chore: upgrade to vitest v4 (#18265)

## Summary

Bumps `vitest` and `@vitest/coverage-v8` from `^2.1.9` to `^4.1.7` (two
major versions). Three small test-harness updates compensate for vitest
4 behavior changes; all 7569 tests still pass.

- **`packages/svelte/tests/runtime-browser/test.ts`** — vitest 4 removed
the deprecated `describe(name, fn, opts)` signature. Pass options as the
second argument.
- **`packages/svelte/tests/runtime-legacy/shared.ts`** — vitest 4's
jsdom env binds `virtualConsole` to the original `globalThis.console`
reference *before* vitest wraps the console, so inline-`<script>` logs
and `jsdomError` events no longer reach per-test `console.{log,error}`
overrides. Restore vitest 2's behavior by re-routing the virtual console
through the live `console` in `beforeAll`. Also promote the window-error
listener to a named function and remove it in `finally` — previously
leaked listeners from earlier tests kept writing to module-level
`unhandled_rejection`, polluting later tests.
- **`vitest.config.js`** — bump `testTimeout` to 10s. The 5s default
trips a handful of dev-mode tests that exercise
`effect_update_depth_exceeded`, whose ~1000 Error-stack captures per
flush are slower under vitest 4's deeper async stacks.

## Test plan

- [x] `pnpm test` — 7569 passed, 63 skipped, 0 failed

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Rich Harris <rich.harris@vercel.com>

.github/workflows/ci.yml

@@ -18,12 +18,12 @@ jobs:
     strategy:
       matrix:
         include:
-          - node-version: 18
+          # Vitest 4 requires Node 20+, so tests run on 20/22/24. The published
+          # Svelte package still supports Node >=18 (see packages/svelte/package.json).
+          - node-version: 20
             os: windows-latest
-          - node-version: 18
+          - node-version: 20
             os: macOS-latest
-          - node-version: 18
-            os: ubuntu-latest
           - node-version: 20
             os: ubuntu-latest
           - node-version: 22

package.json

@@ -32,7 +32,7 @@
     "@svitejs/changesets-changelog-github-compact": "^1.1.0",
     "@types/node": "^20.11.5",
     "@types/picomatch": "^4.0.2",
-    "@vitest/coverage-v8": "^2.1.9",
+    "@vitest/coverage-v8": "^4.1.7",
     "eslint": "^10.0.0",
     "eslint-plugin-lube": "^0.5.1",
     "eslint-plugin-svelte": "^3.15.0",
@@ -44,6 +44,6 @@
     "typescript": "^5.5.4",
     "typescript-eslint": "^8.56.0",
     "v8-natives": "^1.2.5",
-    "vitest": "^2.1.9"
+    "vitest": "^4.1.7"
   }
 }

packages/svelte/package.json

@@ -166,7 +166,7 @@
     "source-map": "^0.7.4",
     "tinyglobby": "^0.2.12",
     "typescript": "^5.5.4",
-    "vitest": "^2.1.9",
+    "vitest": "^4.1.7",
     "web-features": "^3.29.0"
   },
   "dependencies": {

packages/svelte/tests/hydration/test.ts

@@ -25,7 +25,21 @@ interface HydrationTest extends BaseTest {
 	expect_hydration_error?: true;
 	snapshot?: (target: HTMLElement) => any;
 	test?: (
-		assert: typeof import('vitest').assert & {
+		// `_config.js` test callbacks rely on inferred parameter types, which
+		// TS treats as non-explicit and rejects for chai 5's assertion-function
+		// signatures (TS2775). Override the assertion methods we actually use
+		// with non-assertion equivalents.
+		assert: Omit<
+			typeof import('vitest').assert,
+			'ok' | 'isOk' | 'isTrue' | 'isFalse' | 'exists' | 'notExists' | 'instanceOf'
+		> & {
+			ok(value: unknown, message?: string): void;
+			isOk(value: unknown, message?: string): void;
+			isTrue(value: unknown, message?: string): void;
+			isFalse(value: unknown, message?: string): void;
+			exists(value: unknown, message?: string): void;
+			notExists(value: unknown, message?: string): void;
+			instanceOf(value: unknown, type: Function, message?: string): void;
 			htmlEqual(a: string, b: string, description?: string): void;
 		},
 		target: HTMLElement,
@@ -152,7 +166,6 @@ const { test, run } = suite<HydrationTest>(async (config, cwd) => {
 
 		if (config.test) {
 			await config.test(
-				// @ts-expect-error TS doesn't get it
 				{
 					...assert,
 					htmlEqual: assert_html_equal

