Webpack 6

webpack.config.js

var webpack = require('webpack');
var path = require('path');
var glob = require('glob');
var ExtractTextPlugin = require('extract-text-webpack-plugin');
var PurifyCSSPlugin = require('purifycss-webpack');

module.exports = {
    entry: './src',
    output: {
        path: path.resolve(__dirname, './public'),
        filename: 'index.js'
    },
    module: {
        rules: [
            {
                test: /\.js$/,
                exclude: /(node_modules|bower_components)/,
                loader: 'babel-loader',
                query: {
                    presets: ['env']
                }
            },
            {
                test: /\.css$/,
                use: ExtractTextPlugin.extract({
                    fallback: 'style-loader',
                    use: 'css-loader'
                })
            }
        ]
    },
    plugins: [
        new ExtractTextPlugin('style.css'),
        new PurifyCSSPlugin({
          paths: glob.sync(path.join(__dirname, './src/*.js')),
          minimize: true
        })
    ]
}