fix: serve prerendered pages when paths.base is set on Vercel

.changeset/tidy-bases-serve.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/adapter-vercel': patch
+---
+
+fix: serve prerendered pages when `paths.base` is set

packages/adapter-vercel/index.js

@@ -400,6 +400,10 @@ function static_vercel_config(builder, config, dir) {
 	/** @type {Record<string, { path: string }>} */
 	const overrides = {};
 
+	// prerendered files are written to `static${base}`, but their paths are relative to `base`,
+	// whereas the keys of `overrides` must be relative to `static`
+	const static_prefix = builder.config.paths.base ? `${builder.config.paths.base.slice(1)}/` : '';
+
 	/** @type {import('./index.js').ImagesConfig | undefined} */
 	const images = config.images;
 
@@ -441,7 +445,7 @@ function static_vercel_config(builder, config, dir) {
 			);
 		}
 
-		overrides[page.file] = { path: overrides_path };
+		overrides[static_prefix + page.file] = { path: overrides_path };
 	}
 
 	const routes = [

packages/adapter-vercel/test/apps/paths-base/build.test.js

@@ -0,0 +1,24 @@
+import fs from 'node:fs';
+import path from 'node:path';
+import { assert, beforeAll, test } from 'vitest';
+import { deploy } from '../../../../../test-utils/deploy.vitest.js';
+
+const app = deploy(import.meta.dirname);
+const output = path.join(app, '.vercel/output');
+
+/** @type {{ overrides: Record<string, { path: string }> }} */
+let config;
+beforeAll(() => (config = JSON.parse(fs.readFileSync(`${output}/config.json`, 'utf8'))));
+
+test('prerendered pages are written below the base path', () => {
+	assert.deepEqual(config.overrides, {
+		'base/index.html': { path: 'base' },
+		'base/prerendered/a.html': { path: 'base/prerendered/a' }
+	});
+});
+
+test('overrides refer to files that exist', () => {
+	for (const file of Object.keys(config.overrides)) {
+		assert.ok(fs.existsSync(`${output}/static/${file}`), `${file} does not exist`);
+	}
+});

packages/adapter-vercel/test/apps/paths-base/package.json

@@ -0,0 +1,21 @@
+{
+	"name": "test-vercel-paths-base",
+	"version": "0.0.1",
+	"private": true,
+	"scripts": {
+		"dev": "vite dev",
+		"preview": "vite preview",
+		"build": "vite build",
+		"test:build": "vitest run build.test.js"
+	},
+	"devDependencies": {
+		"@sveltejs/adapter-vercel": "workspace:*",
+		"@sveltejs/kit": "workspace:*",
+		"@sveltejs/vite-plugin-svelte": "catalog:",
+		"svelte": "catalog:",
+		"typescript": "catalog:",
+		"vite": "catalog:",
+		"vitest": "catalog:"
+	},
+	"type": "module"
+}

packages/adapter-vercel/test/apps/paths-base/src/app.d.ts

@@ -0,0 +1,13 @@
+// See https://svelte.dev/docs/kit/types#app.d.ts
+// for information about these interfaces
+declare global {
+	namespace App {
+		// interface Error {}
+		// interface Locals {}
+		// interface PageData {}
+		// interface PageState {}
+		// interface Platform {}
+	}
+}
+
+export {};

packages/adapter-vercel/test/apps/paths-base/src/app.html

@@ -0,0 +1,11 @@
+<!doctype html>
+<html lang="en">
+	<head>
+		<meta charset="utf-8" />
+		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		%sveltekit.head%
+	</head>
+	<body data-sveltekit-preload-data="hover">
+		<div style="display: contents">%sveltekit.body%</div>
+	</body>
+</html>

packages/adapter-vercel/test/apps/paths-base/src/routes/+page.js

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

packages/adapter-vercel/test/apps/paths-base/src/routes/+page.svelte

@@ -0,0 +1 @@
+<h1>Hello from SvelteKit on Vercel</h1>

packages/adapter-vercel/test/apps/paths-base/src/routes/prerendered/[slug]/+page.server.ts

@@ -0,0 +1,9 @@
+export const prerender = true;
+
+export function entries() {
+	return [{ slug: 'a' }];
+}
+
+export function load({ params }) {
+	return { slug: params.slug };
+}

packages/adapter-vercel/test/apps/paths-base/src/routes/prerendered/[slug]/+page.svelte

@@ -0,0 +1,5 @@
+<script lang="ts">
+	let { data } = $props();
+</script>
+
+<h1>prerendered: {data.slug}</h1>

packages/adapter-vercel/test/apps/paths-base/tsconfig.json

@@ -0,0 +1,15 @@
+{
+	"extends": "$app/tsconfig",
+	"compilerOptions": {
+		"rewriteRelativeImportExtensions": true,
+		"allowJs": true,
+		"checkJs": true,
+		"esModuleInterop": true,
+		"forceConsistentCasingInFileNames": true,
+		"resolveJsonModule": true,
+		"skipLibCheck": true,
+		"sourceMap": true,
+		"moduleResolution": "bundler"
+	},
+	"include": ["src", "build.test.js", "vite.config.ts"]
+}

packages/adapter-vercel/test/apps/paths-base/vite.config.ts

@@ -0,0 +1,12 @@
+import { sveltekit } from '@sveltejs/kit/vite';
+import { defineConfig } from 'vite';
+import adapter from '@sveltejs/adapter-vercel';
+
+export default defineConfig({
+	plugins: [
+		sveltekit({
+			paths: { base: '/base' },
+			adapter: adapter()
+		})
+	]
+});