create-react-app ssr dev script

start-server.js

// @remove-on-eject-begin
/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
// @remove-on-eject-end
'use strict';

// Do this as the first thing so that any code reading it knows the right env.
process.env.REACT_APP_ENV = 'server';
process.env.BABEL_ENV = 'development';
process.env.NODE_ENV = 'development';

// Makes the script crash on unhandled rejections instead of silently
// ignoring them. In the future, promise rejections that are not handled will
// terminate the Node.js process with a non-zero exit code.
process.on('unhandledRejection', err => {
  throw err;
});

// Ensure environment variables are read.
require('../config/env');
// @remove-on-eject-begin
// Do the preflight check (only happens before eject).
const verifyPackageTree = require('./utils/verifyPackageTree');
if (process.env.SKIP_PREFLIGHT_CHECK !== 'true') {
  verifyPackageTree();
}
// @remove-on-eject-end

const chalk = require('chalk');
const webpack = require('webpack');
const WebpackDevServer = require('webpack-dev-server');
const clearConsole = require('react-dev-utils/clearConsole');
const checkRequiredFiles = require('react-dev-utils/checkRequiredFiles');
const {
  choosePort,
  createCompiler,
  prepareProxy,
  prepareUrls,
} = require('react-dev-utils/WebpackDevServerUtils');
const openBrowser = require('react-dev-utils/openBrowser');
const paths = require('../config/paths');
const config = require('../config/webpack.config.dev');
const serverConfig = require('../config/webpack.config.dev-server');
const createDevServerConfig = require('../config/webpackDevServer.config');

const isInteractive = process.stdout.isTTY;

// Warn and crash if required files are missing
if (!checkRequiredFiles([paths.appHtml, paths.appIndexJs])) {
  process.exit(1);
}

// Tools like Cloud9 rely on this.
const DEFAULT_PORT = parseInt(process.env.PORT, 10) || 3000;
const HOST = process.env.HOST || '0.0.0.0';

if (process.env.HOST) {
  console.log(
    chalk.cyan(
      `Attempting to bind to HOST environment variable: ${chalk.yellow(
        chalk.bold(process.env.HOST)
      )}`
    )
  );
  console.log(
    `If this was unintentional, check that you haven't mistakenly set it in your shell.`
  );
  console.log(
    `Learn more here: ${chalk.yellow('http://bit.ly/CRA-advanced-config')}`
  );
  console.log();
}

// Copy the defaultFeatures logic from webpack-dev-server. We need to hack the
// default features in order to ensure that our custom serverSideRender
// middleware always runs after contentBaseFiles but before the other default
// features. Running "after" last will result in the public index always being
// rendered (for some reason).
const createDefaultFeatures = (options) => {
  const { after, contentBase } = options
  const defaultFeatures = ['before', 'setup', 'headers', 'middleware'];
  if (options.proxy) { defaultFeatures.push('proxy', 'middleware'); }
  // Ensure "after" runs after "middleware" and "contentBaseFiles" but before evertything else
  if (contentBase !== false) { defaultFeatures.push('contentBaseFiles', 'after'); }
  else if (after) { defaultFeatures.push('after'); }
  if (options.watchContentBase) { defaultFeatures.push('watchContentBase'); }
  if (options.historyApiFallback) {
    defaultFeatures.push('historyApiFallback', 'middleware');
    // Ensure "after" runs after "middleware" and "contentBaseFiles" but before evertything else
    if (contentBase !== false) { defaultFeatures.push('contentBaseFiles', 'after'); }
    else if (after) { defaultFeatures.push('after'); }
  }
  defaultFeatures.push('magicHtml');
  // NOTE: contentBaseIndex is the devil 😈. *Never* enable it.
  // if (contentBase !== false) { defaultFeatures.push('contentBaseIndex'); }
  // compress is placed last and uses unshift so that it will be the first middleware used
  if (options.compress) { defaultFeatures.unshift('compress'); }
  return defaultFeatures
}

