jest-transform-styl

A Jest transformer which enables importing stylus into Jest's jsdom. If you are looking for just Jest CSS transformer, use jest-transform-css or use this package without stylus setting. Details below.

If you are not here for Visual Regression Testing, but just want to make your tests work with CSS Modules, then you are likley looking for https://github.com/keyanzhang/identity-obj-proxy/.

Description

When you want to do Visual Regression Testing in Jest, it is important that the styles used components is available to the test setup. So far, CSS was not part of tests as it was mocked away by using moduleNameMapper like a file-mock or identity-obj-proxy. Package jest-transform-css is a great starting point if your app is using CSS or CSS modules. However, if your app is using stylus, you may need additional stylus transformer for Jest.

jest-transform-styl is forked from jest-transform-css with additional transformer feature for stylus. The package is intended to be used in an jsdom environment. When any component imports CSS in the test environment, then the loaded CSS will get added to jsdom using style-inject - just like the Webpack CSS loader would do in a production environment. This means the full styles are added to jsdom.

This doesn't make much sense at first, as jsdom is headless (non-visual). However, we can copy the resulting document markup ("the HTML") of jsdom and copy it to a puppeteer instance. We can let the markup render there and take a screenshot there. The jsdom-screenshot package does exactly this.

Once we obtained a screenshot, we can compare it to the last version of that screenshot we took, and make tests fail in case they did. The jest-image-snapshot plugin does that.

Setup

Installation

npm install jest-transform-styl --save-dev

The old setup of CSS in jest needs to be removed, and the new setup needs to be added next.

Removing module name mapping

If your project is using plain CSS imported in the components, then you're likely using a mock file. You can remove that configuration.

// in the Jest config
"moduleNameMapper": {
- "\\.(styl|css|less)$": "<rootDir>/__mocks__/styleMock.js"
},

If your project is using CSS Modules, then it's likely that identity-obj-proxy is configured. It needs to be removed in order for the styles of the jest-transform-css to apply.

So, remove these lines from jest.config.js:

// in the Jest config
"moduleNameMapper": {
-  "\\.(s?css|less)$": "identity-obj-proxy"
},

Adding transform

Open jest.config.js and modify the transform:

// in the Jest config
transform: {
  "^.+\\.js$": "babel-jest",
  "^.+\\.css$": "jest-transform-css",
  "^.+\\.styl$": "jest-transform-styl"
}

Notice that babel-jest gets added as well.

The babel-jest code preprocessor is enabled by default, when no other preprocessors are added. As jest-transform-css and jest-transform-styl are code preprocessors, babel-jest gets disabled when jest-transform-css and jest-transform-styl are added.

So it needs to be added again manually.

See https://github.com/facebook/jest/tree/master/packages/babel-jest#setup

Enabling Stylus

This package supports CSS, CSS Modules and Stylus. If you are not using Stylus, no need to provide stylus: true setting.

If you are using both modules and stylus, follow steps below:

// jesttransformcss.config.js
 
module.exports = {
  modules: true,
  stylus: true,
  pathToStyles: 'src/styles'  // this is the path where your styles reside. used for resolving path provided by `composes` keyword in stylus
};

• modules: This will enable CSS module transformation for all CSS and Stylus files transformed by jest-transform-styl.
• stylus: This tells the transformer that the application uses stylus files and it needs to transform stylus to CSS before doing rest of CSS transformation for Jest.
• pathToStyles is an optional path to styles which points to your styles file. This path is used to compile the keywords like composes used in Stylus files to inherit styles from other stylus files.

If your setup uses both, CSS modules and regular CSS, then you can determine how to load each file individually by specifying a function:

// jesttransformcss.config.js
 
module.exports = {
  modules: filename => filename.endsWith(".mod.css")
};

This will load all files with .mod.css as CSS modules and load all other files as regular CSS. Notice that the function will only be called for whichever regex you provided in the transform option of the Jest config.

Note: If your stylus files are using require to import styles from other stylus files, you will run into some issues since stylus cannot compile a relative path. You either will have to provide an absolute path of the file or will need to change the path into a webpack alias, eg. ~styles, and regex the ~ with the root path. This change will have to be done in this package itself.

PostCSS

If your setup is using PostCSS then you should add a postcss.config.js at the root of your folder.

You can apply certain plugins only when process.env.NODE_ENV === 'test'. Ensure that valid CSS can be generated.

jest-transform-css is likley not flexible enough yet to support more sophisticated PostCSS configurations. However, we should be able to add this functionality by extending the configuration file. Feel free to open an issue with your setup and we'll try to support it.

css-loader

If your setup is using css-loader only, without PostCSS then you should be fine. If you have modules: true enabled in css-loader, you need to also enable it for jest-transform-css (see "Enabling CSS modules"). When components import CSS modules in the test environment, then the CSS is transformed through PostCSS's cssModules plugin to generate the classnames. It also injects the styles into jsdom.