// postcss.config.js / npm install postcss-loader autoprefixer css-mqpacker cssnano --save-dev
module.exports = {
  plugins: [
    require('autoprefixer'),
    require('css-mqpacker'),
    require('cssnano')({
      preset: [
        'default', {
          discardComments: {
            removeAll: true,
          }
        }
      ]
    })
  ]
}

// webpack.config.js

{
      test: /\.css$/,
      use: [
        'style-loader',
        MiniCssExtractPlugin.loader,
        {
          loader: 'css-loader',
          options: { sourceMap: true }
        }, {
          loader: 'postcss-loader',
          options: { sourceMap: true, config: { path: 'src/js/postcss.config.js' } }
        }
      ]
},

{
      test: /\.scss$/,
      use: [
        'style-loader',
        MiniCssExtractPlugin.loader,
        {
          loader: 'css-loader',
          options: { sourceMap: true }
        }, {
          loader: 'postcss-loader',
          options: { sourceMap: true, config: { path: 'src/js/postcss.config.js' } }
        }, {
          loader: 'sass-loader',
          options: { sourceMap: true }
        }
      ]
}