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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.js y colocarse en el directorio models/__mocks__. Note que la carpeta __mocks__ diferencia entre mayúsculas y minúsculas, así que al nombrar el directorio __MOCKS__ se romperá en algunos sistemas.

note

Cuando necesitemos ese módulo en nuestros tests (entendiendo que queremos usar la simulación en lugar de la implementación real), será necesario llamarlo explícitamente jest.mock('./nombreModulo').

Mocking Node modules

Si el módulo que estamos simulando es un módulo de Node (ej: lodash), la simulación (mock) debería colocarse en el directorio __mocks__ adyacente a node_modules (a menos que se haya confiturado roots apuntando a otra carpeta que no sea la raíz del proyecto) y será automáticamente simulado. No es necesario llamar explícitamente a jest.mock('nombre_modulo')

Scoped modules (also known as scoped packages) 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.

caution

If we want to mock Node's build-in modules (e.g.: fs or path), then explicitly calling e.g. jest.mock('path') is required, because build-in 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.unmock('nombreModulo') en pruebas donde se desee usar la implementación real del módulo.

info

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. In this case jest.mock('fs') must be called explicitly, because fs is Node’s build-in 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.

caution

jest.mock calls cannot be hoisted to the top of the module if you enabled ECMAScript modules support. The ESM module loader always evaluates the static imports before executing code. See ECMAScriptModules for details.

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...
});