title: ERROR in build.js from UglifyJs Unexpected token: punc (()
body: <p>I have developed an application on my local. The client and server run on different ports. The application runs fine on my local system. I have now pushed the code onto the server. When I try running the code using npm run build.</p>
  
  <p>I get an error that looks like this</p>
  
  <p><a href="https://i.stack.imgur.com/OMdXg.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/OMdXg.png" alt="enter image description here"></a></p>
  
  <p>My urlMixin.js looks like this</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="true" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>let path = require('path');
  
  let webpack = require('webpack');
  
  module.exports = {
      entry: './src/main.js',
  output: {
      path: path.resolve(__dirname, './dist'),
      publicPath: '/dist/',
      filename: 'build.js'
  },
  module: {
      rules: [{
              test: /\.vue$/,
              loader: 'vue-loader',
              options: {
                  loaders: {}
                  // other vue-loader options go here
              }
          },
          {
              test: /\.js$/,
              loader: 'babel-loader',
              exclude: /node_modules/
          },
          {
              test: /\.(png|jpg|gif|svg)$/,
              loader: 'file-loader',
              options: {
                  name: '[name].[ext]?[hash]'
              }
          }
      ]
  },
  resolve: {
      alias: {
          'vue$': 'vue/dist/vue.esm.js',
      },
      extensions: ['*', '.js', '.vue', '.json']
  },
  
  devServer: {
      proxy: {
          '/api': {
              target: 'http://localhost:3000'
          }
      },
      historyApiFallback: true,
      noInfo: true
  },
  performance: {
      hints: false
  },
  devtool: '#eval-source-map'
  }
  
  if (process.env.NODE_ENV === 'production') {
       module.exports.devtool = '#source-map'
      // http://vue-loader.vuejs.org/en/workflow/production.html
  module.exports.plugins = (module.exports.plugins || []).concat([
      new webpack.DefinePlugin({
          'process.env': {
              NODE_ENV: '"production"'
          }
      }),
      new webpack.optimize.UglifyJsPlugin({
          sourceMap: true,
          compress: {
              warnings: false
          }
      }),
      new webpack.LoaderOptionsPlugin({
          minimize: true
      })
  ])
  }</code></pre>
  </div>
  </div>
  </p>
  
  <p>Is there something wrong with my webpack? What wrong am I doing? Can someone please help me out?</p>