Remove empty CSS modules workaround

mini-css-extract-plugin-cleanup.js

const EMPTY_MODULES_CLEANUP_DEP_TYPES = new Set([
  'harmony side effect evaluation',
  'import()',
  'single entry',
]);

class SetMap extends Map {
  add (key, value) {
    const set = this.get(key);
    if (set === undefined) {
      super.set(key, new Set([value]));
    } else {
      set.add(value);
    }
  }
}

/**
 * Temporary workaround to clean up empty .js files left behind by
 * mini-css-extract-plugin. This is already fixed in webpack@5.
 *
 * @see {@link https://github.com/webpack-contrib/mini-css-extract-plugin/issues/357}
 */
module.exports = class MiniCssExtractPluginCleanup {
  apply (compiler) {
    compiler.hooks.thisCompilation.tap('mini-css-extract-plugin-cleanup', (compilation) => {
      compilation.hooks.finishModules.tap('mini-css-extract-plugin-cleanup', (modules) => {
        const toRemoveMap = new SetMap();

        for (const module of modules) {
          if (module.type === 'css/mini-extract') {
            for (const cssModuleReason of module.reasons) {
              for (const reason of cssModuleReason.module.reasons) {
                if (EMPTY_MODULES_CLEANUP_DEP_TYPES.has(reason.dependency.type)) {
                  toRemoveMap.add(
                    reason.module || cssModuleReason.module,
                    reason.dependency,
                  );
                }
              }
            }
          }
        }

        for (const [module, set] of toRemoveMap) {
          // collapse transitive CSS module dependencies
          module.dependencies = module.dependencies.flatMap(d => set.has(d) ? d.module.dependencies : [d]);
        }
      });

      compilation.hooks.optimizeChunkAssets.tapAsync('mini-css-extract-plugin-cleanup', (chunks, callback) => {
        for (const chunk of chunks) {
          if (!chunk.getModules().some(module => /^javascript\/(auto|cjs|esm)$/.test(module.type))) {
            // chunk doesn't contain actual JS code, remove the file
            const chunkJs = chunk.files.find(file => /\.js$/.test(file));
            if (!chunk.hasRuntime()) {
              // this is a split chunk, merge it into the runtime chunks that depend on it
              for (const group of compilation.chunkGroups) {
                if (chunk.isInGroup(group)) {
                  const runtimeJs = group.runtimeChunk.files.find(file => /\.js$/.test(file));
                  compilation.assets[runtimeJs]._source.add(compilation.assets[chunkJs]);
                }
              }
            }
            chunk.files = chunk.files.filter(file => {
              if (/\.js(\.map)?$/.test(file)) {
                delete compilation.assets[file];
                return false;
              }
              return true;
            });
          }
        }
        callback();
      });
    });
  }
};