NathanWalker · Oct 22, 2026 · Oct 22, 2026 · Oct 22, 2026
diff --git a/packages/vite/hmr/server/websocket-ns-m.spec.ts b/packages/vite/hmr/server/websocket-ns-m.spec.ts
@@ -1,4 +1,4 @@
-import { describe, expect, it, vi } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 
 import { registerNsModuleServerRoute, type RegisterNsModuleServerRouteOptions } from './websocket-ns-m.js';
 import { interceptNgComponentRequest } from '../frameworks/angular/server/ng-component-route.js';
@@ -67,7 +67,7 @@ function mount(overrides: Partial<RegisterNsModuleServerRouteOptions> = {}) {
 	};
 	registerNsModuleServerRoute(server, options);
 	expect(handlers).toHaveLength(1);
-	return { handler: handlers[0], ensureInitialGraphPopulationStarted, upsertGraphModule };
+	return { handler: handlers[0], server, ensureInitialGraphPopulationStarted, upsertGraphModule };
 }
 
 describe('registerNsModuleServerRoute', () => {
@@ -204,3 +204,81 @@ describe('registerNsModuleServerRoute', () => {
 		});
 	});
 });
+
+describe('native default-import link checking', () => {
+	beforeEach(() => vi.stubEnv('NODE_ENV', 'development'));
+	afterEach(() => vi.unstubAllEnvs());
+
+	const importer = "import value from 'http://localhost:5173/ns/m/src/target.mjs'; export { value };";
+
+	function mountLinkCheck(source: string) {
+		let targetCode = source;
+		let graphVersion = 0;
+		const fixture = mount({
+			sharedTransformRequest: async () => ({ code: importer, map: null }),
+			getGraphVersion: () => graphVersion,
+		});
+		fixture.server.transformRequest.mockImplementation(async () => ({ code: targetCode }));
+		return {
+			async rejects() {
+				fixture.server.transformRequest.mockClear();
+				const res = makeRes();
+				await fixture.handler({ url: '/ns/m/src/importer.mjs' }, res, vi.fn());
+				expect(res.statusCode).toBe(200);
+				expect(fixture.server.transformRequest).toHaveBeenCalled();
+				expect(res.body).toContain('export');
+				return res.body!.includes('[link-check] Missing default export');
+			},
+			edit(source: string) {
+				targetCode = source;
+				graphVersion++;
+			},
+		};
+	}
+
+	it.each([
+		['direct', 'export default 17;', false],
+		['function', 'export default function named() {}', false],
+		['class', 'export default class Named {}', false],
+		['alias', 'const named = 23; export { named as default };', false],
+		['multiple specifiers', 'const named = 23, other = 1; export { other, named as default };', false],
+		['quoted name', 'const named = 23; export { named as "default" };', false],
+		['default re-export', 'export { default } from "./base.mjs";', false],
+		['aliased re-export', 'export { other as default } from "./base.mjs";', false],
+		['namespace re-export', 'export * as default from "./base.mjs";', false],
+		['named only', 'export const named = 99;', true],
+		['star re-export', 'export * from "./base.mjs";', true],
+		['comment', '// export default 17;\nexport const named = 99;', true],
+		['string', 'export const text = "export default 17";', true],
+		['type-only declaration', 'export type { Value as default } from "./types";', true],
+		['type-only specifier', 'export { type Value as default } from "./types";', true],
+		['CommonJS module.exports', 'module.exports = 17;', false],
+		['CommonJS exports', 'exports.named = 17;', false],
+	] as const)('%s', async (_name, source, rejected) => {
+		expect(await mountLinkCheck(source).rejects()).toBe(rejected);
+	});
+
+	it('keeps checking later imports when a target cannot be parsed', async () => {
+		const fixture = mount({
+			sharedTransformRequest: async () => ({
+				code: "import a from 'http://localhost:5173/ns/m/src/a.mjs'; import b from 'http://localhost:5173/ns/m/src/b.mjs'; export { a, b };",
+				map: null,
+			}),
+		});
+		fixture.server.transformRequest.mockImplementation(async (url: string) => ({ code: url.includes('/a.mjs') ? 'export default (;' : 'export const named = 99;' }));
+		const res = makeRes();
+		await fixture.handler({ url: '/ns/m/src/importer.mjs' }, res, vi.fn());
+		expect(res.body).toContain('[link-check] Missing default export in /ns/m/src/b.mjs');
+	});
+
+	it('rechecks the same importer after its target gains or loses a default', async () => {
+		const fixture = mountLinkCheck('export const named = 99;');
+		expect(await fixture.rejects()).toBe(true);
+		fixture.edit('const named = 23; export { named as default };');
+		expect(await fixture.rejects()).toBe(false);
+		fixture.edit('export const named = 99;');
+		expect(await fixture.rejects()).toBe(true);
+		fixture.edit('export default 17;');
+		expect(await fixture.rejects()).toBe(false);
+	});
+});
diff --git a/packages/vite/hmr/server/websocket-ns-m.ts b/packages/vite/hmr/server/websocket-ns-m.ts
@@ -12,7 +12,7 @@ import { createNsMRequestContext, resolveFsAnchoredSpec, resolveNsMTransformedMo
 import { setDeviceModuleHeaders } from './route-helpers.js';
 import { RAW_QUERY_PATTERN } from './constants.js';
 import { CSS_MODULE_RE, buildCssRegisterSnippetFromVar, normalizeCssForDevice } from './css-device-module.js';
-import { assertNoOptimizedArtifacts, buildBootProgressSnippet, canonicalizeRtImports, dedupeRtNamedImportsAgainstDestructures, deduplicateLinkerImports, ensureDestructureCoreImports, ensureGuardPlainDynamicImports, ensureVariableDynamicImportHelper, expandStarExports, hoistTopLevelStaticImports, MODULE_IMPORT_ANALYSIS_PLUGINS, wrapCommonJsModuleForDevice, ensureWorkerEntryGlobalsImport, classifyServedRequest } from './websocket-served-module-helpers.js';
+import { assertNoOptimizedArtifacts, buildBootProgressSnippet, canonicalizeRtImports, dedupeRtNamedImportsAgainstDestructures, deduplicateLinkerImports, ensureDestructureCoreImports, ensureGuardPlainDynamicImports, ensureVariableDynamicImportHelper, expandStarExports, hasEsmDefaultExport, hoistTopLevelStaticImports, MODULE_IMPORT_ANALYSIS_PLUGINS, wrapCommonJsModuleForDevice, ensureWorkerEntryGlobalsImport, classifyServedRequest } from './websocket-served-module-helpers.js';
 import { cleanCode, collectImportDependencies, isWorkerEntryModuleId, processCodeForDevice, rewriteImports } from './websocket-device-transform.js';
 import { REQUIRE_GUARD_SNIPPET } from './require-guard.js';
 import { getServerOrigin } from './server-origin.js';
@@ -565,7 +565,8 @@ export function registerNsModuleServerRoute(server: ViteDevServer, options: Regi
 								}
 							} catch {}
 							if (!targetCode) continue;
-							const hasDefault = /\bexport\s+default\b/.test(targetCode) || /export\s*\{\s*default\s*(?:as\s*default)?\s*\}/.test(targetCode);
+							const hasDefault = hasEsmDefaultExport(targetCode);
+							if (hasDefault === null) continue;
 							if (!hasDefault) {
 								// CJS/UMD modules won't have `export default` — they get CJS-wrapped
 								// by the serving pipeline. Only warn, don't fatally block the importer.

