webpack client.config.js

const webpack = require('webpack');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const config = require('sapper/webpack/config.js');
const { svelteMinifyHtml, sveltePostcss } = require('./utils.js');

const mode = process.env.NODE_ENV;
const isDev = mode === 'development';

module.exports = {
  entry: config.client.entry(),
  output: config.client.output(),
  resolve: {
    extensions: ['.js', '.json', '.html'],
  },
  module: {
    rules: [
      {
        test: /\.html$/,
        exclude: /node_modules/,
        use: {
          loader: 'svelte-loader',
          options: {
            preprocess: {
              // only remove whitespace in production; better feedback during development
              ...(isDev ? {} : { markup: svelteMinifyHtml({ safe: false }) }),
              style: sveltePostcss(),
            },
            emitCss: true, // pass CSS to webpack
            hydratable: true,
            hotReload: isDev,
            dev: isDev,
          },
        },
      },
      {
        test: /\.css$/,
        use: [
          isDev
            ? { loader: 'style-loader', options: { sourceMap: true }} // CSS HMR
            : MiniCssExtractPlugin.loader, // extract CSS in production builds
          { loader: 'css-loader', options: { sourceMap: isDev }},
          { loader: 'postcss-loader', options: { sourceMap: isDev }},
        ],
      },
    ],
  },
  mode,
  plugins: [
    isDev && new webpack.HotModuleReplacementPlugin(),
    new webpack.DefinePlugin({
      'process.browser': true,
      'process.env.NODE_ENV': JSON.stringify(mode),
    }),

    new MiniCssExtractPlugin({
      filename: '[hash]/[name].css',
      chunkFilename: '[hash]/[name].[id].css',
    }),
  ].filter(Boolean),
  ...(isDev ? {} : { optimization: {
    // combine all CSS into a single file
    splitChunks: {
      cacheGroups: {
        styles: {
          name: 'main',
          test: /\.css$/,
          chunks: 'all',
          enforce: true,
        },
      },
    },
  }}),
  devtool: isDev && 'eval-source-map',
};

What's the contents of './utils.js'?