How to redirect to a URL in Sapper

I was working on a Svelte+Sapper application when I had the need to redirect to a page, with URL /spreadsheet/1, when the user visited the root domain /, instead of showing the home page.

So I opened src/routes/index.svelte, removed everything in that file, and I added this code:

<script context="module">
  export async function preload(page, session) {
    return this.redirect(301, 'spreadsheet/1')
  }
</script>