feat: expose base via $service-worker, make paths relative (#9250)

closes #4714

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>
Co-authored-by: Ben McCann <322311+benmccann@users.noreply.github.com>

.changeset/great-toes-wash.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': minor
+---
+
+feat: expose `base` via `$service-worker`, make paths relative

documentation/docs/30-advanced/40-service-workers.md

@@ -18,7 +18,7 @@ if ('serviceWorker' in navigator) {
 
 ## Inside the service worker
 
-Inside the service worker you have access to the [`$service-worker` module](modules#$service-worker), which provides you with the paths to all static assets, build files and prerendered pages. You're also provided with an app version string which you can use for creating a unique cache name. If your Vite config specifies `define` (used for global variable replacements), this will be applied to service workers as well as your server/client builds.
+Inside the service worker you have access to the [`$service-worker` module](modules#$service-worker), which provides you with the paths to all static assets, build files and prerendered pages. You're also provided with an app version string, which you can use for creating a unique cache name, and the deployment's `base` path. If your Vite config specifies `define` (used for global variable replacements), this will be applied to service workers as well as your server/client builds.
 
 The following example caches the built app and any files in `static` eagerly, and caches all other requests as they happen. This would make each page work offline once visited.
 

packages/kit/src/exports/vite/build/build_service_worker.js

@@ -33,24 +33,30 @@ export async function build_service_worker(
 
 	const service_worker = `${kit.outDir}/generated/service-worker.js`;
 
+	// in a service worker, `location` is the location of the service worker itself,
+	// which is guaranteed to be `<base>/service-worker.js`
+	const base = `location.pathname.split('/').slice(0, -1).join('/')`;
+
 	fs.writeFileSync(
 		service_worker,
 		dedent`
+			export const base = /*@__PURE__*/ ${base};
+
 			export const build = [
 				${Array.from(build)
-					.map((file) => `${s(`${kit.paths.base}/${file}`)}`)
+					.map((file) => `base + ${s(`/${file}`)}`)
 					.join(',\n')}
 			];
 
 			export const files = [
 				${manifest_data.assets
 					.filter((asset) => kit.serviceWorker.files(asset.file))
-					.map((asset) => `${s(`${kit.paths.base}/${asset.file}`)}`)
+					.map((asset) => `base + ${s(`/${asset.file}`)}`)
 					.join(',\n')}
 			];
 
 			export const prerendered = [
-				${prerendered.paths.map((path) => s(path)).join(',\n')}
+				${prerendered.paths.map((path) => `base + ${s(path.replace(kit.paths.base, ''))}`).join(',\n')}
 			];
 
 			export const version = ${s(kit.version.name)};

packages/kit/test/apps/options-2/src/routes/hello/+page.js

@@ -0,0 +1 @@
+export const prerender = true;

packages/kit/test/apps/options-2/src/routes/hello/+page.svelte

@@ -0,0 +1 @@
+<h1>Prerendered</h1>

packages/kit/test/apps/options-2/src/service-worker.js

@@ -1,4 +1,7 @@
-import { build, version } from '$service-worker';
+import { base, build, files, prerendered, version } from '$service-worker';
+
+self.base = base;
+self.build = build;
 
 const name = `cache-${version}`;
 

packages/kit/test/apps/options-2/test/test.js

@@ -46,7 +46,20 @@ test.describe('Service worker', () => {
 		const response = await request.get('/basepath/service-worker.js');
 		const content = await response.text();
 
-		expect(content).toMatch(/\/_app\/immutable\/entry\/start\.[a-z0-9]+\.js/);
+		const fn = new Function('self', 'location', content);
+
+		const self = {
+			addEventListener: () => {},
+			base: null,
+			build: null
+		};
+
+		fn(self, {
+			pathname: '/basepath/service-worker.js'
+		});
+
+		expect(self.base).toBe('/basepath');
+		expect(self.build[0]).toMatch(/\/basepath\/_app\/immutable\/entry\/start\.[a-z0-9]+\.js/);
 	});
 
 	test('does not register /basepath/service-worker.js', async ({ page }) => {

packages/kit/types/ambient.d.ts

@@ -326,6 +326,11 @@ declare module '$app/stores' {
  * This module is only available to [service workers](https://kit.svelte.dev/docs/service-workers).
  */
 declare module '$service-worker' {
+	/**
+	 * The `base` path of the deployment. Typically this is equivalent to `config.kit.paths.base`, but it is calculated from `location.pathname` meaning that it will continue to work correctly if the site is deployed to a subdirectory.
+	 * Note that there is a `base` but no `assets`, since service workers cannot be used if `config.kit.paths.assets` is specified.
+	 */
+	export const base: string;
 	/**
 	 * An array of URL strings representing the files generated by Vite, suitable for caching with `cache.addAll(build)`.
 	 * During development, this is an empty array.