Custom Volto configuration to fix Babel problems with react-leaflet

I’ve started working on a new Leaflet-powered Volto map block and the first thing that happened while loading react-leaftlet was an error reported by the browser:

Module parse failed: Unexpected token (10:41) in @react-leaflet/core/esm/path.js
const options = props.pathOptions ?? {};

The problem is that is, for some reasons, the transpiled JS bundle includes code using the nulish coalescing operator

This is already a problem reported in react-leaflet and it happens because the distributed transpiled library includes that code. Now, it could be fixed by doing a safer transpilation in their build process, but I don’t care. This is a problem that can be taken care easily, in Volto.

Volto’s Webpack (and, by extension, BabelJS) behavior can be customized through the razzle.config.js file. My solution to fix the problem is this custom razzle.config.js file included in my project:

const path = require('path');
const makeLoaderFinder = require('razzle-dev-utils/makeLoaderFinder');

const babelLoaderFinder = makeLoaderFinder('babel-loader');

const jsConfig = require('./jsconfig').compilerOptions;

const pathsConfig = jsConfig.paths;
let voltoPath = './node_modules/@plone/volto';
Object.keys(pathsConfig).forEach((pkg) => {
  if (pkg === '@plone/volto') {
    voltoPath = `./${jsConfig.baseUrl}/${pathsConfig[pkg][0]}`;

const { modifyWebpackConfig, plugins } = require(`${voltoPath}/razzle.config`);

const customModifyWebpackConfig = ({ env, webpackConfig, webpackObject }) => {
  const config = modifyWebpackConfig({
  const babelLoader = config.module.rules.find(babelLoaderFinder);
  const { include } = babelLoader;
  const corePath = path.join(
  const esmPath = path.join(

  return config;

module.exports = { modifyWebpackConfig: customModifyWebpackConfig, plugins };

The solution envolves pushing the @react-leaflet package paths to Babel’s webpack loader’s include option, so that they are included in the transpilation process. By default all packages in node_modules are excluded from the transpilation.