Snapshots

Ephemeral DOM state — like scroll positions on sidebars, the content of <input> elements and so on — is discarded when you navigate from one page to another.

For example, if the user fills out a form but clicks a link before submitting, then hits the browser's back button, the values they filled in will be lost. In cases where it's valuable to preserve that input, you can take a snapshot of DOM state, which can then be restored if the user navigates back.

To do this, export a snapshot object with capture and restore methods from a +page.svelte or +layout.svelte:

<script>
    let comment = '';

    /** @type {import('./$types').Snapshot<string>} */
    export const snapshot = {
        capture: () => comment,
        restore: (value) => comment = value
    };
</script>

<form method="POST">
    <label for="comment">Comment</label>
    <textarea id="comment" bind:value={comment} />
    <button>Post comment</button>
</form>

<script lang="ts">
    import type { Snapshot } from './$types';
    
    let comment = '';
    
    export const snapshot: Snapshotstring = {
        capture: () => comment,
        restore: (value) => (comment = value),
    };
</script>

<form method="POST">
    <label for="comment">Comment</label>
    <textarea id="comment" bind:value={comment} />
    <button>Post comment</button>
</form>

When you navigate away from this page, the capture function is called immediately before the page updates, and the returned value is associated with the current entry in the browser's history stack. If you navigate back, the restore function is called with the stored value as soon as the page is updated.

The data must be serializable as JSON so that it can be persisted to sessionStorage. This allows the state to be restored when the page is reloaded, or when the user navigates back from a different site.

Avoid returning very large objects from capture — once captured, objects will be retained in memory for the duration of the session, and in extreme cases may be too large to persist to sessionStorage.