// We require that you explictly set browsers and do not fall back to
// browserslist defaults.
const { checkBrowsers } = require('react-dev-utils/browsersHelper');
checkBrowsers(paths.appPath)
  .then(() => {
    // We attempt to use the default port but if it is busy, we offer the user to
    // run on a different port. `choosePort()` Promise resolves to the next free port.
    return choosePort(HOST, DEFAULT_PORT);
  })
  .then(port => {
    if (port == null) {
      // We have not found a port.
      return;
    }
    const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
    const appName = require(paths.appPackageJson).name;

    const urls = prepareUrls(protocol, HOST, port);
    // Create a webpack compiler that is configured with custom messages.
    const compiler = createCompiler(
      webpack,
      [config, serverConfig],
      appName,
      urls,
      paths.useYarn
    );
    // Load proxy config
    const proxySetting = require(paths.appPackageJson).proxy;
    const proxyConfig = prepareProxy(proxySetting, paths.appPublic);
    // Serve webpack assets generated by the compiler over a web server.
    const devServerConfig = createDevServerConfig(
      proxyConfig,
      urls.lanUrlForConfig
    );

    // While webpack-dev-middleware supports a serverSideRender option,
    // webpack-dev-server does not. We're passing in the serverSideRender
    // option from webpackDevServer.config.js but there's no good way to have
    // our reactApp middleware run in the right place. So, we need to override
    // the features option to include our "after" middleware in the right spots.
    // NOTE: this will break if you're already using the features option.
    // TODO: emit a warning if you're using the features option.
    if (devServerConfig.serverSideRender) {
      devServerConfig.features = createDefaultFeatures(devServerConfig)
    }
    const devServer = new WebpackDevServer(compiler, devServerConfig);

    // Launch WebpackDevServer.
    devServer.listen(port, HOST, err => {
      if (err) {
        return console.log(err);
      }
      if (isInteractive) {
        clearConsole();
      }
      console.log(chalk.cyan('Starting the development server...\n'));
      openBrowser(urls.localUrlForBrowser);
    });

    ['SIGINT', 'SIGTERM'].forEach(function(sig) {
      process.on(sig, function() {
        devServer.close();
        process.exit();
      });
    });
  })
  .catch(err => {
    if (err && err.message) {
      console.log(err.message);
    }
    process.exit(1);
  });

webpack.config.dev-server.js

// @remove-on-eject-begin
/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
// @remove-on-eject-end
'use strict';

const autoprefixer = require('autoprefixer');
const path = require('path');
const webpack = require('webpack');
const CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
const WatchMissingNodeModulesPlugin = require('react-dev-utils/WatchMissingNodeModulesPlugin');
const eslintFormatter = require('react-dev-utils/eslintFormatter');
const ModuleScopePlugin = require('react-dev-utils/ModuleScopePlugin');
const getClientEnvironment = require('./env');
const paths = require('./paths');

// Webpack uses `publicPath` to determine where the app is being served from.
// In development, we always serve from the root. This makes config easier.
const publicPath = '/';
// `publicUrl` is just like `publicPath`, but we will provide it to our app
// as %PUBLIC_URL% in `index.html` and `process.env.PUBLIC_URL` in JavaScript.
// Omit trailing slash as %PUBLIC_PATH%/xyz looks better than %PUBLIC_PATH%xyz.
const publicUrl = '';
// Get environment variables to inject into our app.
const env = getClientEnvironment(publicUrl);

// Options for PostCSS as we reference these options twice
// Adds vendor prefixing based on your specified browser support in
// package.json
const postCSSLoaderOptions = {
  // Necessary for external CSS imports to work
  // https://github.com/facebook/create-react-app/issues/2677
  ident: 'postcss',
  plugins: () => [
    require('postcss-flexbugs-fixes'),
    autoprefixer({
      flexbox: 'no-2009',
    }),
  ],
};

