fix(@angular/build): prevent externalizing builder-injected i18n locale data

packages/angular/build/src/builders/application/execute-build.ts

@@ -15,6 +15,7 @@ import { BundleContextResult, BundlerContext } from '../../tools/esbuild/bundler
 import { ExecutionResult, RebuildState } from '../../tools/esbuild/bundler-execution-result';
 import { BuildOutputFileType } from '../../tools/esbuild/bundler-files';
 import { checkCommonJSModules } from '../../tools/esbuild/commonjs-checker';
+import { LOCALE_DATA_BASE_MODULE } from '../../tools/esbuild/i18n-locale-plugin';
 import { extractLicenses } from '../../tools/esbuild/license-extractor';
 import { profileAsync } from '../../tools/esbuild/profiling';
 import {
@@ -210,6 +211,12 @@ export async function executeBuild(
     const explicitExternal = new Set<string>();
 
     const isExplicitExternal = (dep: string): boolean => {
+      // Locale-data imports are injected by the builder itself (see i18n-locale-plugin) and must stay resolvable
+      // even when the containing package is externalized by the user.
+      if (dep.startsWith(`${LOCALE_DATA_BASE_MODULE}/`)) {
+        return false;
+      }
+
       if (exclusions.has(dep)) {
         return true;
       }

packages/angular/build/src/builders/application/tests/options/external-dependencies_spec.ts

@@ -74,5 +74,33 @@ describeBuilder(buildApplication, APPLICATION_BUILDER_INFO, (harness) => {
       // If not externalized, build will fail with a Node.js platform builtin error
       expect(result?.success).toBeTrue();
     });
+
+    it('should not externalize builder-injected i18n locale-data imports when @angular/common is external', async () => {
+      harness.useProject('test', {
+        root: '.',
+        sourceRoot: 'src',
+        cli: {
+          cache: {
+            enabled: false,
+          },
+        },
+        i18n: {
+          sourceLocale: 'fr',
+        },
+      });
+
+      harness.useTarget('build', {
+        ...BASE_OPTIONS,
+        externalDependencies: ['@angular/common'],
+      });
+
+      const { result } = await harness.executeOnce();
+      expect(result?.success).toBeTrue();
+
+      harness.expectFile('dist/browser/polyfills.js').toExist();
+      harness
+        .expectFile('dist/browser/polyfills.js')
+        .content.not.toMatch(/['"]@angular\/common\/locales\/global\/fr['"]/);
+    });
   });
 });

packages/angular/build/src/tools/esbuild/i18n-locale-plugin.ts

@@ -7,7 +7,7 @@
  */
 
 import type { Plugin, ResolveResult } from 'esbuild';
-import { createRequire } from 'node:module';
+import { createProjectResolver } from '../../utils/resolve-project';
 
 /**
  * The internal namespace used by generated locale import statements and Angular locale data plugin.
@@ -50,7 +50,7 @@ export function createAngularLocaleDataPlugin(): Plugin {
         }
 
         let exact = true;
-        let localeRequire: NodeJS.Require | undefined;
+        let projectResolve: ((packageName: string) => string) | undefined;
         while (partialLocaleTag) {
           // Angular embeds the `en`/`en-US` locale into the framework and it does not need to be included again here.
           // The onLoad hook below for the locale data namespace has an `empty` loader that will prevent inclusion.
@@ -73,10 +73,10 @@ export function createAngularLocaleDataPlugin(): Plugin {
           let result: ResolveResult | undefined;
           const { packages, absWorkingDir } = build.initialOptions;
           if (packages === 'external' && absWorkingDir) {
-            localeRequire ??= createRequire(absWorkingDir + '/');
+            projectResolve ??= createProjectResolver(absWorkingDir);
 
             try {
-              localeRequire.resolve(potentialPath);
+              projectResolve(potentialPath);
 
               result = {
                 errors: [],
@@ -94,6 +94,17 @@ export function createAngularLocaleDataPlugin(): Plugin {
               kind: 'import-statement',
               resolveDir: absWorkingDir,
             });
+            if (result && result.external && absWorkingDir) {
+              projectResolve ??= createProjectResolver(absWorkingDir);
+              try {
+                const resolvedPath = projectResolve(potentialPath);
+                result = {
+                  ...result,
+                  external: false,
+                  path: resolvedPath,
+                };
+              } catch {}
+            }
           }
 
           if (result?.path) {