Today I Learned

Babel loader transpilation for Jest

Let’s imagine a following situation: we need to create a new npm package with create-react-app, in order to bundle some files in a package to have them available in a project (it’s possible to use create-component-lib for this task). The aforementioned sometimes requires from us to update our babel config.

In my latest task I had a situation where my babel config contained invalid (for a situation) presets:

module.exports =
  "presets": ["@babel/preset-env", ["react-app", { "absoluteRuntime": false }]]

In the above example, the build process creates a transpiled code, which contains babel runtime for minimalizing package size:

var _interopRequireWildcard = require("@babel/runtime/helpers/interopRequireWildcard");

var _interopRequireDefault = require("@babel/runtime/helpers/interopRequireDefault");

Object.defineProperty(exports, "__esModule", {
  value: true
exports.default = void 0;

var _defineProperty2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/defineProperty"));

var _objectSpread4 = _interopRequireDefault(require("@babel/runtime/helpers/esm/objectSpread"));

var _classCallCheck2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/classCallCheck"));

var _createClass2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/createClass"));

var _possibleConstructorReturn2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/possibleConstructorReturn"));

var _getPrototypeOf3 = _interopRequireDefault(require("@babel/runtime/helpers/esm/getPrototypeOf"));

var _inherits2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/inherits"));

Now when we start our test which uses a transpiled component it’s possible we receive error code like this:

Jest encountered an unexpected token

    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 to 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:


    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){export default function _defineProperty(obj, key, value) {

    SyntaxError: Unexpected token export

      10 | exports.default = void 0;
      11 |
    > 12 | var _defineProperty2 = _interopRequireDefault(require("@babel/runtime/helpers/esm/defineProperty"));
         |                                               ^

      at ScriptTransformer._transformAndBuildScript (node_modules/jest-runtime/build/script_transformer.js:403:17)
      at Object.require (dist/FormGenerator.js:12:47)

This happens because by default jest ignores transformations for everything in node_modules directory.

Now we have two possibilities to resolve this problem. In the first scenario we can add transformIgnorePatterns for our jest config to transpile babel runtime module:

transformIgnorePatterns: [

In the above example transpiled code still will be smaller in application runtime but it doesn’t crash our tests.

The second option is using another babel preset for proper transpilation:

module.exports = {
  plugins: [
  presets: [

Now after build, we get transpiled code working in a test environment, however, our code will be bloated with helpers required to mimic transpiled features (for example class inheritance):

Object.defineProperty(exports, "__esModule", {
  value: true
exports.default = void 0;

var _react = _interopRequireWildcard(require("react"));

var _propTypes = _interopRequireDefault(require("prop-types"));

var _formik = require("formik");

var _yup = require("yup");

var _classnames = _interopRequireDefault(require("classnames"));

var _moment = _interopRequireDefault(require("moment"));

var _BooleanField = _interopRequireDefault(require("./FieldTypes/BooleanField"));

var _EnumField = _interopRequireDefault(require("./FieldTypes/EnumField"));

var _MoneyField = _interopRequireDefault(require("./FieldTypes/MoneyField"));

var _TextField = _interopRequireDefault(require("./FieldTypes/TextField"));

var _QuarterDateField = _interopRequireDefault(require("./FieldTypes/QuarterDateField"));