fix: correctly deserialize on the server

.changeset/silly-berries-repair.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: correctly run `deserialize` on the server

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

@@ -31,7 +31,8 @@ export function deserialize(result) {
 	const parsed = JSON.parse(result);
 
 	if (parsed.data) {
-		parsed.data = devalue.parse(parsed.data, app.decoders);
+		// app is uninitialised on the server
+		parsed.data = devalue.parse(parsed.data, app?.decoders);
 	}
 
 	return parsed;

packages/kit/test/apps/basics/src/routes/deserialize/+page.server.js

@@ -0,0 +1,5 @@
+export const actions = {
+	default: () => {
+		return { a: 1 };
+	}
+};

packages/kit/test/apps/basics/src/routes/deserialize/+server.js

@@ -0,0 +1,13 @@
+import { deserialize } from '$app/forms';
+
+export async function GET({ fetch }) {
+	const response = await fetch('/deserialize', {
+		method: 'POST',
+		body: new FormData(),
+		headers: {
+			'x-sveltekit-action': 'true'
+		}
+	});
+	const result = deserialize(await response.text());
+	return Response.json(result);
+}

packages/kit/test/apps/basics/test/server.test.js

@@ -716,3 +716,10 @@ test.describe('getRequestEvent', () => {
 		expect(await response.text()).toBe('hello from hooks.server.js');
 	});
 });
+
+test.describe('$app/forms', () => {
+	test('deserialize works on the server', async ({ request }) => {
+		const response = await request.get('/deserialize');
+		expect(await response.json()).toEqual({ type: 'success', status: 200, data: { a: 1 } });
+	});
+});