mocking mailgun.js with jest/typescript

I've got this emailService module in my project:

import config from "../config";
import formData from "form-data";
import Mailgun from "mailgun.js";
import MailOptions from "../@types/MailOptions";

const mailgun = new Mailgun(formData);
const mg = mailgun.client({
  username: "api",
  key: config.mailApiKey,
  url: config.mailApiUrl,
});

export const sendMail = (mailOptions: MailOptions): Promise<unknown> => {
  return new Promise((resolve, reject) => {
    mg.messages
      .create(config.mailDomain, mailOptions)
      .then((msg) => resolve(msg))
      .catch((err) => reject(err));
  });
};

export const setMailOptions = (
  recipientEmail: string,
  subject: string,
  body: string
): MailOptions => {
  const mailOptions: MailOptions = {
    from: config.senderEmail,
    to: recipientEmail,
    subject: subject,
    html: body,
  };

  return mailOptions;
};

Normally I import this emailService.ts in any other module I want to send emails in.

I'm trying to figure out how to test the sendMail() in emailService, and assert that mg.messages.create() was called.

I have a feeling I need to refactor this code so that I can mock the config values and Mailgun instance being used by emailService, unless there's an alternative way to mock Mailgun in the test suite in such a way that that the emailService would use it?

You'll need to export your mailgun client instance in order to mock it.

mailgun.ts:

import config from "../config";
import formData from "form-data";
import Mailgun from "mailgun.js";

const mailgun = new Mailgun(formData);
const mg = mailgun.client({
  username: "api",
  key: config.mailApiKey,
  url: config.mailApiUrl,
});

export default mg;

mailgunMock.ts:

import { mockDeep, DeepMockProxy } from 'jest-mock-extended';
import Client from 'mailgun.js/client';
import mg from '../mailgun';

jest.mock('../mailgun', () => ({
    __esModule: true,
    default: mockDeep<Client>(),
}));

export const mailgunMock = mg as unknown as DeepMockProxy<Client>;

You can then use this mock in your test suite:

emailService.test.ts:

import { mailgunMock } from './mocks/mailgunMock';
import { sendMail } from '../emailService';

describe('send email', () => {

     const emailMessage = {
            id: 'test',
            message: 'test',
            status: 200,
            details: 'n/a',
        };

     test('setup mocks', async () => {
          mailgunMock.messages.create.mockResolvedValue(emailMessage);
     });

     test('should send email', async () => {
          await expect(
               sendMail({from: 'test', to: 'test', subject: 'test', html: 'test'}),
          ).resolves.toEqual(emailMessage);

          expect(mailgunMock.messages.create.mock.calls[0][1]).toEqual(
                expect.objectContaining({
                    from: 'test',
                    to: 'test',
                    subject: 'test',
                    html: 'test',
                }),
            );
     });
});