babel/packages/babel-plugin-transform-node-env-inline/src/index.js

export default function ({ types: t }) {
  return {
    visitor: {
      MemberExpression(path) {
        if (path.matchesPattern("process.env.NODE_ENV")) {
          path.replaceWith(t.valueToNode(process.env.NODE_ENV));

          if (path.parentPath.isBinaryExpression()) {
            let evaluated = path.parentPath.evaluate();
            if (evaluated.confident) {
              path.parentPath.replaceWith(t.valueToNode(evaluated.value));
            }
          }
        }
      }
    }
  };
}