docs additions for external add-ons usage

docs/source/addons/index.md

@@ -455,6 +455,74 @@ volto-slate can declare in its `package.json`:
 And of course, the dependency add-on can depend, on its turn, on other add-ons
 which will be loaded as well. Circular dependencies should be avoided.
 
+## Problems with untranspiled add-on dependencies
+
+When using external add-ons in your project, sometimes you will run into add-ons
+that are not securely transpiled or haven't been transpiled at all. In that case
+you might see an error like the following:
+
+```console
+Module parse failed: Unexpected token (10:41) in @react-leaflet/core/esm/path.js
+...
+const options = props.pathOptions ?? {};
+...
+```
+
+Babel automatically transpiles the code in your add-on, but `node_modules` are
+excluded from this process, so we need to include the add-on path in the list of
+modules to be transpiled. This can be accomplished by customizing the webpack
+configuration in the `razzle.config.js` file in your add-on. For example,
+suppose that we want to use react-leaflet, which has a known transpilation
+issue:
+
+```js
+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, options }) => {
+  const config = modifyWebpackConfig({
+    env,
+    webpackConfig,
+    webpackObject,
+    options,
+  });
+  const babelLoader = config.module.rules.find(babelLoaderFinder);
+  const { include } = babelLoader;
+  const corePath = path.join(
+    path.dirname(require.resolve('@react-leaflet/core')),
+    '..',
+  );
+  const esmPath = path.join(
+    path.dirname(require.resolve('react-leaflet')),
+    '..',
+  );
+
+  include.push(corePath);
+  include.push(esmPath);
+  return config;
+};
+
+module.exports = { modifyWebpackConfig: customModifyWebpackConfig, plugins };
+```
+
+First we need some setup to get the webpack configuration from Volto's configuration.
+Once we have that, we need to resolve the path to the desired add-ons and push it
+into the Babel loader include list. After this, the add-ons will load correctly.
+
 ## Testing add-ons
 
 We should let jest know about our aliases and make them available to it to

docs/source/recipes/lazyload.md

@@ -91,9 +91,15 @@ You can call the hook like:
 ```jsx
 import { useLazyLibs } from '@plone/volto/helpers/Loadable/Loadable';
 
-useLazyLibs(['toastify', 'reactDnd'])
+const loaded = useLazyLibs(['toastify', 'reactDnd'])
 // or:
-useLazyLibs(['toastify', 'reactDnd'], {shouldRerender: false})
+const loaded = useLazyLibs(['toastify', 'reactDnd'], {shouldRerender: false})
+
+const reactDnd = loaded?.reactDnd;
+
+if (reactDnd) {
+  // The library is now loaded and can be used.
+}
 ```
 
 Passing the `shouldRerender` as false as options will cause the component to

news/4233.documentation

@@ -0,0 +1 @@
+Add some instructions for dealing with untranspiled add-ons and a lazy loading example for functional components. [cguardia]