javascript - 我如何对 JavaScript 中 promise 的 'then' 结果进行单元测试？

Question

我正在使用 TypeScript 编写一个非常简单的服务，该服务利用了 AWS 开发工具包。我的 Jest 单元测试通过了，但是覆盖率报告说“return result.Items”这一行没有被覆盖。谁能说出这是为什么？这是 Jest 错误吗？

// service file

/**
 * Gets an array of documents.
 */
function list(tableName) {
  const params = {
    TableName: tableName,
  };
  return docClient
    .scan(params)
    .promise()
    .then((result) => {
      return result.Items;
    });
}

// test file

const stubAwsRequestWithFakeArrayReturn = () => {
  return {
    promise: () => {
      return { then: () => ({ Items: 'fake-value' }) };
    },
  };
};

it(`should call docClient.scan() at least once`, () => {
  const mockAwsCall = jest.fn().mockImplementation(stubAwsRequest);
  aws.docClient.scan = mockAwsCall;
  db.list('fake-table');
  expect(mockAwsCall).toBeCalledTimes(1);
});

it(`should call docClient.scan() with the proper params`, () => {
  const mockAwsCall = jest.fn().mockImplementation(stubAwsRequest);
  aws.docClient.scan = mockAwsCall;
  db.list('fake-table');
  expect(mockAwsCall).toBeCalledWith({
    TableName: 'fake-table',
  });
});

it('should return result.Items out of result', async () => {
  const mockAwsCall = jest
    .fn()
    .mockImplementation(stubAwsRequestWithFakeArrayReturn);
  aws.docClient.get = mockAwsCall;
  const returnValue = await db.get('fake-table', 'fake-id');
  expect(returnValue).toEqual({ Items: 'fake-value' });
});

Answer 1

未覆盖的行是传递给 then 的成功回调。

您的模拟将 then 替换为不接受任何参数且仅返回一个对象的函数。来自您代码的回调在测试期间被传递给 then 模拟，但它不会调用回调，因此 Jest 正确地报告回调未被您的测试覆盖。

与其尝试返回一个看起来像Promise的模拟对象，不如从您的模拟中返回一个实际解析的Promise:

const stubAwsRequestWithFakeArrayReturn = () => ({
  promise: () => Promise.resolve({ Items: 'fake-value' })
});

...这样 then 仍然是实际的 Promise.prototype.then并且您的回调将按预期调用。

---

您还应该await 返回的Promise 以确保在测试完成之前调用回调:

it(`should call docClient.scan() at least once`, async () => {
  const mockAwsCall = jest.fn().mockImplementation(stubAwsRequest);
  aws.docClient.scan = mockAwsCall;
  await db.list('fake-table');  // await the Promise
  expect(mockAwsCall).toBeCalledTimes(1);
});

it(`should call docClient.scan() with the proper params`, async () => {
  const mockAwsCall = jest.fn().mockImplementation(stubAwsRequest);
  aws.docClient.scan = mockAwsCall;
  await db.list('fake-table');  // await the Promise
  expect(mockAwsCall).toBeCalledWith({
    TableName: 'fake-table',
  });
});