Use webpack-merge

webpack.config.js

-const path = require('path');
-const HtmlWebpackPlugin = require('html-webpack-plugin');
-const CleanWebpackPlugin = require('clean-webpack-plugin');
-const webpack = require('webpack');
-
-const outputDirectory = 'dist';
-
-module.exports = {
-  //mode: 'development',
-  entry: {
-    app: './src/client/index.js',
-  },
-  devtool: 'eval-source-map',
-  devServer: {
-    // contentBase: './dist',
-    // hot: true,
-    port: 3000,
-    open: true,
-    proxy: {
-      '/api': 'http://localhost:8080'
-    }
-  },
-  plugins: [
-    new CleanWebpackPlugin(['dist']),
-    new HtmlWebpackPlugin({
-      title: 'Hipla',
-      // Load a custom template
-      template: 'src/client/index.html',
-    }),
-    new webpack.NamedModulesPlugin(),
-    new webpack.HotModuleReplacementPlugin(),
-  ],
-  output: {
-    filename: '[name].bundle.js',
-    path: path.resolve(__dirname, outputDirectory),
-  },
-  module: {
-    rules: [
-      {
-        test: /\.js$/,
-        exclude: /node_modules/,
-        loader: 'babel-loader',
-        query: {
-          presets: ['es2015', 'react', 'stage-2']
-        },
-      },
-      {
-        test: /\.css$/,
-        use: [
-          'style-loader',
-          'css-loader',
-        ],
-      },
-      {
-        test: /\.(png|svg|jpg|gif)$/,
-        use: [
-          'file-loader',
-        ],
-      },
-    ],
-  },
-  resolve: {
-    extensions: ['.js', '.jsx'],
-  },
-};