How to fix modules with "ModuleConcatenation bailout: Module is not an ECMAScript module" bailout message?

Webpack3 comes with the ModuleConcatenation plugin that when used with the --display-optimization-bailout flag will give you the reason for bailout.

The bailout messages not that easy to understand, and it's hard to understand why they happened to a specific module.

Here is my output for the webpack command on a very simplified version of my project:

> webpack --bail --display-optimization-bailout

Hash: 4a9a55dc2883e82a017e
Version: webpack 3.4.1
Child client:
    Hash: 4a9a55dc2883e82a017e
    Time: 594ms
                                   Asset       Size  Chunks                    Chunk Names
        a.d3ade61d21d5cb8dd426.bundle.js  712 bytes       0  [emitted]         a
    a.d3ade61d21d5cb8dd426.bundle.js.map    6.57 kB       0  [emitted]         a
                           manifest.json  102 bytes          [emitted]         
                              index.html     299 kB          [emitted]  [big]  
       [0] multi ./src/client/bootstrap/ErrorTrap.js 28 bytes {0} [built]
           ModuleConcatenation bailout: Module is not an ECMAScript module
       [1] ./src/client/bootstrap/ErrorTrap.js 199 bytes {0} [built]
           ModuleConcatenation bailout: Module is not an ECMAScript module

I simplified the contents of ./src/client/bootstrap/ErrorTrap.js as much as I could, and I still get the ModuleConcatenation bailout: Module is not an ECMAScript module. Here are its contents:

class ErrorTrap {
}

export default ErrorTrap;

I looked into understanding this bailout message, and one of the reasons it happens is when the module doesn't have imports or exports, as seen at https://github.com/webpack/webpack/blob/93ac8e9c3699bf704068eaccaceec57b3dd45a14/lib/dependencies/HarmonyDetectionParserPlugin.js#L12-L14, but I don't know why it's not considering this module a ECMAScript module.

.babelrc

{
  "presets": [
    "es2015"
  ]
}

webpack.config.js representation:

{ target: 'web',
  devtool: 'source-map',
  entry: { a: [ './src/client/bootstrap/ErrorTrap.js' ] },
  output:
   { path: '/project/build/client/assets',
     filename: '[name].[chunkhash].bundle.js',
     chunkFilename: '[name].[chunkhash].chunk.js',
     publicPath: '/assets/' },
  module: { rules: [ [Object], [Object], [Object], [Object], [Object] ] },
  resolve: { alias: { 'lodash.defaults': 'lodash' } },
  plugins:
   [ ModuleConcatenationPlugin { options: {} },
     CommonsChunkPlugin {
       chunkNames: [Object],
       filenameTemplate: undefined,
       minChunks: Infinity,
       selectedChunks: undefined,
       children: undefined,
       async: undefined,
       minSize: undefined,
       ident: '/project/node_modules/webpack/lib/optimize/CommonsChunkPlugin.js0' },
     ManifestPlugin { opts: [Object] },
     ChunkManifestPlugin {
       manifestFilename: 'chunk-manifest.json',
       manifestVariable: 'webpackManifest',
       inlineManifest: false },
     OccurrenceOrderPlugin { preferEntry: undefined },
     DefinePlugin { definitions: [Object] },
     VisualizerPlugin { opts: [Object] },
     ExtractTextPlugin { filename: '[name].[contenthash].css', id: 1, options: {} },
     UglifyJsPlugin { options: [Object] },
     LoaderOptionsPlugin { options: [Object] } ],
  name: 'client' }

Solution

You're using Babel to transpile your JavaScript files and by default the es2015 preset transforms ES modules (import/export) to CommonJS (what Node uses, require). Webpack receives the CommonJS modules, but the ModuleConcatenationPlugin relies on ES modules. You can configure Babel to not transform the modules with the modules option.

{
  "presets": [
    ["es2015", { "modules": false }]
  ]
}

Webpack 2+ supports ES modules out of the box and it's best to leave them to webpack, because it enables features such as Tree Shaking.