明明已调用模拟函数，Jest却断言未调用？问题排查

业务代码 #

const logger = require('../../../common/lib/logger')();

const displayErrors = async (errors) => {
  const promisesToWriteData = [];

  errors.forEach((error) => {
    console.log({error});
    promisesToWriteData.push(logger.error(`${error[0]}: ${error[1]} - Customer: ${error[2]}`));
  });
  await Promise.all(promisesToWriteData);
};

测试代码 #

const logger = require('../../../../common/lib/logger');

jest.mock('../../../../common/lib/logger');

const logInfoMock = jest.fn();
const logErrorMock = jest.fn();
logger.mockImplementation(() => ({
  info: logInfoMock,
  error: logErrorMock,
}));

const { displayErrors } = require('../../lib/verification-utils');

const errorArray = [
  [
    'customerData.PaymentDetails.CasePaymentGroup.PAXNominee',
    'NomineeID missing',
    'example4',
  ],
  [
    'customerData.PaymentDetails.CasePaymentGroup.PAXNominee',
    'Comments missing',
    'example4',
  ],
];

describe('Display errors', () => {
  it('should send errors to the logger when there are errors in the error array', async () => {
    const flushPromises = () => new Promise(setImmediate);
    await displayErrors(errorArray);
    await flushPromises();
    expect(logger).toHaveBeenCalledTimes(2);
  });
});

测试报错 #

expect(jest.fn()).toHaveBeenCalled()

    Expected number of calls: >= 2
    Received number of calls:    0

      34 |     await displayErrors(errorArray);
      35 |     await flushPromises();
    > 36 |     expect(logger).toHaveBeenCalled();
         |                    ^
      37 |   });
      38 | });
      39 |

已知业务代码中的console.log已输出错误数组，说明逻辑执行到了调用logger.error的位置，但测试断言始终失败。

问题根源 #

1. 断言对象错误：测试中错误断言了logger工厂函数的调用次数，但业务代码里logger工厂函数在模块加载时仅执行一次（require('...')()），后续调用的是实例上的error方法，正确的断言对象应该是logErrorMock。
2. Mock函数返回值不符合异步要求：业务代码将logger.error的返回值加入Promise数组并通过await Promise.all等待，但默认Jest mock函数返回undefined而非Promise，导致异步流程无法正确等待。

修复方案 #

修改测试代码 #

1. 将断言对象改为logErrorMock，验证其被调用2次；
2. 让logErrorMock返回resolved状态的Promise，匹配业务代码的异步场景；
3. 移除不必要的flushPromises，因为await Promise.all已能确保所有异步操作完成。

修改后的测试代码：

const logger = require('../../../../common/lib/logger');

jest.mock('../../../../common/lib/logger');

const logInfoMock = jest.fn();
// 让mock返回Promise，适配业务代码的异步逻辑
const logErrorMock = jest.fn(() => Promise.resolve());
logger.mockImplementation(() => ({
  info: logInfoMock,
  error: logErrorMock,
}));

const { displayErrors } = require('../../lib/verification-utils');

const errorArray = [
  [
    'customerData.PaymentDetails.CasePaymentGroup.PAXNominee',
    'NomineeID missing',
    'example4',
  ],
  [
    'customerData.PaymentDetails.CasePaymentGroup.PAXNominee',
    'Comments missing',
    'example4',
  ],
];

describe('Display errors', () => {
  it('should send errors to the logger when there are errors in the error array', async () => {
    await displayErrors(errorArray);
    // 断言error方法的调用次数与参数
    expect(logErrorMock).toHaveBeenCalledTimes(2);
    expect(logErrorMock).toHaveBeenNthCalledWith(1, 
      'customerData.PaymentDetails.CasePaymentGroup.PAXNominee: NomineeID missing - Customer: example4'
    );
    expect(logErrorMock).toHaveBeenNthCalledWith(2, 
      'customerData.PaymentDetails.CasePaymentGroup.PAXNominee: Comments missing - Customer: example4'
    );
  });
});

业务代码可选优化 #

如果logger.error本身不是异步方法，业务代码无需保持异步逻辑，可简化为：

const logger = require('../../../common/lib/logger')();

const displayErrors = (errors) => {
  errors.forEach((error) => {
    console.log({error});
    logger.error(`${error[0]}: ${error[1]} - Customer: ${error[2]}`);
  });
};

对应的测试代码也可去掉Promise相关处理，直接断言logErrorMock的调用次数即可。

内容的提问来源于stack exchange，提问作者Bragon