fix: correctly access app.decoders in inline and single output apps

.changeset/stupid-singers-talk.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly access transport decoders on the client when building for a single or inline output app

packages/kit/src/runtime/app/forms.js

@@ -6,8 +6,6 @@ import { app as server_app } from '../server/app.js';
 
 export { applyAction };
 
-const decoders = BROWSER ? client_app.decoders : server_app?.decoders;
-
 /**
  * Use this function to deserialize the response from a form submission.
  * Usage:
@@ -34,7 +32,9 @@ export function deserialize(result) {
 	const parsed = JSON.parse(result);
 
 	if (parsed.data) {
-		parsed.data = devalue.parse(parsed.data, decoders);
+		// the decoders should never be initialised at the top-level because `app`
+		// will not initialised yet if `kit.output.bundleStrategy` is 'single' or 'inline'
+		parsed.data = devalue.parse(parsed.data, BROWSER ? client_app.decoders : server_app.decoders);
 	}
 
 	return parsed;

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

@@ -0,0 +1,5 @@
+<script>
+	export let data;
+</script>
+
+<p>{data.data.text}</p>

packages/kit/test/apps/options-2/src/routes/deserialize/+page.ts

@@ -0,0 +1,15 @@
+import { deserialize } from '$app/forms';
+
+export const ssr = false;
+
+export function load() {
+	const result = deserialize(
+		JSON.stringify({
+			type: 'success',
+			status: 200,
+			data: '[{"text":1},"Hello world!"]'
+		})
+	);
+
+	return result;
+}

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

@@ -108,7 +108,7 @@ test.describe('Service worker', () => {
 		});
 
 		expect(self.base).toBe('/basepath');
-		expect(self.build[0]).toMatch(/\/basepath\/_app\/immutable\/bundle\.[\w-]+\.js/);
+		expect(self.build?.[0]).toMatch(/\/basepath\/_app\/immutable\/bundle\.[\w-]+\.js/);
 		expect(self.image_src).toMatch(/\/basepath\/_app\/immutable\/assets\/image\.[\w-]+\.jpg/);
 	});
 
@@ -120,6 +120,7 @@ test.describe('Service worker', () => {
 
 test.describe("bundleStrategy: 'single'", () => {
 	test.skip(({ javaScriptEnabled }) => !javaScriptEnabled || !!process.env.DEV);
+
 	test('loads a single js file and a single css file', async ({ page }) => {
 		/** @type {string[]} */
 		const requests = [];
@@ -135,4 +136,9 @@ test.describe("bundleStrategy: 'single'", () => {
 		expect(requests.filter((req) => req.endsWith('.js')).length).toBe(1);
 		expect(requests.filter((req) => req.endsWith('.css')).length).toBe(1);
 	});
+
+	test('app.decoders is accessed only after app has been initialised', async ({ page }) => {
+		await page.goto('/basepath/deserialize');
+		await expect(page.locator('p')).toHaveText('Hello world!');
+	});
 });