Jest测试返回Promise的axios流方法时出现超时错误如何解决

你的downloadMtgJsonZip方法调整后返回了一个自定义Promise，该Promise仅在writer触发close/error事件时才会resolve/reject。你的测试用例中存在两个核心问题导致Promise永远不会被解决，最终触发超时：

1. mock的axios返回的是普通对象，而非异步Promise，await axios()之后拿到的值不符合预期
2. mock的createWriteStream返回的on方法仅做了函数记录，没有触发你注册的close事件回调，导致返回的Promise一直处于pending状态

1. 基础依赖mock声明 #

首先确认测试文件顶部已经对依赖做了全局mock：

jest.mock('axios');
jest.mock('fs');
jest.mock('progress', () => jest.fn()); // 如果你用的是progress库的话

2. 修正测试用例逻辑 #

第一个测试修改后代码： #

it('fetches successfully data from an URL', async () => {
  const onFn = jest.fn();
  // axios返回的是resolve后的响应对象
  const response = { 
    status: 200, 
    data: { pipe: jest.fn(), on: onFn }, 
    headers: { 'content-length': 100 } 
  };

  // 存储writer的事件回调
  const writerEventHandlers = {};
  const writerOnFn = jest.fn((event, callback) => {
    writerEventHandlers[event] = callback;
  });

  // 注意这里返回Promise.resolve，符合axios异步调用的逻辑
  axios.mockImplementationOnce(() => Promise.resolve(response));
  fs.createWriteStream.mockImplementationOnce(() => ({ 
    on: writerOnFn,
    close: jest.fn()
  }));

  // 先拿到Promise引用，手动触发close事件后再await
  const downloadPromise = downloadMtgJsonZip();
  writerEventHandlers['close']();
  await downloadPromise;

  expect(onFn).toHaveBeenCalledWith('data', expect.any(Function));
  expect(axios).toHaveBeenCalledWith(
    expect.objectContaining({ url: 'https://mtgjson.com/api/v5/AllPrintings.json.zip' }),
  );
  expect(axios).toHaveBeenCalledWith(
    expect.objectContaining({ responseType: 'stream' }),
  );
});

第二个测试修改后代码： #

it('ticks up the progress bar', async () => {
  const tickFn = jest.fn();
  // 传入的chunk长度为1，匹配测试预期
  const dataOnFn = jest.fn((name, func) => func({ length: 1 }));
  const response = { 
    status: 200, 
    data: { pipe: jest.fn(), on: dataOnFn }, 
    headers: { 'content-length': 1 } 
  };

  const writerEventHandlers = {};
  const writerOnFn = jest.fn((event, callback) => {
    writerEventHandlers[event] = callback;
  });

  ProgressBar.mockImplementationOnce(() => ({ tick: tickFn }));
  axios.mockImplementationOnce(() => Promise.resolve(response));
  fs.createWriteStream.mockImplementationOnce(() => ({ 
    on: writerOnFn,
    close: jest.fn()
  }));

  const downloadPromise = downloadMtgJsonZip();
  writerEventHandlers['close']();
  await downloadPromise;

  expect(ProgressBar).toHaveBeenCalledWith(
    expect.stringContaining('downloading'),
    expect.objectContaining({
      total: 1,
    }),
  );
  expect(tickFn).toHaveBeenCalledWith(1);
});

3. 解决VSCode类型报错 #

出现「表达式不可调用」和「mockImplementationOnce不存在」的提示是类型问题，按以下步骤修复：

• 确认axios的导入方式为默认导入：import axios from 'axios'，不要用import * as axios from 'axios'
• 给mock的axios做类型断言，示例：

import axios from 'axios';
const mockedAxios = axios as jest.MockedFunction<typeof axios>;
// 后续调用mock方法用mockedAxios即可
mockedAxios.mockImplementationOnce(() => Promise.resolve(response));

内容的提问来源于stack exchange，提问作者AncientSwordRage