fix(@angular/build): load zone.js dynamically for library unit tests

When unit testing library targets with the unit-test builder, the build
target (ng-packagr) does not have a 'polyfills' configuration. This
caused the builder to fall back to the 'dynamic' zone testing strategy,
which only dynamically imports zone.js/testing at runtime if Zone is
already defined. Since the main zone.js library was never loaded, Zone
was undefined, and library tests relying on TestBed/fakeAsync failed.

This commit introduces a new 'dynamic-zone' zone testing strategy. If
polyfills is undefined (meaning we are running a library target) and
zone.js is installed/resolvable, both zone.js and zone.js/testing are
dynamically imported at startup, resolving the failure.

Fixes #33477

Author: clydin

packages/angular/build/src/builders/unit-test/runners/vitest/build-options.ts

@@ -33,7 +33,7 @@ function createTestBedInitVirtualFile(
   providersFile: string | undefined,
   projectSourceRoot: string,
   teardown: boolean,
-  zoneTestingStrategy: 'none' | 'static' | 'dynamic',
+  zoneTestingStrategy: 'none' | 'static' | 'dynamic' | 'dynamic-zone',
   hasLocalize: boolean,
 ): string {
   let providersImport = 'const providers = [];';
@@ -53,6 +53,13 @@ function createTestBedInitVirtualFile(
       // It must be imported dynamically to avoid a static dependency on 'zone.js'.
       await import('zone.js/testing');
     }`;
+  } else if (zoneTestingStrategy === 'dynamic-zone') {
+    zoneTestingSnippet = `try {
+      await import('zone.js');
+      await import('zone.js/testing');
+    } catch (e) {
+      console.error('DYNAMIC IMPORT ERROR:', e);
+    }`;
   }
 
   // The DynamicDOMTestComponentRenderer is used to avoid stale document references
@@ -150,12 +157,12 @@ function adjustOutputHashing(hashing?: OutputHashing): OutputHashing {
  *
  * @param buildOptions The partial application builder options.
  * @param projectSourceRoot The root directory of the project source.
- * @returns The resolved zone testing strategy ('none', 'static', 'dynamic').
+ * @returns The resolved zone testing strategy ('none', 'static', 'dynamic', 'dynamic-zone').
  */
 function getZoneTestingStrategy(
   buildOptions: Partial<ApplicationBuilderInternalOptions>,
   projectSourceRoot: string,
-): 'none' | 'static' | 'dynamic' {
+): 'none' | 'static' | 'dynamic' | 'dynamic-zone' {
   if (buildOptions.polyfills?.includes('zone.js/testing')) {
     return 'none';
   }
@@ -168,6 +175,12 @@ function getZoneTestingStrategy(
     const projectRequire = createRequire(path.join(projectSourceRoot, 'package.json'));
     projectRequire.resolve('zone.js');
 
+    // If polyfills is undefined (e.g. library build target), load zone.js dynamically.
+    // If polyfills is defined but doesn't include zone.js (e.g. zoneless application), do NOT load zone.js.
+    if (buildOptions.polyfills === undefined) {
+      return 'dynamic-zone';
+    }
+
     return 'dynamic';
   } catch {
     return 'none';

packages/angular/build/src/builders/unit-test/tests/behavior/vitest-zone-init_spec.ts

@@ -67,5 +67,48 @@ describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
       const { result } = await harness.executeOnce();
       expect(result?.success).toBe(true);
     });
+
+    it('should load Zone and Zone testing support when testing a library and zone.js is installed', async () => {
+      harness.withBuilderTarget(
+        'build',
+        async () => ({ success: true }),
+        {
+          project: 'ng-package.json',
+        },
+        {
+          builderName: '@angular/build:ng-packagr',
+        },
+      );
+
+      await harness.writeFile(
+        'ng-package.json',
+        JSON.stringify({
+          lib: {
+            entryFile: 'src/public-api.ts',
+          },
+        }),
+      );
+
+      harness.useTarget('test', {
+        ...BASE_OPTIONS,
+        include: ['src/app.component.spec.ts'],
+      });
+
+      await harness.writeFile(
+        'src/app.component.spec.ts',
+        `
+        import { describe, it, expect } from 'vitest';
+
+        describe('Library Zone Test', () => {
+          it('should have Zone defined', () => {
+            expect((globalThis as any).Zone).toBeDefined();
+          });
+        });
+      `,
+      );
+
+      const { result } = await harness.executeOnce();
+      expect(result?.success).toBeTrue();
+    });
   });
 });