javascript - 如何对JavaScript中的promise的“then”结果进行单元测试？

我正在使用TypeScript编写一个利用AWS开发工具包的非常简单的服务。我的Jest单元测试通过了，但是覆盖率报告说'return result.Items'行未覆盖。谁能说出为什么呢？这是开玩笑的错误吗？

// 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' });
});

最佳答案

未覆盖的行是传递给then的成功回调。

模拟将then替换为不接受任何参数而仅返回对象的函数。在测试期间，来自代码的回调将传递给then模拟，但它不会调用该回调，因此Jest正确地报告测试未覆盖该回调。

与其尝试返回看起来像Promise的模拟对象，不如从模拟中返回实际解析的Promise：

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

...那样，then仍将是实际的Promise.prototype.then，您的回调将按预期方式被调用。



您还应该await返回的Promise以确保在测试完成之前已调用了回调：

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',
  });
});