in packages/babel-preset-react-app/index.js [11:20]
module.exports = function (api, 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.
const env = process.env.BABEL_ENV || process.env.NODE_ENV;
return create(api, opts, env);
};