fix parsing endpoint json body (#1272)

* fix parsing endpoint json body

Converting to JSON string also when the 'content-type' header is 'application/json'
prevents the endpoint from functioning as a relay, that is, it can't fetch
an API that returns a JSON string and then forwards the headers and the body
as they are, because svelte kit would call JSON.stringify on the string again.

* Do call JSON.stringify if the body is not string and header denotes json

.changeset/real-bikes-matter.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Not calling JSON.stringify on endpoint's body if it's a string and the content-type header denotes json

packages/kit/src/runtime/server/endpoint.js

@@ -32,8 +32,8 @@ export default async function render_route(request, route) {
 			headers = lowercase_keys(headers);
 
 			if (
-				(typeof body === 'object' && !('content-type' in headers)) ||
-				headers['content-type'] === 'application/json'
+				typeof body === 'object' &&
+				(!('content-type' in headers) || headers['content-type'] === 'application/json')
 			) {
 				headers = { ...headers, 'content-type': 'application/json' };
 				body = JSON.stringify(body);

packages/kit/test/apps/basics/src/routes/load/__tests__.js

@@ -29,6 +29,10 @@ export default function (test, is_dev) {
 		);
 	});
 
+	test('json string is returned', '/load/relay', async ({ page }) => {
+		assert.equal(await page.textContent('h1'), '42');
+	});
+
 	test('prefers static data over endpoint', '/load/foo', async ({ page }) => {
 		assert.equal(await page.textContent('h1'), 'static file');
 	});

packages/kit/test/apps/basics/src/routes/load/relay.json.js

@@ -0,0 +1,8 @@
+export function get() {
+	return {
+		headers: {
+			'content-type': 'application/json'
+		},
+		body: '42'
+	};
+}

packages/kit/test/apps/basics/src/routes/load/relay.svelte

@@ -0,0 +1,17 @@
+<script context="module">
+	/** @type {import('@sveltejs/kit').Load} */
+	export async function load({ fetch }) {
+		const res = await fetch('/load/relay.json');
+		return {
+			props: {
+				answer: await res.json()
+			}
+		}
+	}
+</script>
+
+<script>
+	export let answer;
+</script>
+
+<h1>{answer}</h1>

packages/kit/test/apps/basics/src/routes/load/serialization.svelte

@@ -11,4 +11,4 @@
 	export let answer;
 </script>
 
-<h1>{answer}</h1>
+<h1>{answer}</h1>