How to mock es6 class using Jest

Question

I am attempting to mock a class Mailer using jest and I can\'t figure out how to do it. The docs don\'t give many examples of how this works. The process is the

Answer 1

Just for Googlers and future visitors, here's how I've setup jest mocking for ES6 classes. I also have a working example at github, with babel-jest for transpiling the ES module syntax so that jest can mock them properly.

__mocks__/MockedClass.js

const stub = {
  someMethod: jest.fn(),
  someAttribute: true
}

module.exports = () => stub;

Your code can call this with new, and in your tests you can call the function and overwrite any default implementation.

example.spec.js

const mockedClass = require("path/to/MockedClass")(); 
const AnotherClass = require("path/to/AnotherClass");
let anotherClass;

jest.mock("path/to/MockedClass");

describe("AnotherClass", () => {
  beforeEach(() => {
    mockedClass.someMethod.mockImplementation(() => {
      return { "foo": "bar" };
    });

    anotherClass = new AnotherClass();
  });

  describe("on init", () => {
    beforeEach(() => { 
      anotherClass.init(); 
    });

    it("uses a mock", () => {
      expect(mockedClass.someMethod.toHaveBeenCalled();
      expect(anotherClass.settings)
        .toEqual(expect.objectContaining({ "foo": "bar" }));
    });
  });

});

Answer 2

You don't have to mock your mailer class but the mailgun-js module. So mailgun is a function that returns the function messages that return the function send. So the mock will look like this.

for the happy path

const happyPath = () => ({
  messages: () => ({
    send: (args, callback) => callback()
  })
})

for the error case

const errorCase = () => ({
  messages: () => ({
    send: (args, callback) => callback('someError')
  })
})

as you have this 2 cases it make sense to mock the module inside your test. First you have to mock it with a simple spy where we later can set the implementation for our cases and then we have to import the module.

jest.mock('mailgun-js', jest.fn())
import mailgun from 'mailgun-js'
import Mailer from '../../../../server/services/emails/mailer'

As your module uses promises we have 2 options either return the promise from the test or use async/await. I use the later one for more info have a look here.

test('test the happy path', async() => {
 //mock the mailgun so it returns our happy path mock
  mailgun.mockImplementation(() => happyPath)
  //we need to use async/awit here to let jest recognize the promise
  const send = await Mailer.send();
  expect(send).toBe('The email was sent successfully!')
});

If you would like to test that the mailgun send method was called with the correct parameter you need to adapt the mock like this:

const send = jest.fn((args, callback) => callback())
const happyPath = () => ({
  messages: () => ({
    send: send
  })
})

Now you could check that the first parameter for send was correct:

expect(send.mock.calls[0][0]).toMatchSnapshot()