fix(@angular/build): disable code splitting for unit test builds

Every spec file is its own entry point, so esbuild code splitting hoists any module
reached from more than one spec into a chunk shared between them. A module placed in a
shared chunk is wrapped in a lazy `__esm` initializer, so its exported value is only
assigned once that initializer runs, and importing chunks read the export as a live ESM
binding.

The unit test runners load the generated output through a module runner rather than the
browser's own ESM implementation, and that does not reliably preserve those bindings. An
importing chunk can therefore observe the export as `undefined`. A component whose class
field initializer reads a `const` exported from a module that was hoisted into a shared
chunk fails with a `TypeError`, while the same value read later, or read from within the
shared chunk itself, is correct. It only appears once a project has more than one spec
file, because a single entry point inlines everything and never splits.

Test bundles are never downloaded by a browser, so splitting has nothing to optimize
here. This disables it for the unit test build only, via an internal option, leaving
application builds unaffected.

The regression test mirrors the reproduction's exact shape: a shared const read during
class-field initialization from a spec with an async test callback, under zone.js
polyfills. That combination is load-bearing: zone.js downlevels async, the spec then
imports the `__async` helper, and esbuild emits the spec entry CommonJS-wrapped with the
component module behind a lazy `__esm` initializer. The fixture file set was verified to
fail against an unpatched 21.2.19 build and pass with this change applied.

Author: jonmarozick

packages/angular/build/src/builders/application/options.ts

@@ -126,6 +126,20 @@ interface InternalOptions {
    * Suppress build summary and stats table.
    */
   quiet?: boolean;
+
+  /**
+   * Disables esbuild code splitting for the browser code bundle.
+   *
+   * Splitting emits shared chunks whose exports are read across chunk boundaries as live ESM
+   * bindings. A module hoisted into a shared chunk is wrapped in a lazy initializer, so its exported
+   * value is only assigned once that initializer runs. Runners that load the generated output
+   * through a module runner rather than the browser's own ESM implementation do not reliably
+   * preserve those bindings, and an importing chunk can observe the export as `undefined`.
+   *
+   * Test bundles are never downloaded by a browser, so there is nothing for splitting to optimize
+   * there. Used exclusively for tests and shouldn't be used for other kinds of builds.
+   */
+  disableCodeSplitting?: boolean;
 }
 
 /** Full set of options for `application` builder. */
@@ -439,6 +453,7 @@ export async function normalizeOptions(
     partialSSRBuild = false,
     externalRuntimeStyles,
     instrumentForCoverage,
+    disableCodeSplitting,
   } = options;
 
   // Return all the normalized options
@@ -475,6 +490,7 @@ export async function normalizeOptions(
     watch,
     workspaceRoot,
     entryPoints,
+    disableCodeSplitting,
     optimizationOptions,
     outputOptions,
     outExtension,

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

@@ -257,6 +257,12 @@ export async function getVitestBuildOptions(
     outputHashing: adjustOutputHashing(baseBuildOptions.outputHashing),
     optimization: false,
     entryPoints,
+    // Every spec file is its own entry point, so splitting hoists any module shared between two
+    // specs into a chunk whose exports are then read across a chunk boundary. Those reads rely on
+    // live ESM bindings, and a module placed in a shared chunk is only assigned its exported value
+    // when that chunk's lazy initializer runs, so an importing chunk can read `undefined`. Nothing
+    // downloads these bundles, so there is no benefit to weigh against that.
+    disableCodeSplitting: true,
     // Enable support for vitest browser prebundling. Excludes can be controlled with a runnerConfig
     // and the `optimizeDeps.exclude` option.
     externalPackages: true,

