fix: ensure version is defined when using $app/env with explicit env vars

.changeset/wide-otters-shout.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: ensure `version` is defined when importing from `$app/env` with explicit environment variables

packages/kit/src/runtime/app/environment/index.js

@@ -5,6 +5,3 @@ if (__SVELTEKIT_EXPERIMENTAL_EXPLICIT_ENVIRONMENT_VARIABLES__) {
 		'Cannot import `$app/environment` when `experimental.explicitEnvironmentVariables` is enabled. Use `$app/env` instead.'
 	);
 }
-
-// force the Vite client to load, so that defines are definitely defined
-import.meta.hot;

packages/kit/src/runtime/server/page/render.js

@@ -634,8 +634,11 @@ export async function render_response({
 					}`);
 		}
 
+		// we need to eagerly import the Vite client module in development to ensure
+		// that Vite global constant replacements are initialised before our code runs
 		const init_app = `
 				{
+					${DEV ? `import('${paths.base}/@vite/client')` : ''}
 					${blocks.join('\n\n\t\t\t\t\t')}
 				}
 			`;

packages/kit/test/apps/options-2/src/hooks.client.js

@@ -0,0 +1,6 @@
+import { version } from '$app/env';
+
+// Regression guard for #15971: this import runs before the router is initialized.
+// Without the fix it throws `__SVELTEKIT_APP_VERSION__ is not defined` when
+// `experimental.explicitEnvironmentVariables` is enabled. `void` keeps the import.
+void version;

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

@@ -184,3 +184,19 @@ test.describe("bundleStrategy: 'single'", () => {
 		await expect(page.locator('h1', { hasText: 'It works!' })).toBeVisible();
 	});
 });
+
+test.describe('$app/env', () => {
+	// regression test for https://github.com/sveltejs/kit/issues/15971:
+	// importing `$app/env` before the router is initialized (e.g. in
+	// hooks.client.js) must not throw `__SVELTEKIT_APP_VERSION__ is not defined`
+	test('version is defined when imported during client init', async ({
+		page,
+		javaScriptEnabled
+	}) => {
+		test.skip(!javaScriptEnabled || !process.env.DEV);
+
+		await page.goto('/basepath', { wait_for_started: false });
+		await expect(page.locator('body.started')).toBeVisible();
+		expect(await page.pageErrors()).toHaveLength(0);
+	});
+});