Mock a custom event emitter with jest

Question

I want to assert that emit from the EventEmitter class was called with specific parameters by using Jest. I have a separate file where I create an instance of eventEmitter to be used, and on the other class I import it and at some point the event is emitted.

// commonEmitter.ts
const events = require('events');
export const commonEmitter = new events.EventEmitter();

// class.ts
import { commonEmitter } from (..)

export class MyClass {
   (...)       

   method(){
     commonEmitter.emit('eventName', { data: true});
   }
}

// class.spec.ts

let commonEmitterMock: any

beforeEach(() => {
  commonEmitterMock = createMock('emit');
});


it('testMyClass', async () => {
   const method = new MyClass().method();
   expect(commonEmitterMock).toHaveBeenCalledWith('eventName', { data: true})
}

With this implementation the emit event is never called.. Cant figure out why, any idea?

Answer 1

To test different branches of your http request events without giving up on a over-engineered code you can do the follow.

This is a stub version of the function I intend to test using Jest:

    function myRequest(resolve, reject) {
        http.request(url, options, (res: IncomingMessage) => {
            response.on('data', (chunk) => {
              // On data event code
            })
            response.on('end', () => {
              // On end event code
              resolve()
            })
            response.on('error', (err) => {
              reject(err)
            })
        }
    }

Firstly, we need to mock the http library an overwrite the request implementation to manually trigger the callback and inject our mocked response object:

    ...
    const mockRes = {
       write: jest.fn(),
       on: jest.fn(),
       end: jest.fn()
    }
        
    jest.mock('http', () => ({
       request: jest.fn().mockImplementation((url, options, cb) => {
          cb(mockRes)
       })
    })

Then, each of our jest test unit, we manually trigger the callbacks on each of the events we desire to test passing data to each of the specific callbacks:

    it('should call request callback and reject for invalid content response', async () => {
        const resolve = jest.fn()
        const reject = jest.fn()
    
        mockRes.on.mockImplementation((event, cb) => {
          if (event === 'end') {
            cb()
          } else if (event === 'data') {
            cb(new Error('invalid_json_string'))
          }
        })
    
        // @ts-ignore
        myRequest(resolve, reject)
    
        // @ts-ignore
        expect(mockRes.on).toHaveBeenCalledWith('data', expect.any(Function))
        expect(mockRes.on).toHaveBeenCalledWith('end', expect.any(Function))
        expect(reject).toHaveBeenCalledWith(expect.any(Error))
      })