Shallow routing

As you navigate around a SvelteKit app, you create history entries. Clicking the back and forward buttons traverses through this list of entries, re-running any load functions and replacing page components as necessary.

Sometimes, it's useful to create history entries without navigating. For example, you might want to show a modal dialog that the user can dismiss by navigating back. This is particularly valuable on mobile devices, where swipe gestures are often more natural than interacting directly with the UI. In these cases, a modal that is not associated with a history entry can be a source of frustration, as a user may swipe backwards in an attempt to dismiss it and find themselves on the wrong page.

SvelteKit makes this possible with the pushState and replaceState functions, which allow you to associate state with a history entry without navigating. For example, to implement a history-driven modal:

<script>
	import { pushState } from '$app/navigation';
	import { page } from '$app/stores';
	import Modal from './Modal.svelte';

	function showModal() {
		pushState('', {
			showModal: true
		});
	}
</script>

{#if $page.state.showModal}
	<Modal close={() => history.back()} />
{/if}

<script lang="ts">
	import { pushState } from '$app/navigation';
	import { page } from '$app/stores';
	import Modal from './Modal.svelte';
	
	function showModal() {
		pushState('', {
			showModal: true,
		});
	}
</script>

{#if $page.state.showModal}
	<Modal close={() => history.back()} />
{/if}

The modal can be dismissed by navigating back (unsetting $page.state.showModal) or by interacting with it in a way that causes the close callback to run, which will navigate back programmatically.

APIpermalink

The first argument to pushState is the URL, relative to the current URL. To stay on the current URL, use ''.

The second argument is the new page state, which can be accessed via the page store as $page.state. You can make page state type-safe by declaring an App.PageState interface (usually in src/app.d.ts).

To set page state without creating a new history entry, use replaceState instead of pushState.

Loading data for a routepermalink

When shallow routing, you may want to render another +page.svelte inside the current page. For example, clicking on a photo thumbnail could pop up the detail view without navigating to the photo page.

For this to work, you need to load the data that the +page.svelte expects. A convenient way to do this is to use preloadData inside the click handler of an <a> element. If the element (or a parent) uses data-sveltekit-preload-data, the data will have already been requested, and preloadData will reuse that request.

<script>
	import { preloadData, pushState, goto } from '$app/navigation';
	import Modal from './Modal.svelte';
	import PhotoPage from './[id]/+page.svelte';

	export let data;
</script>

{#each data.thumbnails as thumbnail}
	<a
		href="/photos/{thumbnail.id}"
		on:click={async (e) => {
			// bail if opening a new tab, or we're on too small a screen
			if (e.metaKey || innerWidth < 640) return;

			// prevent navigation
			e.preventDefault();

			const { href } = e.currentTarget;

			// run `load` functions (or rather, get the result of the `load` functions
			// that are already running because of `data-sveltekit-preload-data`)
			const result = await preloadData(href);

			if (result.type === 'loaded' && result.status === 200) {
				pushState(href, { selected: result.data });
			} else {
				// something bad happened! try navigating
				goto(href);
			}
		}}
	>
		<img alt={thumbnail.alt} src={thumbnail.src} />
	</a>
{/each}

{#if $page.state.selected}
	<Modal on:close={() => history.goBack()}>
		<!-- pass page data to the +page.svelte component,
			 just like SvelteKit would on navigation -->
		<PhotoPage data={$page.state.selected} />
	</Modal>
{/if}

<script lang="ts">
	import { preloadData, pushState, goto } from '$app/navigation';
	import Modal from './Modal.svelte';
	import PhotoPage from './[id]/+page.svelte';
	
	export let data;
</script>

{#each data.thumbnails as thumbnail}
	<a
		href="/photos/{thumbnail.id}"
		on:click={async (e) => {
			// bail if opening a new tab, or we're on too small a screen
			if (e.metaKey || innerWidth < 640) return;

			// prevent navigation
			e.preventDefault();

			const { href } = e.currentTarget;

			// run `load` functions (or rather, get the result of the `load` functions
			// that are already running because of `data-sveltekit-preload-data`)
			const result = await preloadData(href);

			if (result.type === 'loaded' && result.status === 200) {
				pushState(href, { selected: result.data });
			} else {
				// something bad happened! try navigating
				goto(href);
			}
		}}
	>
		<img alt={thumbnail.alt} src={thumbnail.src} />
	</a>
{/each}

{#if $page.state.selected}
	<Modal on:close={() => history.goBack()}>
		<!-- pass page data to the +page.svelte component,
			 just like SvelteKit would on navigation -->
		<PhotoPage data={$page.state.selected} />
	</Modal>
{/if}

Caveatspermalink

During server-side rendering, $page.state is always an empty object. The same is true for the first page the user lands on — if the user reloads the page, state will not be applied until they navigate.

Shallow routing is a feature that requires JavaScript to work. Be mindful when using it and try to think of sensible fallback behavior in case JavaScript isn't available.