dependencies.js

module.exports = function (api, opts) {
  if (!opts) {
    opts = {};
  }

  // This is similar to how `env` works in Babel:
  // https://babeljs.io/docs/usage/babelrc/#env-option
  // We are not using `env` because it’s ignored in versions > babel-core@6.10.4:
  // https://github.com/babel/babel/issues/4539
  // https://github.com/facebook/create-react-app/issues/720
  // It’s also nice that we can enforce `NODE_ENV` being specified.
  var env = process.env.BABEL_ENV || process.env.NODE_ENV;
  var isEnvDevelopment = env === 'development';
  var isEnvProduction = env === 'production';
  var isEnvTest = env === 'test';
  if (!isEnvDevelopment && !isEnvProduction && !isEnvTest) {
    throw new Error(
      'Using `babel-preset-react-app` requires that you specify `NODE_ENV` or ' +
        '`BABEL_ENV` environment variables. Valid values are "development", ' +
        '"test", and "production". Instead, received: ' +
        JSON.stringify(env) +
        '.'
    );
  }

  return {
    presets: [
      isEnvTest && [
        // ES features necessary for user's Node version
        require('@babel/preset-env').default,
        {
          targets: {
            node: 'current',
          },
          // Do not transform modules to CJS
          modules: false,
        },
      ],
      (isEnvProduction || isEnvDevelopment) && [
        // Latest stable ECMAScript features
        require('@babel/preset-env').default,
        {
          // Do not transform modules to CJS
          modules: false,
        },
      ],
    ].filter(Boolean),
  };
};