jest test emitting events for eventemitter objects (http)

问题

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)?

回答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

来源：https://stackoverflow.com/questions/60435647/jest-test-emitting-events-for-eventemitter-objects-http

标签

javascript

node.js

jestjs