How to fix modules with "ModuleConcatenation bailout: Module is not an ECMAScript module" bailout message? : <p>Webpack3 comes with the <code>ModuleConcatenation</code> plugin that when used with the <code>--display-optimization-bailout</code> flag will give you the reason for bailout.</p>

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

<p>Here is my output for the <code>webpack</code> command on a very simplified version of my project:</p>

<pre><code>&gt; 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
</code></pre>

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

<pre><code>class ErrorTrap {
}

export default ErrorTrap;
</code></pre>

<p>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 <a href="https://github.com/webpack/webpack/blob/93ac8e9c3699bf704068eaccaceec57b3dd45a14/lib/dependencies/HarmonyDetectionParserPlugin.js#L12-L14" rel="noreferrer">https://github.com/webpack/webpack/blob/93ac8e9c3699bf704068eaccaceec57b3dd45a14/lib/dependencies/HarmonyDetectionParserPlugin.js#L12-L14</a>, but I don't know why it's not considering this module a <code>ECMAScript</code> module.</p>

<p><strong>.babelrc</strong></p>

<pre><code>{
  "presets": [
    "es2015"
  ]
}
</code></pre>

<p><strong>webpack.config.js</strong> representation:</p>

<pre><code>{ 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' }
</code></pre>
