Contournement des modules simulés

Jest vous permet de simuler des modules entiers dans vos tests, ce qui peut être utile pour vérifier si votre code appelle correctement les fonctions de ce module. 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. Vous pouvez donc, dans un premier temps, essayer d'écrire un test comme celui-ci :

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. Pour que le test ci-dessus fonctionne, apportez la modification suivante aux importations dans le fichier de test :

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

// APRES
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. Cela signifie que le test va maintenant passer correctement.