Webpack 4 - Unable to properly transpile ES6 to UMD

I'm trying to migrate an older library to use ES6 imports/exports (eg. replacing const SomeClass = require('./SomeClass') to import SomeClass from './SomeClass').

The problem is that when I try to use the generated umd lib package, a Module instance is returned instead of the library itself.

<script src="lib.min.js"></script>
<script>
var lib = new Lib();
// Should be Lib instance, instead receive the following:
Module { {default: ƒ, __esModule: true, Symbol(Symbol.toStringTag): "Module" }
<script>

Here are my package.json devDependencies:

"@babel/core": "^7.4.3",
"@babel/preset-env": "^7.4.3",
"babel-loader": "^8.0.5",
"babel-plugin-syntax-dynamic-import": "^6.18.0",
"webpack": "^4.30.0",
"webpack-cli": "^3.3.1"

Here is my webpack config:

module.exports = {
    mode: 'production',
    entry: './src/index.js',
    output: {
        libraryTarget: 'umd',
        library:       'Lib',
        filename:      'lib.min.js',
        path:          (__dirname + '/dist')
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                include: [__dirname + '/src'],
                loader: 'babel-loader',
                options: {
                    presets: ['@babel/preset-env']
                }
            }
        ]
    }
};

And here is a sample code from the library I'm working on:

// /src/SomeClass.js
export default class SomeClass {...};

// /src/index.js
import SomeClass from './SomeClass';
export default SomeClass;

Probably I messed up something with the webpack/babel-loader configuration but can't find what exactly I'm not doing correct.

Solution

After following various github issue threads, it seems that this is intended behavior for Babel6+, since they dropped export default modules.export (https://github.com/babel/babel/issues/2212).

If you need to add it (eg. to prevent calling Lib.default), there is a babel-plugin-add-module-exports that should fix it. My updated babel loader configuration looks like this:

presets: [['@babel/env', { 'modules': 'umd' }]],
plugins: ['add-module-exports']