Testing AWS UI components with Jest

Developers who have recently installed AWS’s new UI component library may quickly run into trouble with their unit tests. AWS UI is written in ECMAScript, and Jest cannot import it by default.

Jest encountered an unexpected token.

The full error will look something like this:

Test suite failed to runJest encountered an unexpected tokenThis 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:
* If you are trying to use ECMAScript Modules, see ... for how to enable it.
* 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:
({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,jest){export ...
SyntaxError: Unexpected token 'export' at Runtime.createScriptFromCode (...)
at Object.<anonymous> (...)

There are a few configurations you need to make to force the transpilation of AWS UI from ECMAScript to CommonJS in order for Jest to adequately test it.

Regenerator runtime 🤖

If you are attempting to support IE11 and are not already providing regeneratorRuntime, check out the documentation on the regenerator-runtime NPM package to get it installed and setup.

If you are not supporting IE11, you do not need to do this step.

Babel 🦜

You will need to install and configure Babel, if you are not already, in order to transpile AWS UI. If you are already using TypeScript, this may sound daunting, but it can luckily be done with Babel’s out-of-the-box presets, requiring little to not configuration on your part.

Install the @babel/preset-env and babel-jest “dev dependencies” with your package manager of choice, e.g. yarn add --dev @babel/present-env babel-jest.

Add the following babel.config.cjs file to your project (or configure your existing Babel configuration to include these changes):

// Don't process node_modules except for AWS UI.
module.exports = {
ignore: [/node_modules\/?!@awsui\/components-react/],
presets: ['@babel/preset-env'],

Important note: The file name is babel.config.cjs, not babel.config.js. This is particularly necessary if your package.json file defines the type property as "module".

Jest 🤡

Now that Babel is installed and configured to transpile AWS UI, we need to simply tell Jest to use it.

Add or extend your transform property to contain the AWS UI node module:

transform: {
'.+\\.js$': 'babel-jest',

Add or extend your transformIgnorePatterns to stop ignoring the AWS UI node module:

transformIgnorePatterns: [

Conclusion 🔚

It’s as simple as that! If you have any questions or feedback, please leave them in the comments below.

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Senior front end engineer / charlesstover.com

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