test(locale): add unit tests for normalizeStartOfWeek function

Author: Pixselve

src/internal/locale/__tests__/normalize-start-of-week.test.ts

@@ -0,0 +1,46 @@
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+import { normalizeStartOfWeek } from '../normalize-start-of-week';
+
+// Mock the weekstart module
+jest.mock('weekstart', () => ({
+  getWeekStartByLocale: jest.fn(),
+}));
+
+import { getWeekStartByLocale } from 'weekstart';
+
+const mockGetWeekStartByLocale = getWeekStartByLocale as jest.MockedFunction<typeof getWeekStartByLocale>;
+
+describe('normalizeStartOfWeek', () => {
+  beforeEach(() => {
+    mockGetWeekStartByLocale.mockReset();
+  });
+
+  describe('when startOfWeek is undefined', () => {
+    test('should call getWeekStartByLocale with the provided locale', () => {
+      mockGetWeekStartByLocale.mockReturnValue(0);
+      const result = normalizeStartOfWeek(undefined, 'en-US');
+
+      expect(mockGetWeekStartByLocale).toHaveBeenCalledWith('en-US');
+      expect(mockGetWeekStartByLocale).toHaveBeenCalledTimes(1);
+      expect(result).toBe(0);
+    });
+
+    test('should return Sunday (0) for US locale', () => {
+      mockGetWeekStartByLocale.mockReturnValue(0);
+      expect(normalizeStartOfWeek(undefined, 'en-US')).toBe(0);
+    });
+
+    test('should return Monday (1) for French locale', () => {
+      mockGetWeekStartByLocale.mockReturnValue(1);
+      expect(normalizeStartOfWeek(undefined, 'fr-FR')).toBe(1);
+    });
+  });
+
+  test('should prioritize provided number over locale', () => {
+    mockGetWeekStartByLocale.mockReturnValue(0);
+    expect(normalizeStartOfWeek(1, 'en-US')).toBe(1);
+    expect(mockGetWeekStartByLocale).not.toHaveBeenCalled();
+  });
+});