jest test emitting events for eventemitter objects (http)

Question

assume the following nodejs code

const server = http.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)?

Answer 1

Since you create a server in module scope, the code will execute immediately when you require or import server.js . You need to stub the http.createServer before you require this module.

For testing .on(error, callback) method, you should use mockImplementation or mockImplementationOnce , so when the mocked server calls the mocked .on('error', callback) , you will get the original callback in your test case. Which means handler is equivalent to callback . When you call handler(mError) , the mocked error object will be passed into the original callback . Then you can use this mError test your code logic.

Here is the unit test solution:

server.js :

const http = require('http');
const server = http.createServer();

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

module.exports = server;

server.test.js :

const http = require('http');

describe('60435647', () => {
  it('should handle error', () => {
    const mError = new Error('network');
    const mServer = {
      listen: jest.fn().mockReturnThis(),
      on: jest.fn().mockImplementationOnce((event, handler) => {
        // handler is the original callback, the mError variable will be passed into the original callback.
        handler(mError);
      }),
    };
    const createServerSpy = jest.spyOn(http, 'createServer').mockImplementationOnce(() => mServer);
    const logSpy = jest.spyOn(console, 'log');
    require('./server');
    expect(createServerSpy).toBeCalledTimes(1);
    expect(mServer.listen).toBeCalledWith(8080, '127.0.0.1');
    expect(mServer.on).toBeCalledWith('error', expect.any(Function));
    expect(logSpy).toBeCalledWith(mError);
  });
});

Unit test results with 100% coverage:

 PASS  stackoverflow/60435647/server.test.js
  60435647
    ✓ should handle error (459ms)

  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/60435647/server.test.js:5:20)
        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:        3.772s, estimated 6s