packages/svelte/tests/runtime-browser/test.ts

@@ -42,9 +42,9 @@ const { run: run_browser_tests } = suite_with_variants<
 
 describe.concurrent(
 	'runtime-browser',
-	() => run_browser_tests(__dirname),
 	// Browser tests are brittle and slow on CI
-	{ timeout: 20000, retry: process.env.CI ? 1 : 0 }
+	{ timeout: 20000, retry: process.env.CI ? 1 : 0 },
+	() => run_browser_tests(__dirname)
 );
 
 const { run: run_ce_tests } = suite<ReturnType<typeof import('./assert').test>>(
@@ -55,9 +55,9 @@ const { run: run_ce_tests } = suite<ReturnType<typeof import('./assert').test>>(
 
 describe.concurrent(
 	'custom-elements',
-	() => run_ce_tests(__dirname, 'custom-elements-samples'),
 	// Browser tests are brittle and slow on CI
-	{ timeout: 20000, retry: process.env.CI ? 1 : 0 }
+	{ timeout: 20000, retry: process.env.CI ? 1 : 0 },
+	() => run_ce_tests(__dirname, 'custom-elements-samples')
 );
 
 async function run_test(

packages/svelte/tests/runtime-legacy/shared.ts

@@ -15,19 +15,35 @@ import { clear } from '../../src/internal/client/reactivity/batch.js';
 import { hydrating } from '../../src/internal/client/dom/hydration.js';
 import { ssr_context } from '../../src/internal/server/context.js';
 
-type Assert = typeof import('vitest').assert & {
-	htmlEqual(a: string, b: string, description?: string): void;
-	htmlEqualWithOptions(
-		a: string,
-		b: string,
-		opts: {
-			preserveComments: boolean;
-			withoutNormalizeHtml: boolean;
-		},
-		description?: string
-	): void;
+// `_config.js` files call `assert.ok` etc. with `assert` typed via parameter
+// inference, which TypeScript treats as non-explicit. chai 5 (pulled in by
+// vitest 4) declares these as assertion functions (`asserts value`), so TS2775
+// fires on every call. Override the affected methods with non-assertion
+// signatures — the runtime behavior is unchanged.
+type NonAssertingMethods = {
+	ok(value: unknown, message?: string): void;
+	isOk(value: unknown, message?: string): void;
+	isTrue(value: unknown, message?: string): void;
+	isFalse(value: unknown, message?: string): void;
+	exists(value: unknown, message?: string): void;
+	notExists(value: unknown, message?: string): void;
+	instanceOf(value: unknown, type: Function, message?: string): void;
 };
 
+type Assert = Omit<typeof import('vitest').assert, keyof NonAssertingMethods> &
+	NonAssertingMethods & {
+		htmlEqual(a: string, b: string, description?: string): void;
+		htmlEqualWithOptions(
+			a: string,
+			b: string,
+			opts: {
+				preserveComments: boolean;
+				withoutNormalizeHtml: boolean;
+			},
+			description?: string
+		): void;
+	};
+
 // TODO remove this shim when we can
 // @ts-expect-error
 Promise.withResolvers = () => {
@@ -75,6 +91,7 @@ export interface RuntimeTest<Props extends Record<string, any> = Record<string,
 		raf: {
 			tick: (ms: number) => void;
 		};
+		snapshot: any;
 		target: HTMLElement;
 		window: Window & {
 			Event: typeof Event;
@@ -126,6 +143,16 @@ const listeners = process.rawListeners('unhandledRejection');
 beforeAll(() => {
 	// @ts-expect-error TODO huh?
 	process.prependListener('unhandledRejection', unhandled_rejection_handler);
+
+	// Route inline-`<script>` console calls through `globalThis.console` at
+	// call time, so per-test `console.{log,warn,error}` overrides see them.
+	// (jsdom scripts run in a VM with their own `console` object, separate
+	// from Node's `globalThis.console` — vitest 4's jsdom env no longer
+	// bridges them per-test.)
+	const vc = (window as any)._virtualConsole;
+	for (const m of ['log', 'warn', 'error'] as const) {
+		vc?.on(m, (...args: any[]) => console[m](...args));
+	}
 });
 
 beforeEach(() => {
@@ -255,6 +282,26 @@ async function run_test_variant(
 	let warnings: string[] = [];
 	let errors: string[] = [];
 	let manual_hydrate = false;
+	let intercept_errors = false;
+
+	// Capture phase so we still see the error if a test installs its own
+	// `stopImmediatePropagation` listener (e.g. event-handler-*). Named so we
+	// can remove it in `finally` — otherwise listeners from earlier tests leak
+	// and keep writing to module-level `unhandled_rejection`. When the test
+	// intercepts `errors`, mirror jsdom's `Uncaught [...]` format into the
+	// array — vitest 4's jsdom env no longer routes `jsdomError` to vitest's
+	// wrapped console reliably (works locally but not in CI's forks pool).
+	const window_error_listener = (e: ErrorEvent) => {
+		if (intercept_errors) {
+			const detail = e.error;
+			const is_error = detail && detail.name && detail.message !== undefined && detail.stack;
+			const error_string = is_error ? `[${detail.name}: ${detail.message}]` : String(detail);
+			errors.push(`Error: Uncaught ${error_string}\n${detail?.stack ?? ''}`, detail);
+		} else {
+			unhandled_rejection = e.error;
+		}
+		e.preventDefault();
+	};
 
