Jest TypeScript Error axios_1.default.post.mockImplementation is not a function

Question

SEE UPDATE BELOW:

Note:

I am not using axios-mock-adapter, unless someone can point me to an example where I can test toHaveBeenCalledWith.

I also want to avoid creating a ̶_̶_̶m̶o̶c̶k̶_̶_̶ __mocks__ ( folder, to mock the entire module.

---

Error:

All my tests pass, but I still get this type error.

TypeError: axios_1.default.post.mockImplementation is not a function

      28 | const axiosPost = jest.fn(() => ({ success: true }));
      29 | jest.mock('axios');
    > 30 | (axios as jest.Mocked<any>).post.mockImplementation(
         |                                           ^
      31 |   jest.fn().mockImplementation(axiosPost),
      32 | );

---

Code:

File: __tests__/index.ts

import axios from 'axios';

// ...

// Before Tests
const axiosPost = jest.fn();
jest.mock('axios');
(axios as jest.Mocked<typeof axios>).post.mockImplementation(
  jest.fn().mockImplementation(axiosPost),
);

// ...

beforeEach(() => {
  jest.clearAllMocks();
});

// Test Performed & Passes
/**
 * Validates send email request
 */
test('test - sendEmail - [email protected], [email protected], my subject, hello there!', async () => {
  // Setup
  const from = '[email protected]';
  const to = '[email protected]';
  const subject = 'my subject';
  const body = 'hello there!';
  const basicAuth = Buffer.from(`api:secret`).toString('base64');

  process.env.MAILGUN_API_URL = 'url';
  process.env.MAILGUN_DOMAIN = 'domain';
  process.env.MAILGUN_SECRET_KEY = 'secret';

  // Pre Expectations
  expect(formData).not.toHaveBeenCalled();
  expect(axiosPost).not.toHaveBeenCalled();

  // Init
  const result = await sendEmail(from, to, subject, body);

  // Post Expectations
  // Form Data
  expect(formData).toHaveBeenCalledTimes(1);
  expect(formDataAppend).toHaveBeenCalledTimes(4);
  expect(formDataAppend.mock.calls[0][0]).toEqual('from');
  expect(formDataAppend.mock.calls[0][1]).toEqual(from);
  expect(formDataAppend.mock.calls[1][0]).toEqual('to');
  expect(formDataAppend.mock.calls[1][1]).toEqual(to);
  expect(formDataAppend.mock.calls[2][0]).toEqual('subject');
  expect(formDataAppend.mock.calls[2][1]).toEqual(subject);
  expect(formDataAppend.mock.calls[3][0]).toEqual('html');
  expect(formDataAppend.mock.calls[3][1]).toEqual(body);
  expect(formDataGetHeaders).toHaveBeenCalledTimes(1);

  // Axios
  expect(axiosPost).toHaveBeenCalledTimes(1);
  expect(axios.post).toHaveBeenCalledWith(
    'url/domain/messages',
    { append: formDataAppend, getHeaders: formDataGetHeaders },
    {
      headers: {
        Authorization: `Basic ${basicAuth}`,
        'Content-Type': 'multipart/form-data',
      },
    },
  );
  expect(result).toStrictEqual({ success: true });
});

---

UPDATE

As per @estus-flask's recommendation and direction I modified the code to mock axios as a returned object which contains post and to not test for a jest.fn() but to use a spyOn.

Modified File: __tests__/index.ts

// Mocks
// ========================================================
//  ̶c̶o̶n̶s̶t̶ ̶a̶x̶i̶o̶s̶P̶o̶s̶t̶ ̶=̶ ̶j̶e̶s̶t̶.̶f̶n̶(̶)̶;̶
jest.mock('axios', () => {
  return Object.assign(jest.fn(), {
    post: jest.fn().mockReturnValue({ success: true }),
  });
});

// ...

/**
 * Validates send email request
 */
test('test - sendEmail - [email protected], [email protected], my subject, hello there!', async () => {
  // Setup
  const spyOnAxiosPost = jest.spyOn(axios, 'post');

  // ...
  // Pre Expectations
  expect(formData).not.toHaveBeenCalled();
  //  ̶e̶x̶p̶e̶c̶t̶(̶a̶x̶i̶o̶s̶P̶o̶s̶t̶)̶.̶n̶o̶t̶.̶t̶o̶H̶a̶v̶e̶B̶e̶e̶n̶C̶a̶l̶l̶e̶d̶(̶)̶;̶
  expect(spyOnAxiosPost).not.toBeCalled();

  // Init
  const result = await sendEmail(from, to, subject, body);

  // Post Expectations
  // ...
  // Axios
  //  ̶e̶x̶p̶e̶c̶t̶(̶a̶x̶i̶o̶s̶P̶o̶s̶t̶)̶.̶t̶o̶H̶a̶v̶e̶B̶e̶e̶n̶C̶a̶l̶l̶e̶d̶T̶i̶m̶e̶s̶(̶1̶)̶;̶
  expect(spyOnAxiosPost).toBeCalledTimes(1);
  // ...
  expect(result).toStrictEqual({ success: true });
});

Answer 1

axios isn't fully handled by Jest auto-mock because axios is a function and not an object, its methods like axios.post are ignored.

The use of __mocks__ (not __mock__) is optional for manual mocks. They can be mocked in-place, although it makes sense to use __mocks__ for reuse:

jest.mock('axios', () => {
  return Object.assign(jest.fn(), {
    get: jest.fn(),
    post: jest.fn(),
    ...
  });
});

There are no problems with axios-mock-adapter and toHaveBeenCalledWith because the use of real Axios with custom adapter allows to spy on methods (but not axios() function itself):

jest.spyOn(axios, 'post');