babel.config.js

// See https://babeljs.io/docs/en/config-files#root-babelconfigjs-files
module.exports = function getBabelConfig(api) {
  const isTest = api.env('test');

  // Config for when running Jest tests
  if (isTest) {
    return {
      plugins: [
        '@babel/plugin-proposal-class-properties',
      ],
      presets: [
        '@babel/env',
        '@babel/preset-react',
      ],
    };
  }

  // We set this in the `build:modules` package.json script
  const esmodules = process.env.BABEL_MODULES === '1';

  const babelEnvOptions = {
    modules: esmodules ? false : 'auto',
    // https://babeljs.io/docs/en/babel-preset-env#targets
    targets: {
      // 'browsers' target is ignored when 'esmodules' is true
      esmodules,
    },
  };

  return {
    ignore: [
      '**/*.test.js',
      '__snapshots__',
    ],
    plugins: [
      '@babel/plugin-proposal-class-properties',
      [
        '@babel/plugin-transform-runtime',
        {
          useESModules: esmodules,
        },
      ],
    ],
    presets: [
      ['@babel/env', babelEnvOptions],
      '@babel/preset-react',
    ],
    sourceMaps: true,
  };
};