babel.config.js

module.exports = function(api) {
  var validEnv = ['development', 'test', 'production']
  var currentEnv = api.env()
  var isDevelopmentEnv = api.env('development')
  var isProductionEnv = api.env('production')
  var isTestEnv = api.env('test')

  if (!validEnv.includes(currentEnv)) {
    throw new Error(
      'Please specify a valid `NODE_ENV` or ' +
        '`BABEL_ENV` environment variables. Valid values are "development", ' +
        '"test", and "production". Instead, received: ' +
        JSON.stringify(currentEnv) +
        '.'
    )
  }

  return {
    presets: [
      isTestEnv && [
        require('@babel/preset-env').default,
        {
          targets: {
            node: 'current',
          },
        },
      ],
      (isProductionEnv || isDevelopmentEnv) && [
        require('@babel/preset-env').default,
        {
          forceAllTransforms: true,
          useBuiltIns: 'entry',
          modules: false,
          exclude: ['transform-typeof-symbol'],
        },
      ],
      '@babel/preset-typescript',
      '@babel/preset-react',
    ].filter(Boolean),
    plugins: [
      require('babel-plugin-macros'),
      require('@babel/plugin-syntax-dynamic-import').default,
      isTestEnv && require('babel-plugin-dynamic-import-node'),
      require('@babel/plugin-transform-destructuring').default,
      [
        require('@babel/plugin-proposal-class-properties').default,
        {
          loose: true,
          // spec: true ?!
        },
      ],
      [
        require('@babel/plugin-proposal-object-rest-spread').default,
        {
          useBuiltIns: true,
        },
      ],
      [
        require('@babel/plugin-transform-runtime').default,
        {
          helpers: false,
          regenerator: true,
        },
      ],
      [
        require('@babel/plugin-transform-regenerator').default,
        {
          async: false,
        },
      ],
      [
        'module-resolver',
        {
          root: ['./app/javascript'],
        },
      ],
      [
        'babel-plugin-styled-components',
        {
          displayName: true,
          minify: true,
        },
      ],
    ].filter(Boolean),
    env: {
      test: {
        presets: [
          [
            '@babel/env',
            {
              modules: 'commonjs',
            },
          ],
        ],
      },
    },
  }
}