Keep modules for Webpack, transpile everything for Jest

.babelrc

{
  "presets": ["./.babelrc.js"]
}

.babelrc.js

// Works like magic™
//
// We point .babelrc to use this file as a preset, and here we could
// use regular Node.js code
//
// Reference: https://fatfisz.com/blog/using-babelrc-js-today

const modules = (/build/.test(process.env.npm_lifecycle_event)) ? false : 'commonjs'
const retainLines = !(/build/.test(process.env.npm_lifecycle_event))

module.exports = {
  presets: [
    ['env', {
      modules,
      targets: {
        node: '4.3'
      }
    }]
  ],
  plugins: ['preval', 'transform-object-rest-spread'],
  retainLines
}

webpack.config.js

module.exports = {
  // ...
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: 'babel-loader'
      }
    ]
  }
}