$app/state

import { 
const navigating: Navigation | {
    from: null;
    to: null;
    type: null;
    willUnload: null;
    delta: null;
    complete: null;
}
A read-only object representing an in-progress navigation, with from, to, type and (if type === 'popstate') delta properties.
Values are null when no navigation is occurring, or during server rendering.
referencenavigating, const page: Page<Record<string, string>, string | null>
A read-only reactive object with information about the current page, serving several use cases:

retrieving the combined data of all pages/layouts anywhere in your component tree (also see loading data)
retrieving the current value of the form prop anywhere in your component tree (also see form actions)
retrieving the page state that was set through goto, pushState or replaceState (also see goto and shallow routing)
retrieving metadata such as the URL you're on, the current route and its parameters, and whether or not there was an error

+layout
<script>
	import { page } from '$app/state';
</script>

<p>Currently at {page.url.pathname}</p>

{#if page.error}
	<span class="red">Problem detected</span>
{:else}
	<span class="small">All systems operational</span>
{/if}
<script lang="ts">
	import { page } from '$app/state';
</script>

<p>Currently at {page.url.pathname}</p>

{#if page.error}
	<span class="red">Problem detected</span>
{:else}
	<span class="small">All systems operational</span>
{/if}
Changes to page are available exclusively with runes. (The legacy reactivity syntax will not reflect any changes)
+page
<script>
	import { page } from '$app/state';
	const id = $derived(page.params.id); // This will correctly update id for usage on this page
	$: badId = page.params.id; // Do not use; will never update after initial load
</script>
<script lang="ts">
	import { page } from '$app/state';
	const id = $derived(page.params.id); // This will correctly update id for usage on this page
	$: badId = page.params.id; // Do not use; will never update after initial load
</script>
On the server, values can only be read during rendering (in other words not in e.g. load functions). In the browser, the values can be read at any time.
referencepage, 
const updated: {
    readonly current: boolean;
    check(): Promise<boolean>;
}
A read-only reactive value that's initially false. If version.pollInterval is a non-zero value, SvelteKit will poll for new versions of the app and update current to true when it detects one. updated.check() will force an immediate check, regardless of polling.
referenceupdated } from '$app/state';

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

<p>Currently at {page.url.pathname}</p>

{#if page.error}
	<span class="red">Problem detected</span>
{:else}
	<span class="small">All systems operational</span>
{/if}

<script lang="ts">
	import { page } from '$app/state';
</script>

<p>Currently at {page.url.pathname}</p>

{#if page.error}
	<span class="red">Problem detected</span>
{:else}
	<span class="small">All systems operational</span>
{/if}

<script>
	import { page } from '$app/state';
	const id = $derived(page.params.id); // This will correctly update id for usage on this page
	$: badId = page.params.id; // Do not use; will never update after initial load
</script>

<script lang="ts">
	import { page } from '$app/state';
	const id = $derived(page.params.id); // This will correctly update id for usage on this page
	$: badId = page.params.id; // Do not use; will never update after initial load
</script>