jest: how to mock a dependency that listens to events?

Question

I came across a very complicated situation. I'll try to keep it as concise as possible.

So I have a code like this in myModule.js:

const lib = require('@third-party/lib');

const myFunction = () => {
  const client = lib.createClient('foo');
  return new Promise((resolve, reject) => {
    client.on('error', (err) => reject(err));
    client.on('success', () => {
      client.as(param1).post(param2, param3, (err, data) => {
        if (err) reject(err);

        // Some important logical processing of data
        resolve(data);
      });
    });
  });
}

module.exports = { myFunction };

There are a few things I am able to mock, like: createClient. What I am not able to mock is the event part I don't even know how to do this. And the .as().post() part.

Here's how my jest test looks like:

const myModule = require('./myModule');
const mockData = require('./mockData');

describe('myFunction', () => {

  it('Should resolve promise when lib calls success event', async () => {
    try {
      const myData = await myModule.myFunction();
      expect(myData).toMatchObject(mockData.resolvedData);
    } catch (err) {
      expect(err).toBeNull();
    }
  })
});

Any help, much Appreciated.

I tried to find similar questions but at this point, my mind has just stopped working... Please let me know if you need any more details.

Answer 1

Here’s what you need to do:

// EventEmitter is here to rescue you
const events = require("events");

// Mock the third party library
const lib = require("@third-party/lib");

lib.createClient.mockImplementationOnce(params => {
  const self = new events.EventEmitter();

  self.as = jest.fn().mockImplementation(() => {
    // Since we're calling post on the same object.
    return self;
  });

  self.post = jest.fn().mockImplementation((arg1, _cb) => {
    // Can have a conditional check for arg 1 if so desird
    _cb(null, { data : "foo" });
  });

  // Finally call the required event with delay.
  // Don't know if the delay is necessary or not.
  setInterval(() => {
    self.emit("success");
  }, 10);
  return self;
}).mockImplementationOnce(params => {
  const self = new events.EventEmitter();

  // Can also simulate event based error like so:
  setInterval(() => {
    self.emit("error", {message: "something went wrong."});
  }, 10);
  return self;
}).mockImplementationOnce(params => {
  const self = new events.EventEmitter();
  self.as = jest.fn().mockImplementation(() => {
    return self;
  });

  self.post = jest.fn().mockImplementation((arg1, _cb) => {
    // for negative callback in post I did:
    _cb({mesage: "Something went wrong"}, null);
  });

  setInterval(() => {
    self.emit("success");
  }, 10);
  return self;
});

This is only the mock object that you need to put in your test.js file.

Not sure if this code will work as is, although won’t require a lot of debugging.

If you just want to positive scenario, remove the second mockImplementationOnce and replace the first mockImplementationOnce with just mockImplementation.