expose base via $service-worker, make paths relative

[Rich-Harris]

• closes Expose paths to $service-worker module #4714
• closes feat: expose paths to service worker #8472

This adds a base export to $service-worker:

import { base } from '$service-worker';

Rather than being hardcoded from the paths.base in svelte.config.js, this is computed when the service worker boots up. This means that if the app is deployed somewhere where we don't know the base path (IPFS being the classic example), it will continue to work.

It also modifies the existing build, files and prerendered exports to use base instead of having the hardcoded prefix.

Note that there's no assets here, because you can't use a service worker alongside assets (since a service worker can only control requests for the same domain/path prefix, i.e. https://cdn.example.com/my-app/service-worker.js can't handle requests for your app).

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[dummdidumm]

Having a hard time understanding how/why this works. If you you visit example.com/foo/bar wouldn't the base be /foo, even if I didn't set a base path?

[Rich-Harris]

In a service worker, location is the location of the service worker, not the URL you're currently visiting. Because SvelteKit guarantees that the service worker is at the root of the app (because a service worker can only handle requests at the same level or below), location.pathname.split('/').slice(0, -1).join('/') is guaranteed to be the base path

[dummdidumm]

Thanks for clearing it up!

[benmccann]

this might be worth a comment?