } from 'svelte';

let userKey = Symbol('user');

export function setUserContext(user: User) {
	setContext(userKey, user);
}

export function getUserContext(): User {
	return getContext(userKey) as User;
}