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How to make the nodemailer mocking object or factory in nestjs test code?

I am using nodemailer in my NestJS application to send emails. To prevent real emails from being sent whenever I run tests, I want to create a mocking object of nodemailer that I can use in my tests. However, I am not sure how to create this object properly.

When I tried to make a mocking object, there is "connect ECONNREFUSED 127.0.0.1:587". This error is that "nodemailer needs an access connect to gmail". So it added the setting values using docenv code.

dotenv.config({
  path: path.resolve(`src/config/env/.development.env`),
});

After that, test is success and it always send the verification mail.But I don't want to send emails when running test and want to use mocking object.

I have uploaded the entire code to CodeSandBox except .env, so I hope it is helpful for reference.

Source code (codeSandbox) : https://codesandbox.io/p/sandbox/crazy-bouman-2tvq79

UserService test code : https://codesandbox.io/p/sandbox/crazy-bouman-2tvq79?file=%2Fsrc%2Fusers%2Fusers.service.spec.ts&selection=%5B%7B%22endColumn%22%3A4%2C%22endLineNumber%22%3A26%2C%22startColumn%22%3A4%2C%22startLineNumber%22%3A26%7D%5D

EmailService : https://codesandbox.io/p/sandbox/crazy-bouman-2tvq79?file=%2Fsrc%2Femail%2Femail.service.ts&selection=%5B%7B%22endColumn%22%3A4%2C%22endLineNumber%22%3A26%2C%22startColumn%22%3A4%2C%22startLineNumber%22%3A26%7D%5D

Solution

  • You can mock your EmailService from your UsersService. Also, if you want to test EmailService you can either use jest.mock(), or you inject nodemailer as a dependency to your service, and then you will be able to mock it easily by using Nest's testing module.

    Mocking EmailService in UsersService

    describe('UsersService', () => {
  let module: TestingModule;
  let service: UsersService;
  let sendUserVerficationEmailMock;

  beforeAll(() => {
    dotenv.config({
      path: path.resolve(`src/config/env/.development.env`),
    });
  });

  beforeEach(async () => {
    sendUserVerficationEmailMock = jest
     .fn()
     .mockImplementation(() => console.log('email sent'));


    module = await Test.createTestingModule({
      imports: [
        TypeOrmModule.forRoot({
          type: 'sqlite',
          database: ':memory:',
          entities: [UserEntity],
          synchronize: true,
        }),
        UsersModule,
      ],
    })
      .overrideProvider(EmailService)
      .useValue({
        sendUserVerficationEmail: sendUserVerficationEmailMock,
      })
      .compile();

    service = module.get(UsersService);
    await service.createUser('tom', '[email protected]', '!@#$');
  });

  it('should throw Error when email is existed', async () => {
    const result = service.createUser('tom', '[email protected]', '!@#$');
    expect(sendUserVerficationEmailMock).toBeCalledTimes(1);
    await expect(result).rejects.toThrowError(
      new UnprocessableEntityException('이미 가입된 이메일 입니다.'),
    );
  });
});

    Mocking NodeMailer in EmailService

    There are many examples online on how to mock third-party libraries with jest. You can try the different approaches mentioned here: https://jestjs.io/docs/jest-object.

    Or even better (IMO), is to inject NodeMailer into your service and follow the same approach as we did in the previous example

    You may also want to consider using a NestJS wrapper module for NodeMailer, something like NestJS Mailer as it offers these dependencies as NestJS modules and providers which are easier to mock.