trying to get an inspiration from jest test emitting events for eventemitter objects (http) didn't solve my pain with express.

assume the following nodejs code

// server.js

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

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

module.exports = server;

how to write a test using jest to emit the http "error" event (to cover the error event handler)?

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));
});

but what i get when running the test

 ● 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);

trying to get an inspiration from jest test emitting events for eventemitter objects (http) didn't solve my pain with express.

assume the following nodejs code

// server.js

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

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

module.exports = server;

how to write a test using jest to emit the http "error" event (to cover the error event handler)?

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));
});

but what i get when running the test

 ● 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);

• javascript
• node.js
• express
• jestjs

1 Answer 1

Update: Using jest.mock in each test case the functional scope is possible, but you should import/require the module after calling 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);
  });
});

Unit test results with 100% coverage:

 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./mrdulin/react-apollo-graphql-starter-kit/stackoverflow/60451082/server.test.js:3:20)
        at Object.asyncJestTest (/Users/ldu020/workspace/github./mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/jasmineAsyncInstall.js:100:37)
        at resolve (/Users/ldu020/workspace/github./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./mrdulin/react-apollo-graphql-starter-kit/node_modules/jest-jasmine2/build/queueRunner.js:26:19)
        at promise.then (/Users/ldu020/workspace/github./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

source code: https://github./mrdulin/react-apollo-graphql-starter-kit/tree/master/stackoverflow/60451082