How to use babel-jest to transpile tests

Question

I'm writing tests for a React application and run into an issue while compiling classes that i import in my test files. When running the test suite

i get the following error:
Jest encountered an unexpected token
This usually means that you are trying to import a file which Jest cannot parse, e.g. it's not plain JavaScript.
By default, if Jest sees a Babel config, it will use that to transform your files, ignoring "node_modules".

Here's what you can do:
 • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
 • If you need a custom transformation specify a "transform" option in your config.
 • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

You'll find more details and examples of these config options in the docs:
https://jestjs.io/docs/en/configuration.html

Details:

/node_modules/react-redux/es/connect/connect.js:5
import connectAdvanced from '../components/connectAdvanced';
^^^^^^ 

I have tried to use babel-jest to transpile using the transform property and the modulePathIgnorePatterns . My jest.config.json looks like:

{
  "moduleNameMapper": {
    ".+\\.(css|styl|less|sass|scss|png|jpg|ttf|woff|woff2)$": "identity-obj-proxy"
  },
  "setupFiles": [
    "raf/polyfill",
    "<rootDir>/test/testSetup.js"
  ],
  "transform": {
    "^.+\\.jsx?$": "babel-jest"
  }
}

my babel.rc file (its in the root directory)

{
    "presets": [ "react", "es2015", "stage-2" ],
    "plugins": [
        "transform-class-properties",
        "transform-object-rest-spread",
    ]
}

I have the following packages in my package.json:

 "devDependencies": {
    "babel-cli": "^6.26.0",
    "babel-core": "^6.26.3",
    "babel-eslint": "^8.2.1",
    "babel-loader": "^7.1.2",
    "babel-plugin-transform-object-rest-spread": "^6.26.0",
    "babel-preset-env": "^1.7.0",
    "babel-preset-es2015": "^6.24.1",
    "babel-preset-react": "^6.24.1",
    "babel-preset-stage-2": "^6.24.1",
    "enzyme": "^3.6.0",
    "enzyme-adapter-react-16": "^1.5.0",
    "eslint": "^5.3.0",
    "eslint-plugin-react": "^7.10.0",
    "identity-obj-proxy": "^3.0.0",
    "jest": "^23.5.0",
    "babel-jest": "^23.6.0"
    "mkdirp": "^0.5.1",
    "prettier": "^1.13.7",
    "prettier-eslint": "^8.8.2",
    "prop-types": "^15.6.0",
    "react-test-renderer": "^16.5.0",
    "webpack": "^4.16.5",
    "webpack-cli": "^3.1.0",
    "webpack-dev-server": "^3.1.5",
    "webpack-merge": "^4.1.4",
    "yargs": "^12.0.1"
  },
  "dependencies": {
    "axios": "^0.18.0",
    "babel-polyfill": "^6.26.0",
    "css-loader": "^1.0.0",
    "eventing-bus": "^1.3.3",
    "filesize": "^3.6.1",
    "i18next": "^11.5",
    "node-sass": "^4.8.3",
    "react": "^16.4",
    "react-circular-progressbar": "^1.0.0",
    "react-dom": "^16.4",
    "react-redux": "^5.0",
    "react-truncate": "^2.4.0",
    "redux": "^4.0",
    "sass-loader": "^7.1.0",
    "style-loader": "^0.22.1"
  }

Who knows how i can modify my jest.config or .babelrc file so that my tests will compile without problem? Thanks in advance.

Answer 1

Your problem is: js files from /node_modules are not compiled by default.

It is totaly fine when you run your code in browser (that is exactly why such modules are not compiled because you have no need for that).

But jest runs using Node which doesn't understand import syntax. The best solution is to enable babel transpilation of node_modules when you run tests .

{"env": {
    "development": {
        "plugins": ["transform-es2015-modules-commonjs"]
    },
    "test": {
        "plugins": ["transform-es2015-modules-commonjs"]
    }
}}

Also use --no-cache when runing jest while fixing such problems.