webpack.config.js

const path = require('path');
const glob = require('glob');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
const CopyWebpackPlugin = require('copy-webpack-plugin');

module.exports = (env, options) => ({
  optimization: {
    minimizer: [
      new UglifyJsPlugin({ cache: true, parallel: true, sourceMap: false }),
      new OptimizeCSSAssetsPlugin({})
    ]
  },
  entry: {
    './js/app.js': ['./js/app.js'].concat(glob.sync('./vendor/**/*.js'))
  },
  output: {
    filename: 'app.js',
    path: path.resolve(__dirname, '../priv/static/js')
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader'
        }
      },
      {
        test: /\.css$/,
        exclude: /node_modules/,
        use: [
          { loader: 'style-loader', },
          { loader: MiniCssExtractPlugin.loader },
          { loader: 'css-loader', options: { importLoaders: 1, url: false } },
          { loader: 'postcss-loader' },
        ]
      },
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({ filename: '../css/app.css' }), // path relative to output path above: priv/static/js
    new CopyWebpackPlugin([{ from: 'static/', to: '../' }])
  ]
});