jest-transform-css

A Jest transformer which enables importing CSS into Jest's jsdom.

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 CSS of 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.

jest-transform-css 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

1yarn add jest-transform-css --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.

1// in the Jest config
2"moduleNameMapper": {
3- "\\.(css|less)$": "<rootDir>/__mocks__/styleMock.js"
4},

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:

1// in the Jest config
2"moduleNameMapper": {
3-  "\\.(s?css|less)$": "identity-obj-proxy"
4},

Adding transform

Open jest.config.js and modify the transform:

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

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 is a code preprocessor, babel-jest gets disabled when jest-transform-css is added.

So it needs to be added again manually.

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

Enabling CSS modules

By default, jest-transform-css will treat every file it transforms as a regular CSS file.

You need to opt into css-modules mode by specifying it in the configuration. Add { modules: true } option to jest-transform-css in jest.config.js:

1// in the Jest config
2transform: {
3-  "^.+\\.css$": "jest-transform-css"
4+  "^.+\\.css$": ["jest-transform-css", { modules: true }]
5}

This will enable CSS module transformation by jest-transform-css for all CSS files matching the pattern.

The config also supports generateScopedName property to customize the generated class names. Helpful when using Jest Snapshots and not wanting unnecessary noise from hash generated classnames.

// in the Jest config
transform: {
  "^.+\\.css$": ["jest-transform-css", {
    modules: true,
    generateScopedName: "[path]_[name]_[local]"
    // Default value is: '[path][local]-[hash:base64:10]'
  }]
}

Link to all available placeholder tokens *Note not all placeholders are working and must be tested.

Further setup

There are many ways to set up styles in a project (CSS modules, global styles, external global styles, local global styles, CSS in JS, LESS, SASS just to name a few). How to continue from here depends on your project.

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.