Utilizando webpack

Jest can be used in projects that use webpack to manage assets, styles, and compilation. webpack does offer some unique challenges over other tools because it integrates directly with your application to allow managing stylesheets, assets like images and fonts, along with the expansive ecosystem of compile-to-JavaScript languages and tools.

Un ejemplo de webpack

Vamos a comenzar con un tipo común de archivo de configuración webpack y traducirlo a una configuración de Jest.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.jsx?$/,
        exclude: ['node_modules'],
        use: ['babel-loader'],
      },
      {
        test: /\.css$/,
        use: ['style-loader', 'css-loader'],
      },
      {
        test: /\.gif$/,
        type: 'asset/inline',
      },
      {
        test: /\.(ttf|eot|svg)$/,
        type: 'asset/resource',
      },
    ],
  },
  resolve: {
    alias: {
      config$: './configs/app-config.js',
      react: './vendor/react-master',
    },
    extensions: ['.js', '.jsx'],
    modules: [
      'node_modules',
      'bower_components',
      'shared',
      '/shared/vendor/modules',
    ],
  },
};

If you have JavaScript files that are transformed by Babel, you can enable support for Babel by installing the babel-jest plugin. Non-Babel JavaScript transformations can be handled with Jest's transform config option.

Manejo de activos estáticos

A continuación, vamos a configurar Jest para utilizar archivos como hojas de estilo e imágenes. Generalmente, estos archivos no son particularmente útiles en las pruebas por lo que con seguridad podemos mockearlos. Sin embargo, si utiliza módulos de CSS entonces es mejor mockear un proxy para las búsquedas de className.

jest.config.js

module.exports = {
  moduleNameMapper: {
    '\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
      '<rootDir>/__mocks__/fileMock.js',
    '\\.(css|less)$': '<rootDir>/__mocks__/styleMock.js',
  },
};

Y mockear los archivos propios:

__mocks__/styleMock.js

module.exports = {};

__mocks__/fileMock.js

module.exports = 'test-file-stub';

Mockear Modulos de CSS

You can use an ES6 Proxy to mock CSS Modules:

npm

npm install --save-dev identity-obj-proxy

Yarn

yarn add --dev identity-obj-proxy

pnpm

pnpm add --save-dev identity-obj-proxy

Bun

bun add --dev identity-obj-proxy

Then all your className lookups on the styles object will be returned as-is (e.g., styles.foobar === 'foobar'). This is pretty handy for React Snapshot Testing.

jest.config.js (for CSS Modules)

module.exports = {
  moduleNameMapper: {
    '\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
      '<rootDir>/__mocks__/fileMock.js',
    '\\.(css|less)$': 'identity-obj-proxy',
  },
};

If moduleNameMapper cannot fulfill your requirements, you can use Jest's transform config option to specify how assets are transformed. For example, a transformer that returns the basename of a file (such that require('logo.jpg'); returns 'logo') can be written as:

fileTransformer.js

const path = require('path');

module.exports = {
  process(sourceText, sourcePath, options) {
    return {
      code: `module.exports = ${JSON.stringify(path.basename(sourcePath))};`,
    };
  },
};

jest.config.js (for custom transformers and CSS Modules)

module.exports = {
  moduleNameMapper: {
    '\\.(css|less)$': 'identity-obj-proxy',
  },
  transform: {
    '\\.(jpg|jpeg|png|gif|eot|otf|webp|svg|ttf|woff|woff2|mp4|webm|wav|mp3|m4a|aac|oga)$':
      '<rootDir>/fileTransformer.js',
  },
};

Si hemos dicho a Jest que omita archivos que coinciden con una extensión de hoja de estilo o imagen, y por el contrario, necesita nuestros archivos de mock. Puedes ajustar a una expresión regular para que coincida con el tipo de archivos de configuración webpack.

tip

Remember to include the default babel-jest transformer explicitly, if you wish to use it alongside with additional code preprocessors:

"transform": {
  "\\.[jt]sx?$": "babel-jest",
  "\\.css$": "some-css-transformer",
}

Configurar Jest para que encuentre nuestros archivos

Now that Jest knows how to process our files, we need to tell it how to find them. For webpack's modules, and extensions options there are direct analogs in Jest's moduleDirectories and moduleFileExtensions options.

jest.config.js

module.exports = {
  moduleFileExtensions: ['js', 'jsx'],
  moduleDirectories: ['node_modules', 'bower_components', 'shared'],

  moduleNameMapper: {
    '\\.(css|less)$': '<rootDir>/__mocks__/styleMock.js',
    '\\.(gif|ttf|eot|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },
};

note

<rootDir> is a special token that gets replaced by Jest with the root of your project. Most of the time this will be the folder where your package.json is located unless you specify a custom rootDir option in your configuration.

Similarly, Jest's counterpart for Webpack's resolve.roots (an alternative to setting NODE_PATH) is modulePaths.

jest.config.js

module.exports = {
  modulePaths: ['/shared/vendor/modules'],
  moduleFileExtensions: ['js', 'jsx'],
  moduleDirectories: ['node_modules', 'bower_components', 'shared'],
  moduleNameMapper: {
    '\\.(css|less)$': '<rootDir>/__mocks__/styleMock.js',
    '\\.(gif|ttf|eot|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },
};

And finally, we have to handle the webpack alias. For that, we can make use of the moduleNameMapper option again.

jest.config.js

module.exports = {
  modulePaths: ['/shared/vendor/modules'],
  moduleFileExtensions: ['js', 'jsx'],
  moduleDirectories: ['node_modules', 'bower_components', 'shared'],

  moduleNameMapper: {
    '\\.(css|less)$': '<rootDir>/__mocks__/styleMock.js',
    '\\.(gif|ttf|eot|svg)$': '<rootDir>/__mocks__/fileMock.js',

    '^react(.*)$': '<rootDir>/vendor/react-master$1',
    '^config$': '<rootDir>/configs/app-config.js',
  },
};

¡Eso es todo! ¡Eso es todo! webPack es una herramienta compleja y flexible, por lo que tendrás que hacer algunos ajustes para manejar las necesidades específicas de tu aplicación. Afortunadamente para la mayoría de los proyectos, Jest debe ser lo suficientemente flexible como para manejar tu configuración webpack.

tip

For more complex webpack configurations, you may also want to investigate projects such as: babel-plugin-webpack-loaders.

Utilizando webpack

In addition to installing babel-jest as described earlier, you'll need to add @babel/preset-env like so:

npm

npm install --save-dev @babel/preset-env

Yarn

yarn add --dev @babel/preset-env

pnpm

pnpm add --save-dev @babel/preset-env

Bun

bun add --dev @babel/preset-env

Then, you'll want to configure Babel as follows:

.babelrc

{
  "presets": ["@babel/preset-env"]
}

tip

Jest caches files to speed up test execution. If you updated .babelrc and Jest is not working as expected, try clearing the cache by running jest --clearCache.

tip

If you use dynamic imports (import('some-file.js').then(module => ...)), you need to enable the dynamic-import-node plugin.

.babelrc

{
  "presets": [["env", {"modules": false}]],

  "plugins": ["syntax-dynamic-import"],

  "env": {
    "test": {
      "plugins": ["dynamic-import-node"]
    }
  }
}

For an example of how to use Jest with webpack with React, you can view one here.