diff --git a/packages/vite/hmr/server/websocket-served-module-helpers.spec.ts b/packages/vite/hmr/server/websocket-served-module-helpers.spec.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 
-import { classifyServedModule, classifyServedRequest } from './websocket-served-module-helpers.js';
+import { classifyServedModule, classifyServedRequest, hasEsmDefaultExport } from './websocket-served-module-helpers.js';
 
 describe('classifyServedRequest', () => {
 	it('treats a package request as library code when its resolved id is a symlink target outside node_modules', () => {
@@ -24,3 +24,23 @@ describe('classifyServedRequest', () => {
 		expect(classifyServedModule('/node_modules/pkg/index.js')).toBe('library');
 	});
 });
+
+describe('hasEsmDefaultExport', () => {
+	it.each([
+		['export default 17;', true],
+		['const named = 23; export { named as default };', true],
+		['export { default } from "./base.mjs";', true],
+		['export * as default from "./base.mjs";', true],
+		['export const named = 99;', false],
+		['export * from "./base.mjs";', false],
+		['// export default 17;\nexport const text = "export default 17";', false],
+		['export { type Value as default } from "./types";', false],
+		['module.exports = 17;', false],
+	])('%s → %s', (code, expected) => {
+		expect(hasEsmDefaultExport(code)).toBe(expected);
+	});
+
+	it('returns null for unparseable code', () => {
+		expect(hasEsmDefaultExport('export default (;')).toBeNull();
+	});
+});
diff --git a/packages/vite/hmr/server/websocket-served-module-helpers.ts b/packages/vite/hmr/server/websocket-served-module-helpers.ts
@@ -106,6 +106,21 @@ export function classifyServedRequest(spec: string | undefined | null, resolvedI
 
 export const MODULE_IMPORT_ANALYSIS_PLUGINS = ['typescript', 'jsx', 'importMeta', 'topLevelAwait', 'classProperties', 'classPrivateProperties', 'classPrivateMethods', 'decorators-legacy'] as any;
 
+/** Whether `code` has an ESM default export, or `null` when it cannot be parsed. */
+export function hasEsmDefaultExport(code: string): boolean | null {
+	let body: t.Statement[];
+	try {
+		body = babelParse(code, { sourceType: 'module', plugins: MODULE_IMPORT_ANALYSIS_PLUGINS }).program.body;
+	} catch {
+		return null;
+	}
+	return body.some((node) => {
+		if (t.isExportDefaultDeclaration(node)) return true;
+		if (!t.isExportNamedDeclaration(node) || node.exportKind === 'type') return false;
+		return node.specifiers.some((spec) => !(t.isExportSpecifier(spec) && spec.exportKind === 'type') && (t.isIdentifier(spec.exported) ? spec.exported.name : spec.exported.value) === 'default');
+	});
+}
+
 export type TopLevelImportRecord = {
 	start: number;
 	end: number;