fix(@angular/build): ensure locale base href retains leading slash

When baseHref is provided, getLocaleBaseHref was incorrectly stripping the leading slash from the joined path. This commit removes the stripLeadingSlash call to ensure the leading slash is preserved when appropriate.

Closes angular#32030

Author: alan-agius4

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

@@ -25,7 +25,7 @@ import {
   loadPostcssConfiguration,
 } from '../../utils/postcss-configuration';
 import { getProjectRootPaths, normalizeDirectoryPath } from '../../utils/project-metadata';
-import { addTrailingSlash, joinUrlParts } from '../../utils/url';
+import { addTrailingSlash, joinUrlParts, stripLeadingSlash } from '../../utils/url';
 import {
   Schema as ApplicationBuilderOptions,
   ExperimentalPlatform,
@@ -681,9 +681,16 @@ export function getLocaleBaseHref(
 
   const baseHrefSuffix = localeData.baseHref ?? localeData.subPath + '/';
 
-  return baseHrefSuffix !== ''
-    ? addTrailingSlash(joinUrlParts(baseHref, baseHrefSuffix))
-    : undefined;
+  let joinedBaseHref: string | undefined;
+  if (baseHrefSuffix !== '') {
+    joinedBaseHref = addTrailingSlash(joinUrlParts(baseHref, baseHrefSuffix));
+
+    if (baseHref && baseHref[0] !== '/') {
+      joinedBaseHref = stripLeadingSlash(joinedBaseHref);
+    }
+  }
+
+  return joinedBaseHref;
 }
 
 /**

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

@@ -0,0 +1,106 @@
+/**
+ * @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 { NormalizedApplicationBuildOptions, getLocaleBaseHref } from './options';
+
+describe('getLocaleBaseHref', () => {
+  const baseI18nOptions: NormalizedApplicationBuildOptions['i18nOptions'] = {
+    inlineLocales: new Set(),
+    sourceLocale: 'en-US',
+    locales: {},
+    flatOutput: false,
+    shouldInline: false,
+    hasDefinedSourceLocale: false,
+  };
+
+  it('should return undefined if flatOutput is true', () => {
+    const result = getLocaleBaseHref(undefined, { ...baseI18nOptions, flatOutput: true }, 'fr');
+    expect(result).toBeUndefined();
+  });
+
+  it('should return undefined if locale is not found', () => {
+    const result = getLocaleBaseHref(undefined, baseI18nOptions, 'fr');
+    expect(result).toBeUndefined();
+  });
+
+  it('should return baseHref from locale data if present', () => {
+    const i18nOptions = {
+      ...baseI18nOptions,
+      locales: {
+        fr: {
+          files: [],
+          translation: {},
+          subPath: 'fr',
+          baseHref: '/fr/',
+        },
+      },
+    };
+    const result = getLocaleBaseHref(undefined, i18nOptions, 'fr');
+    expect(result).toBe('/fr/');
+  });
+
+  it('should join baseHref and locale subPath if baseHref is provided', () => {
+    const i18nOptions = {
+      ...baseI18nOptions,
+      locales: {
+        fr: {
+          files: [],
+          translation: {},
+          subPath: 'fr',
+        },
+      },
+    };
+    const result = getLocaleBaseHref('/app/', i18nOptions, 'fr');
+    expect(result).toBe('/app/fr/');
+  });
+
+  it('should handle missing baseHref (undefined) correctly', () => {
+    const i18nOptions = {
+      ...baseI18nOptions,
+      locales: {
+        fr: {
+          files: [],
+          translation: {},
+          subPath: 'fr',
+        },
+      },
+    };
+    const result = getLocaleBaseHref(undefined, i18nOptions, 'fr');
+    expect(result).toBe('/fr/');
+  });
+
+  it('should handle empty baseHref correctly', () => {
+    const i18nOptions = {
+      ...baseI18nOptions,
+      locales: {
+        fr: {
+          files: [],
+          translation: {},
+          subPath: 'fr',
+        },
+      },
+    };
+    const result = getLocaleBaseHref('', i18nOptions, 'fr');
+    expect(result).toBe('/fr/');
+  });
+
+  it('should strip leading slash if baseHref does not start with slash', () => {
+    const i18nOptions = {
+      ...baseI18nOptions,
+      locales: {
+        fr: {
+          files: [],
+          translation: {},
+          subPath: 'fr',
+        },
+      },
+    };
+    const result = getLocaleBaseHref('app/', i18nOptions, 'fr');
+    expect(result).toBe('app/fr/');
+  });
+});