Webpack Tree-Shaking Dynamic Imports seems not to be working

Question

History: I recently discovered an odd behaviour with using Webpack and dynamic imports. First I thought it might be the 3rd-party library 'Loadable Components' I used, so I opened a bug issue (https://github.com/gregberge/loadable-components/issues/517) on their end. The author replied telling me that the behaviour is coming from Webpack and the dynamic imports themselves.

I can stand the fact that it does not tree-shake the dynamic import, for me it is more important to understand why that is the case.

Demo repository to demonstrate the behaviour can be found here: https://github.com/dazlious/treeshaking-dynamic-imports

Short description of the problem: From my perspective, an imported named export should not force all the exported code to be bundled within it.

In the demo case we have a component (./lib/index.jsx) that has two sub components called module1 (./lib/module1/module1.jsx) and module2 (./lib/module1/module2.jsx). Module1 exports a constant called FOO_BAR that is then imported by Module2 as a named import.

When looking at the build output, you'll find Module2 containing Module1 in whole and not only the string that is specifically imported.

Is anyone with deep knowledge of Webpack and/or dynamic imports around here? Would be happy to learn more about the behaviour.

Answer 1

I edited the webpack.config to be:

const path = require('path');
const TerserPlugin = require('terser-webpack-plugin');
const { BundleAnalyzerPlugin } = require('webpack-bundle-analyzer');

const baseDir = path.resolve(__dirname);

const config = {
  mode: process.env.NODE_ENV,
  stats: 'minimal',
  resolve: {
    extensions: ['.js', '.jsx'],
    symlinks: false,
  },
  plugins: [
    new BundleAnalyzerPlugin({
      analyzerMode: 'static',
      openAnalyzer: false,
      reportFilename: 'analyze.html',
    }),
  ],
  target: 'web',
  devtool: 'hidden-source-map',
  entry: {
    bundle: [path.resolve(baseDir, 'lib')],
  },
  optimization: {
    minimize: true,
    minimizer: [
      new TerserPlugin({
        parallel: true,
      }),
    ],
    mangleWasmImports: true,
    splitChunks: {
      cacheGroups: {
        default: false,
        vendors: false,
        vendor: {
          name: 'vendor',
          chunks: 'all',
          test: /node_modules/,
          priority: 20
        },
        common: {
          name: 'common',
          minChunks: 2,
          chunks: 'all',
          priority: 10,
          reuseExistingChunk: true,
          enforce: true
        }
      }
    },
  },
  output: {
    chunkFilename: '[name].[chunkhash].js',
    publicPath: '/',
    path: path.join(baseDir, 'dist'),
    filename: '[name].[hash].js',
  },
  module: {
    rules: [
      {
        test: /^.*\.jsx?$/,
        include: [path.resolve(baseDir, 'lib')],
        loader: 'babel-loader?cacheDirectory',
      },
      {
        test: /\.mjs$/,
        include: /node_modules/,
        type: 'javascript/auto',
      },
    ],
  },
};

module.exports = config;

I think this has the result you are looking for? image of bunde analyzer showing modules in their own bundles

I think it requires the splitChunks option to actually tree-shake the components properly. I have spend a lot of time trying to figure webpack out, but I'm still guessing here.