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.

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/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 }),