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webpack.prod.js
const merge = require('webpack-merge');
const common = require('./webpack.config.js');
const OptimizeCssAssetsPlugin = require('optimize-css-assets-webpack-plugin');

module.exports = merge(common, {
  plugins: [
    new OptimizeCssAssetsPlugin({
      assetNameRegExp: /\.css$/,
      cssProcessorOptions: { discardComments: { removeAll: true } }
    })
  ]
});