Cannot access url.searchParams Prerendering - SvelteKit

Here is a comment from Rich explaining why search parameters cannot be used during the pre-rendering:

When you prerender, you're writing files to the filesystem so that a simple webserver can serve them. Query parameters can't exist on the filesystem (both in the literal sense that ? isn't an allowed character on all filesystems, and in the sense that the webserver will ignore them), so if you have a prerendered endpoint called /api/posts/get then it doesn't matter how many times you call it with different parameters, it can only be saved as a file once. The files need to have different paths (/api/posts/[slug]), or it won't work.

A way around this is to use beforeUpdate:

<script>
  let myParameter = null;
  beforeUpdate(() => {
    myParameter =  url.searchParams.get('my-parameter');
  });
</script>

<div>
  <!-- Use `myParameter` -->
</div>

Accessing url.searchParams during prerendering is forbidden - on the server. As written in the documentation, you can get around this by retriving the parameters in the client, using onMount():

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

let page;

onMount(() => {
  params = url.searchParams.get('page');
})
</script>

If you run into this error for adapter-static / SSG, you can detect the build process via building and avoid accessing page.url.searchParams while building, Svelte v5 and SvelteKit v2:

<script>
import { building } from '$app/environment';

let results = $derived.by(() => {
    const searchText = !building ? page.url.searchParams.get('searchText') || "" : "";

Thanks very much to Scott (Svelte Discord) and Pablopang.svelte.