reset stack traces to avoid double fix - closes #3371

Author: Rich-Harris

.changeset/plenty-radios-turn.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Reset stack traces to avoid double-fix

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

@@ -72,7 +72,9 @@ export async function render_response({
 	/** @type {import('types').NormalizedLoadOutputCache | undefined} */
 	let cache;
 
-	if (error) {
+	const stack = error?.stack;
+
+	if (__SVELTEKIT_DEV__ && error) {
 		error.stack = options.get_stack(error);
 	}
 
@@ -315,6 +317,11 @@ export async function render_response({
 		}
 	}
 
+	if (__SVELTEKIT_DEV__ && error) {
+		// reset stack, otherwise it may be 'fixed' a second time
+		error.stack = stack;
+	}
+
 	return new Response(html, {
 		status,
 		headers

packages/kit/test/apps/basics/src/routes/errors/stack-trace/_bad.js

@@ -0,0 +1,6 @@
+const bad = foo().toUpperCase();
+export default bad;
+
+// @ts-expect-error
+/** @returns {string} */
+function foo() {}

packages/kit/test/apps/basics/src/routes/errors/stack-trace/index.svelte

@@ -0,0 +1,5 @@
+<script>
+	import bad from './_bad.js';
+</script>
+
+<h1>{bad}</h1>

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

@@ -380,6 +380,19 @@ test.describe('Errors', () => {
 			expect(/** @type {Response} */ (response).status()).toBe(400);
 		}
 	});
+
+	test('stack traces are not fixed twice', async ({ page }) => {
+		await page.goto('/errors/stack-trace');
+		expect(await page.textContent('#message')).toBe(
+			'This is your custom error page saying: "Cannot read properties of undefined (reading \'toUpperCase\')"'
+		);
+
+		// check the stack wasn't mutated
+		await page.goto('/errors/stack-trace');
+		expect(await page.textContent('#message')).toBe(
+			'This is your custom error page saying: "Cannot read properties of undefined (reading \'toUpperCase\')"'
+		);
+	});
 });
 
 test.describe('Load', () => {