 	{
 		// use some crude static analysis to determine if logs/warnings are intercepted.
@@ -317,6 +364,7 @@ async function run_test_variant(
 		}
 
 		if (str.slice(0, i).includes('errors') || config.errors) {
+			intercept_errors = true;
 			// eslint-disable-next-line no-console
 			console.error = (...args) => {
 				errors.push(...args);
@@ -348,10 +396,7 @@ async function run_test_variant(
 		window.document.head.innerHTML = styles ? `<style>${styles}</style>` : '';
 		window.document.body.innerHTML = '<main></main>';
 
-		window.addEventListener('error', (e) => {
-			unhandled_rejection = e.error;
-			e.preventDefault();
-		});
+		window.addEventListener('error', window_error_listener, true);
 
 		globalThis.requestAnimationFrame = globalThis.setTimeout;
 
@@ -423,7 +468,6 @@ async function run_test_variant(
 				await config.test_ssr({
 					logs,
 					warnings,
-					// @ts-expect-error
 					assert: {
 						...assert,
 						htmlEqual: assert_html_equal,
@@ -515,7 +559,6 @@ async function run_test_variant(
 					}
 
 					await config.test({
-						// @ts-expect-error TS doesn't get it
 						assert: {
 							...assert,
 							htmlEqual: assert_html_equal,
@@ -525,6 +568,7 @@ async function run_test_variant(
 						component: runes ? props : instance,
 						instance,
 						mod,
+						ok,
 						target,
 						snapshot,
 						window,
@@ -593,6 +637,8 @@ async function run_test_variant(
 			throw new Error('Hydration state was not cleared');
 		}
 
+		window.removeEventListener('error', window_error_listener, true);
+
 		config.after_test?.();
 
 		// Free up the microtask queue

packages/svelte/tests/sourcemaps/test.ts

@@ -31,7 +31,21 @@ interface SourcemapTest extends BaseTest {
 	/** The expected `sources` array in the source map */
 	css_map_sources?: string[];
 	test?: (obj: {
-		assert: typeof assert;
+		// chai 5's `asserts value` signatures trip TS2775 in `_config.js` files
+		// where `assert` is a destructured parameter (non-explicit). Override
+		// the assertion methods we use with non-assertion equivalents.
+		assert: Omit<
+			typeof assert,
+			'ok' | 'isOk' | 'isTrue' | 'isFalse' | 'exists' | 'notExists' | 'instanceOf'
+		> & {
+			ok(value: unknown, message?: string): void;
+			isOk(value: unknown, message?: string): void;
+			isTrue(value: unknown, message?: string): void;
+			isFalse(value: unknown, message?: string): void;
+			exists(value: unknown, message?: string): void;
+			notExists(value: unknown, message?: string): void;
+			instanceOf(value: unknown, type: Function, message?: string): void;
+		};
 		input: string;
 		map_preprocessed: any;
 		code_preprocessed: string;

vitest-xhtml-environment.ts

@@ -1,4 +1,4 @@
-import { type Environment, builtinEnvironments } from 'vitest/environments';
+import { type Environment, builtinEnvironments } from 'vitest/runtime';
 
 const xhtml_page = `<?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML Basic 1.0//EN" "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd">
@@ -6,7 +6,7 @@ const xhtml_page = `<?xml version="1.0" encoding="UTF-8"?>
 
 export default <Environment>{
 	name: 'jsdom-xhtml',
-	transformMode: 'web',
+	viteEnvironment: 'client',
 	setup(global, { jsdom = {} }) {
 		return builtinEnvironments.jsdom.setup(global, {
 			jsdom: {

vitest.config.js

@@ -29,6 +29,11 @@ export default defineConfig({
 	test: {
 		dir: '.',
 		reporters: ['dot'],
+		// A handful of dev-mode tests trigger Svelte's `effect_update_depth_exceeded`
+		// guard, which involves ~1000 Error objects per flush for stack tracking —
+		// slow enough under vitest 4's deeper async stacks (and CI's slower workers)
+		// to overrun the 5s default.
+		testTimeout: 30_000,
 		include: [
 			'packages/svelte/**/*.test.ts',
 			'packages/svelte/tests/*/test.ts',