玩笑测试为 eventemitter 对象发出事件（express）

试图从中获得灵感玩笑测试为 eventemitter 对象发出事件 (http)没有解决我的痛苦express.

假设有以下nodejs代码

// server.js

const express = require("express");
const app = express();

const server = app.listen(8080,'127.0.0.1')
  .on("error", err => {
    // ...
  });

module.exports = server;

如何使用 jest 编写测试来发出 http“错误”事件（以覆盖错误事件处理程序）？

i tried:

// server.test.js

it("should handle error", () => {
  jest.mock("express", () => () => ({
    listen: jest.fn().mockReturnThis(),
    on: jest.fn().mockImplementationOnce((event, handler) => {
      handler(new Error("network"));
    })
  }))
  const express = require("express");
  const app = express();
  const appListenSpy = jest.spyOn(app, "listen")
  require("./server");
  expect(appListenSpy).toBeCalledTimes(1);
  expect(app.listen).toBeCalledWith(8080,'127.0.0.1');
  expect(app.on).toBeCalledWith("error", expect.any(Function));
});

但运行测试时我得到了什么

 ● server › should handle listen error

    expect(jest.fn()).toBeCalledTimes(expected)

    Expected number of calls: 1
    Received number of calls: 0

    > 29 |     expect(appListenSpy).toBeCalledTimes(1);

Update: Using jest.mock在每个测试用例中，功能范围都是可能的，但您应该在调用后导入/需要模块jest.mock().

You can't use jest.mock in the function scope. It should be used in the module scope. Instead of using jest.mock inside the test case function, you should use jest.doMock(moduleName, factory, options)

E.g. server.js:

const express = require('express');
const app = express();

const server = app.listen(8080, '127.0.0.1').on('error', (err) => {
  console.log(err);
});

module.exports = server;

server.test.js:

describe('60451082', () => {
  it('should pass', () => {
    const mError = new Error('network');
    const appMock = {
      listen: jest.fn().mockReturnThis(),
      on: jest.fn().mockImplementationOnce((event, handler) => {
        handler(mError);
      }),
    };
    jest.doMock('express', () => jest.fn(() => appMock));
    const logSpy = jest.spyOn(console, 'log');
    const express = require('express');
    require('./server');
    expect(express).toBeCalledTimes(1);
    expect(appMock.listen).toBeCalledWith(8080, '127.0.0.1');
    expect(appMock.on).toBeCalledWith('error', expect.any(Function));
    expect(logSpy).toBeCalledWith(mError);
  });
});

100%覆盖率的单元测试结果：

 PASS  stackoverflow/60451082/server.test.js
  60451082
    ✓ should pass (19ms)

  console.log node_modules/jest-environment-enzyme/node_modules/jest-mock/build/index.js:866
    Error: network
        at Object.<anonymous> (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60451082/server.test.js:3:20)
        at Object.asyncJestTest (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/jasmineAsyncInstall.js:100:37)
        at resolve (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:43:12)
        at new Promise (<anonymous>)
        at mapper (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:26:19)
        at promise.then (/Users/ldu020/workspace/github.com/mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:73:41)
        at process._tickCallback (internal/process/next_tick.js:68:7)

-----------|---------|----------|---------|---------|-------------------
File       | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
-----------|---------|----------|---------|---------|-------------------
All files  |     100 |      100 |     100 |     100 |                   
 server.js |     100 |      100 |     100 |     100 |                   
-----------|---------|----------|---------|---------|-------------------
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        4.444s, estimated 10s

源代码：https://github.com/mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/60451082