fix webpack 4 production issues

horseman/horsemanadmin/frontend/webpack/prod.config.babel.js

@@ -5,6 +5,7 @@ import webpack from 'webpack';
 import ExtractTextPlugin from 'extract-text-webpack-plugin';
 import BundleTracker from 'webpack-bundle-tracker';
 import CleanWebpackPlugin from 'clean-webpack-plugin';
+import UglifyJSPlugin from 'uglifyjs-webpack-plugin';
 
 import config, { cssModulesGeneratedScopedName } from './base.config.babel';
 
@@ -25,17 +26,6 @@ config.plugins = config.plugins.concat([
     },
   }),
 
-  // minifies code
-  new webpack.optimize.UglifyJsPlugin({
-    sourceMap: true,
-    output: {
-      comments: false,
-    },
-    compressor: {
-      warnings: false,
-    },
-  }),
-
   new ExtractTextPlugin({
     filename: 'styles/[name]-[contenthash].css',
     allChunks: true,
@@ -47,6 +37,14 @@ config.plugins = config.plugins.concat([
   }),
 ]);
 
+config.optimization = {
+  minimizer: [
+    new UglifyJSPlugin({
+      sourceMap: true,
+    }),
+  ],
+};
+
 const cssNano = {
   autoprefixer: false,
   discardComments: { removeAll: true },

package.json

@@ -22,7 +22,7 @@
     "eslint-plugin-import": "^2.9.0",
     "eslint-plugin-jsx-a11y": "^6.0.3",
     "eslint-plugin-react": "^7.7.0",
-    "extract-text-webpack-plugin": "^3.0.2",
+    "extract-text-webpack-plugin": "^4.0.0-beta.0",
     "image-loader": "0.0.1",
     "image-webpack-loader": "^4.1.0",
     "node-sass": "^4.7.2",
@@ -34,6 +34,7 @@
     "precss": "^3.1.2",
     "sass-loader": "^6.0.7",
     "style-loader": "^0.20.3",
+    "uglifyjs-webpack-plugin": "^1.2.3",
     "webpack": "^4.1.1",
     "webpack-bundle-tracker": "^0.3.0",
     "webpack-cli": "^2.0.11",