单元测试报错：Cannot read property 'map' of undefined 问题排查

我尝试对一个方法做单元测试，通过mock addressList来测试，但报错提示“Cannot read property 'map' of undefined”。

待测试方法 #

async paginate(
  user: User,
  options: IPaginationOptions,
): Promise<Pagination<AddressResponse>> {
  const pagination = await this.addressRepository.paginate(user, options);

  return new Pagination(
    await Promise.all(
      pagination.items.map(async (address) => new AddressResponse(address)),
    ),
    pagination.meta,
    pagination.links,
  );
}

问题现象 #

用console.log查看pagination变量时，控制台显示它是一个非空数组：

console.log
    addressList:  [ Address {} ]

这是repository返回的数据。

测试代码 #

describe('AddressService', () => {
  let addressService: AddressService;
  const user = new User();
  const addressList: Address[] = [new Address()];

  console.log('addressList: ', addressList);

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        AddressService,
        {
          provide: getRepositoryToken(AddressRepository),
          useValue: {
            paginate: jest.fn().mockResolvedValue(addressList),
            create: jest.fn(),
            save: jest.fn(),
            findById: jest.fn(),
            findOne: jest.fn(),
            update: jest.fn(),
            delete: jest.fn(),
          },
        },
      ],
    }).compile();

    addressService = module.get<AddressService>(AddressService);
  });

  it('should be defined', () => {
    expect(addressService).toBeDefined();
  });

  describe('paginate', () => {
    it('should return an address list successfully', async () => {
      // Act
      const result = await addressService.paginate(user, {
        page: 1,
        limit: 10,
      });

      // Assert
      expect(result).toEqual(addressList);
    });
  });
});

核心问题 #

测试代码中mock的paginate方法返回的是纯数组addressList，但待测试方法里，pagination变量被期望是一个包含items、meta、links属性的分页对象。当调用pagination.items.map()时，因为pagination是数组，没有items属性，所以pagination.items为undefined，触发报错。

修复步骤 #

1. 调整mock返回值结构，让它匹配Pagination类型的要求：

   const mockPaginationResult = {
     items: addressList,
     meta: { page: 1, limit: 10, total: 1, totalPages: 1 }, // 按业务需求定义meta字段
     links: {
       first: '/addresses?page=1&limit=10',
       last: '/addresses?page=1&limit=10',
       prev: null,
       next: null,
     } // 按业务需求定义links字段
   };
   

2. 更新mock的paginate方法，返回上述结构的对象：

   paginate: jest.fn().mockResolvedValue(mockPaginationResult),
   

3. 修正测试断言，因为待测试方法返回的是Pagination<AddressResponse>实例，不是原始数组：

   it('should return an address list successfully', async () => {
     // Act
     const result = await addressService.paginate(user, {
       page: 1,
       limit: 10,
     });
   
     // Assert
     expect(result.items).toBeDefined();
     expect(result.items.length).toBe(addressList.length);
     expect(result.meta).toEqual(mockPaginationResult.meta);
     expect(result.links).toEqual(mockPaginationResult.links);
   });
   

额外说明 #

你看到的console.log显示addressList: [ Address {} ]，是因为mock的返回值就是这个数组，但待测试方法需要的是带items属性的分页对象，结构不匹配才导致了错误。

内容的提问来源于stack exchange，提问作者Lucifer's Heritage