SyntaxError with Jest and React and importing CSS files

Question

I am trying to get my first Jest Test to pass with React and Babel.

I am getting the following error:

SyntaxError: /Users/manueldupont/test/avid-sibelius-publishing-viewer/src/components/TransportButton/TransportButton.less: Unexpected token

    >  7 | @import '../variables.css';
          | ^

My package.json config for jest look like this:

"babel": {
    "presets": [
      "es2015",
      "react"
    ],
    "plugins": [
      "syntax-class-properties",
      "transform-class-properties"
    ]
  },
  "jest": {
    "moduleNameMapper": {
      "^image![a-zA-Z0-9$_-]+$": "GlobalImageStub",
      "^[./a-zA-Z0-9$_-]+\\.png$": "RelativeImageStub"
    },
    "testPathIgnorePatterns": [
      "/node_modules/"
    ],
    "collectCoverage": true,
    "verbose": true,
    "modulePathIgnorePatterns": [
      "rpmbuild"
    ],
    "unmockedModulePathPatterns": [
      "<rootDir>/node_modules/react/",
      "<rootDir>/node_modules/react-dom/",
      "<rootDir>/node_modules/react-addons-test-utils/",
      "<rootDir>/node_modules/fbjs",
      "<rootDir>/node_modules/core-js"
    ]
  },

So what am I missing?

Answer 1

moduleNameMapper is the setting that tells Jest how to interpret files with different extension. You need to tell it how to handle Less files.

Create a file like this in your project (you can use a different name or path if you’d like):

config/CSSStub.js

module.exports = {};

This stub is the module we will tell Jest to use instead of CSS or Less files. Then change moduleNameMapper setting and add this line to its object to use it:

'^.+\\.(css|less)$': '<rootDir>/config/CSSStub.js'

Now Jest will treat any CSS or Less file as a module exporting an empty object. You can do something else too—for example, if you use CSS Modules, you can use a Proxy so every import returns the imported property name.

Read more in this guide.

Answer 2

I solved this by using the moduleNameMapper key in the jest configurations in the package.json file

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

After this you will need to create the two files as described below

__mocks__/styleMock.js

module.exports = {};

__mocks__/fileMock.js

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

If you are using CSS Modules then it's better to mock a proxy to enable className lookups. hence your configurations will change to:

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

But you will need to install identity-obj-proxy package as a dev dependancy i.e.

yarn add identity-obj-proxy -D

For more information. You can refer to the jest docs