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

@@ -0,0 +1,172 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import { execute } from '../../index';
+import {
+  BASE_OPTIONS,
+  describeBuilder,
+  UNIT_TEST_BUILDER_INFO,
+  setupApplicationTarget,
+} from '../setup';
+
+describeBuilder(execute, UNIT_TEST_BUILDER_INFO, (harness) => {
+  describe('Behavior: "Vitest shared chunk initialization"', () => {
+    // Regression test for https://github.com/angular/angular-cli/issues/33728.
+    //
+    // Without `disableCodeSplitting`, esbuild hoists a module imported by more than one spec
+    // entry point into a shared chunk behind a lazy `__esm` initializer, and a class-field
+    // initializer in another chunk reads the exported value as `undefined` under the jsdom
+    // runner. All four trigger conditions are required and encoded below:
+    //   1. two spec entry points import the shared module (so it lands in a shared chunk);
+    //   2. a component in one entry reads the export during class-field initialization;
+    //   3. that component's spec file contains an `async` test callback (no `await` needed);
+    //   4. zone.js is in the polyfills (the `setupApplicationTarget` default), which downlevels
+    //      async and makes esbuild emit the spec entry CommonJS-wrapped.
+    //
+    // NOTE: the failure this guards against is sensitive to inert content — adding a top-level
+    // side effect (even a `console.log`) to the shared or importing module below defused it
+    // during reduction. Mirror https://github.com/jonmarozick/ng-shared-chunk-repro when
+    // modifying these fixtures.
+    it('should provide shared-module exports to class-field initializers in async specs', async () => {
+      setupApplicationTarget(harness);
+
+      harness.useTarget('test', {
+        ...BASE_OPTIONS,
+      });
+
+      // Keep the default project's spec deterministic; a third spec entry that does not touch
+      // the shared module does not affect the reproduction (verified in a fresh workspace).
+      await harness.writeFile(
+        'src/app/app.component.spec.ts',
+        `
+        import { describe, it, expect } from 'vitest';
+
+        describe('AppComponent placeholder', () => {
+            it('runs', () => {
+                expect(1 + 1).toBe(2);
+            });
+        });
+      `,
+      );
+
+      // The shared `const`. Reached from both spec entry points, so it is hoisted into a chunk
+      // shared between them.
+      await harness.writeFile(
+        'src/environments/env-config.ts',
+        `
+        export interface DealerConfig {
+            dealerId: string;
+            clientKey: string;
+        }
+
+        export const DEALERS: DealerConfig[] = [
+            { dealerId: 'dealer-one', clientKey: 'KEY-ONE' },
+            { dealerId: 'dealer-two', clientKey: 'KEY-TWO' },
+            { dealerId: 'dealer-three', clientKey: 'KEY-THREE' },
+            { dealerId: 'dealer-four', clientKey: 'KEY-FOUR' },
+        ];
+      `,
+      );
+
+      // Reached by both spec entries, so it and env-config.ts land in the shared chunk. Reads the
+      // const inside a method — after module initialization — and is the passing control.
+      await harness.writeFile(
+        'src/app/features/lead-generator/services/lead.service.ts',
+        `
+        import { Injectable } from '@angular/core';
+
+        import { DEALERS } from '../../../../environments/env-config';
+
+        @Injectable({ providedIn: 'root' })
+        export class LeadService {
+            resolveClientKey(dealerId: string, clientKey?: string): string {
+                return clientKey ?? DEALERS.find((d) => d.dealerId === dealerId)?.clientKey ?? '';
+            }
+        }
+      `,
+      );
+
+      await harness.writeFile(
+        'src/app/features/lead-generator/services/index.ts',
+        `export * from './lead.service';\n`,
+      );
+
+      // Spec entry point 1 — the second importer that causes the chunk to be shared at all.
+      await harness.writeFile(
+        'src/app/features/lead-generator/services/lead.service.spec.ts',
+        `
+        import { describe, it, expect } from 'vitest';
+        import { TestBed } from '@angular/core/testing';
+
+        import { LeadService } from './lead.service';
+
+        describe('LeadService', () => {
+            it('reads DEALERS inside a method', () => {
+                TestBed.configureTestingModule({ providers: [LeadService] });
+
+                expect(TestBed.inject(LeadService).resolveClientKey('dealer-one')).toBe('KEY-ONE');
+            });
+        });
+      `,
+      );
+
+      // In the other chunk; reads the shared export eagerly during class-field initialization.
+      await harness.writeFile(
+        'src/app/features/lead-generator/lead-generator.container.ts',
+        `
+        import { Component, inject } from '@angular/core';
+
+        import { LeadService } from './services';
+        import { DEALERS, DealerConfig } from '../../../environments/env-config';
+
+        @Component({
+            selector: 'app-lead-generator',
+            standalone: true,
+            template: '',
+        })
+        export class LeadGeneratorContainer {
+            private readonly leadService = inject(LeadService);
+
+            readonly dealers: DealerConfig[] = DEALERS;
+            readonly dealerOptions = this.dealers.map((d) => d.dealerId);
+
+            hasService(): boolean {
+                return this.leadService != null;
+            }
+        }
+      `,
+      );
+
+      // Spec entry point 2 — the failing case without the fix. The `async` is load-bearing:
+      // zone.js makes the builder downlevel it, the spec then imports the `__async` helper, and
+      // esbuild emits this entry CommonJS-wrapped with the component module behind a lazy
+      // `__esm` initializer. No `await` is needed; a synchronous callback hides the defect.
+      await harness.writeFile(
+        'src/app/features/lead-generator/lead-generator.container.spec.ts',
+        `
+        import { describe, it, expect } from 'vitest';
+        import { TestBed } from '@angular/core/testing';
+
+        import { LeadGeneratorContainer } from './lead-generator.container';
+
+        describe('LeadGeneratorContainer', () => {
+            it('reads DEALERS in a class-field initialiser', async () => {
+                const fixture = TestBed.createComponent(LeadGeneratorContainer);
+
+                expect(fixture.componentInstance.dealers.length).toBe(4);
+            });
+        });
+      `,
+      );
+
+      const { result } = await harness.executeOnce();
+
+      expect(result?.success).toBeTrue();
+    });
+  });
+});

packages/angular/build/src/tools/esbuild/application-code-bundle.ts

@@ -71,6 +71,12 @@ export function createBrowserCodeBundleOptions(
       supported: getFeatureSupport(zoneless),
     };
 
+    if (options.disableCodeSplitting) {
+      // Splitting emits shared chunks that are read across chunk boundaries as live ESM bindings,
+      // which the unit-test runners' module loading does not reliably preserve.
+      buildOptions.splitting = false;
+    }
+
     buildOptions.plugins ??= [];
     buildOptions.plugins.push(
       createWasmPlugin({ allowAsync: zoneless, cache: loadCache }),