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Manual Mocks

Las simulaciones mock manuales son usadas para sustituir funcionalidad con datos falsos. Por ejemplo, en lugar de acceder a un recurso remoto como un sitio web o una base de datos, puede que se desee crear una simulación manual que permita usar datos falsos. Esto asegura que las pruebas serán rápidas y estables.

Mocking user modules

Las simulaciones mock manuales se definen escribiendo un modulo en el subdirectorio __mocks__ el cual se debe encontrar adyacente al módulo. Por ejemplo, para simular un módulo llamado user en el directorio models, se debe crear un archivo user.sj y colocarse en el directorio models/__mocks__. Note that the __mocks__ folder is case-sensitive, so naming the directory __MOCKS__ will break on some systems.

When we require that module in our tests, explicitly calling jest.mock('./moduleName') is required.

Mocking Node modules

If the module you are mocking is a Node module (e.g.: lodash), the mock should be placed in the __mocks__ directory adjacent to node_modules (unless you configured roots to point to a folder other than the project root) and will be automatically mocked. There's no need to explicitly call jest.mock('module_name').

Scoped modules can be mocked by creating a file in a directory structure that matches the name of the scoped module. For example, to mock a scoped module called @scope/project-name, create a file at __mocks__/@scope/project-name.js, creating the @scope/ directory accordingly.

Warning: If we want to mock Node's core modules (e.g.: fs or path), then explicitly calling e.g. jest.mock('path') is required, because core Node modules are not mocked by default.

Ejemplos

.
├── config
├── __mocks__
│   └── fs.js
├── models
│   ├── __mocks__
│   │   └── user.js
│   └── user.js
├── node_modules
└── views

Cuando una simulación mock manual existe para un módulo, el sistema de módulos de Jest usará dicho mock cuando se llame a la función jest.mock('nombreModulo'). However, when automock is set to true, the manual mock implementation will be used instead of the automatically created mock, even if jest.mock('moduleName') is not called. Para omitir este comportamiento, se debe llamar de manera explicita a jest.umock('nombreModulo') en pruebas donde se desee usar la implementación real del módulo.

Note: In order to mock properly, Jest needs jest.mock('moduleName') to be in the same scope as the require/import statement.

Here's a contrived example where we have a module that provides a summary of all the files in a given directory. In this case, we use the core (built in) fs module.

// FileSummarizer.js
'use strict';

const fs = require('fs');

function summarizeFilesInDirectorySync(directory) {
  return fs.readdirSync(directory).map(fileName => ({
    directory,
    fileName,
  }));
}

exports.summarizeFilesInDirectorySync = summarizeFilesInDirectorySync;

Since we'd like our tests to avoid actually hitting the disk (that's pretty slow and fragile), we create a manual mock for the fs module by extending an automatic mock. Our manual mock will implement custom versions of the fs APIs that we can build on for our tests:

// __mocks__/fs.js
'use strict';

const path = require('path');

const fs = jest.createMockFromModule('fs');

// This is a custom function that our tests can use during setup to specify
// what the files on the "mock" filesystem should look like when any of the
// `fs` APIs are used.
let mockFiles = Object.create(null);
function __setMockFiles(newMockFiles) {
  mockFiles = Object.create(null);
  for (const file in newMockFiles) {
    const dir = path.dirname(file);

    if (!mockFiles[dir]) {
      mockFiles[dir] = [];
    }
    mockFiles[dir].push(path.basename(file));
  }
}

// A custom version of `readdirSync` that reads from the special mocked out
// file list set via __setMockFiles
function readdirSync(directoryPath) {
  return mockFiles[directoryPath] || [];
}

fs.__setMockFiles = __setMockFiles;
fs.readdirSync = readdirSync;

module.exports = fs;

Now we write our test. Note that we need to explicitly tell that we want to mock the fs module because it’s a core Node module:

// __tests__/FileSummarizer-test.js
'use strict';

jest.mock('fs');

describe('listFilesInDirectorySync', () => {
  const MOCK_FILE_INFO = {
    '/path/to/file1.js': 'console.log("file1 contents");',
    '/path/to/file2.txt': 'file2 contents',
  };

  beforeEach(() => {
    // Set up some mocked out file info before each test
    require('fs').__setMockFiles(MOCK_FILE_INFO);
  });

  test('includes all files in the directory in the summary', () => {
    const FileSummarizer = require('../FileSummarizer');
    const fileSummary = FileSummarizer.summarizeFilesInDirectorySync(
      '/path/to',
    );

    expect(fileSummary.length).toBe(2);
  });
});

The example mock shown here uses jest.createMockFromModule to generate an automatic mock, and overrides its default behavior. This is the recommended approach, but is completely optional. If you do not want to use the automatic mock at all, you can export your own functions from the mock file. One downside to fully manual mocks is that they're manual – meaning you have to manually update them any time the module they are mocking changes. Because of this, it's best to use or extend the automatic mock when it works for your needs.

To ensure that a manual mock and its real implementation stay in sync, it might be useful to require the real module using jest.requireActual(moduleName) in your manual mock and amending it with mock functions before exporting it.

The code for this example is available at examples/manual-mocks.

Usando con imports de módulos ES

If you're using ES module imports then you'll normally be inclined to put your import statements at the top of the test file. But often you need to instruct Jest to use a mock before modules use it. For this reason, Jest will automatically hoist jest.mock calls to the top of the module (before any imports). To learn more about this and see it in action, see this repo.

Mocking methods which are not implemented in JSDOM

If some code uses a method which JSDOM (the DOM implementation used by Jest) hasn't implemented yet, testing it is not easily possible. This is e.g. the case with window.matchMedia(). Jest returns TypeError: window.matchMedia is not a function and doesn't properly execute the test.

In this case, mocking matchMedia in the test file should solve the issue:

Object.defineProperty(window, 'matchMedia', {
  writable: true,
  value: jest.fn().mockImplementation(query => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: jest.fn(), // deprecated
    removeListener: jest.fn(), // deprecated
    addEventListener: jest.fn(),
    removeEventListener: jest.fn(),
    dispatchEvent: jest.fn(),
  })),
});

This works if window.matchMedia() is used in a function (or method) which is invoked in the test. If window.matchMedia() is executed directly in the tested file, Jest reports the same error. In this case, the solution is to move the manual mock into a separate file and include this one in the test before the tested file:

import './matchMedia.mock'; // Must be imported before the tested file
import {myMethod} from './file-to-test';

describe('myMethod()', () => {
  // Test the method here...
});