Mock reassigning properties in mocked interface

Question

I have the following interface which I'm trying to mock:

export interface IEmail {
  from: string;
  body: string;
  to: string;
}

This interface is used in a function like:

async function sendEmail(emailData: IEmail): Promise<void> {
  await this.send({
    emailBody: emailData.body,
    emailBodyFrom: emailData.from,
    emailTo: emailData.to,
  });
}

async function send(email) {
  verifyEmail(email)
  emailprovider.send(email)
}

I tried mocking the interface with jest-mock-extended like this:

it('should send email', async () => {
  const options = mock<IEmail>({
    emailBody: "You got mail",
    from: "marc@meta.com",
    to: "jane.doe@gmail.com",
  });

  mockemailService.send.mockResolvedValueOnce("");
  await emailService.sendEmail(options);
  expect(mockemailService.send).toHaveBeenCalledWith({
    emailBody: 'You got mail',
    emailBodyFrom: 'marc@meta.com',
    emailTo: 'jane.doe@gmail.com',
  });
});

Running the test gives the following diff:

- emailBody: 'You got mail',
- emailBodyFrom: 'marc@meta.com',
- emailTo: 'jane.doe@gmail.com',
+ emailBody: undefined,
+ emailBodyFrom: undefined,
+ emailTo: undefined,

I have debugged and it seems like when I copy the vales from one object to another emailBody: emailData.body is causing the value to be undefined.

Is there a better way to create the mock perhaps?

Answer 1

I was able to resolve it by setting the fields like this instead:

const options = mock<IEmail>({});
options.emailBody = "You got mail"
options.from = "marc@meta.com"
options.to = "jane.doe@gmail.com"