A sample spec demonstrating Arrange, Act, Assert

preferences.service.spec.ts

describe('PreferencesService', () => {
  describe('.getPreference', () => {
    it('should call getItem from LocalStorage', () => {
      // Arrange: initialize spy and test subject
      const getItemSpy = jasmine.createSpy('getItem');
      const service = new PreferencesService({ getItem: getItemSpy } as LocalStorage);

      // Act: call getPreference
      service.getPreference('test key');

      // Assert: make sure it called the spy
      expect(getItemSpy).toHaveBeenCalled();
    });
  });
});