Обходим иммитации модулей

Jest позволяет имитировать целые модули. Это полезно, когда нужно протестировать, что ваш код корректно вызывает функции из других модулей. However, sometimes you may want to use parts of a mocked module in your test file, in which case you want to access the original implementation, rather than a mocked version.

Consider writing a test case for this createUser function:

createUser.js

import fetch from 'node-fetch';

export const createUser = async () => {
  const response = await fetch('https://website.com/users', {method: 'POST'});
  const userId = await response.text();
  return userId;
};

Your test will want to mock the fetch function so that we can be sure that it gets called without actually making the network request. However, you'll also need to mock the return value of fetch with a Response (wrapped in a Promise), as our function uses it to grab the created user's ID. Изначально, тест может выглядеть следующим образом:

jest.mock('node-fetch');

import fetch, {Response} from 'node-fetch';
import {createUser} from './createUser';

test('createUser calls fetch with the right args and returns the user id', async () => {
  fetch.mockReturnValue(Promise.resolve(new Response('4')));

  const userId = await createUser();

  expect(fetch).toHaveBeenCalledTimes(1);
  expect(fetch).toHaveBeenCalledWith('https://website.com/users', {
    method: 'POST',
  });
  expect(userId).toBe('4');
});

However, if you ran that test you would find that the createUser function would fail, throwing the error: TypeError: response.text is not a function. This is because the Response class you've imported from node-fetch has been mocked (due to the jest.mock call at the top of the test file) so it no longer behaves the way it should.

To get around problems like this, Jest provides the jest.requireActual helper. Чтобы тест заработал, добавьте следующие изменения в тестовом файле:

// BEFORE
jest.mock('node-fetch');
import fetch, {Response} from 'node-fetch';

// AFTER
jest.mock('node-fetch');
import fetch from 'node-fetch';
const {Response} = jest.requireActual('node-fetch');

This allows your test file to import the actual Response object from node-fetch, rather than a mocked version. А это значит, что тест должен выполниться успешно.