// This is the development configuration.
// It is focused on developer experience and fast rebuilds.
// The production configuration is different and lives in a separate file.
module.exports = {
  name: 'server',
  mode: 'development',
  // We need to target node
  target: 'node',
  // You may want 'eval' instead if you prefer to see the compiled output in DevTools.
  // See the discussion in https://github.com/facebook/create-react-app/issues/343.
  devtool: 'cheap-module-source-map',
  // These are the "entry points" to our application.
  // This means they will be the "root" imports that are included in JS bundle.
  // The first two entry points enable "hot" CSS and auto-refreshes for JS.
  entry: [
    // We ship a few polyfills by default:
    require.resolve('./polyfills'),
    // NOTE: we don't need a WebpackDevServer client on the server ;)
    // Finally, this is your app's code:
    paths.appIndexServerJs,
    // We include the app code last so that if there is a runtime error during
    // initialization, it doesn't blow up the WebpackDevServer client, and
    // changing JS code would still trigger a refresh.
  ],
  output: {
    // Add /* filename */ comments to generated require()s in the output.
    pathinfo: true,
    // This does not produce a real file. It's just the virtual path that is
    // served by WebpackDevServer in development. This is the JS bundle
    // containing code from all our entry points, and the Webpack runtime.
    filename: 'bundle.js',
    // There are also additional JS chunk files if you use code splitting.
    chunkFilename: '[name].chunk.js',
    // This is the URL that app is served from. We use "/" in development.
    publicPath: publicPath,
    // We need to output a node library
    libraryTarget: 'commonjs2',
    // Point sourcemap entries to original disk location (format as URL on Windows)
    devtoolModuleFilenameTemplate: info =>
      path.resolve(info.absoluteResourcePath).replace(/\\/g, '/'),
  },
  optimization: {
    // Node targets only support async splitting
    splitChunks: {
      chunks: 'async',
    },
    // Node targets do not support runtimeChunk
    runtimeChunk: false,
  },
  resolve: {
    // This allows you to set a fallback for where Webpack should look for modules.
    // We placed these paths second because we want `node_modules` to "win"
    // if there are any conflicts. This matches Node resolution mechanism.
    // https://github.com/facebook/create-react-app/issues/253
    modules: ['node_modules'].concat(
      // It is guaranteed to exist because we tweak it in `env.js`
      process.env.NODE_PATH.split(path.delimiter).filter(Boolean)
    ),
    // These are the reasonable defaults supported by the Node ecosystem.
    // We also include JSX as a common component filename extension to support
    // some tools, although we do not recommend using it, see:
    // https://github.com/facebook/create-react-app/issues/290
    // `web` extension prefixes have been added for better support
    // for React Native Web.
    extensions: ['.web.js', '.mjs', '.js', '.json', '.web.jsx', '.jsx'],
    alias: {
      // @remove-on-eject-begin
      // Resolve Babel runtime relative to react-scripts.
      // It usually still works on npm 3 without this but it would be
      // unfortunate to rely on, as react-scripts could be symlinked,
      // and thus @babel/runtime might not be resolvable from the source.
      '@babel/runtime': path.dirname(
        require.resolve('@babel/runtime/package.json')
      ),
      // @remove-on-eject-end
      // Support React Native Web
      // https://www.smashingmagazine.com/2016/08/a-glimpse-into-the-future-with-react-native-for-web/
      'react-native': 'react-native-web',
    },
    plugins: [
      // Prevents users from importing files from outside of src/ (or node_modules/).
      // This often causes confusion because we only process files within src/ with babel.
      // To fix this, we prevent you from importing files out of src/ -- if you'd like to,
      // please link the files into your node_modules/ and let module-resolution kick in.
      // Make sure your source files are compiled, as they will not be processed in any way.
      new ModuleScopePlugin(paths.appSrc, [paths.appPackageJson]),
    ],
  },
  module: {
    strictExportPresence: true,
    rules: [
      // Disable require.ensure as it's not a standard language feature.
      { parser: { requireEnsure: false } },

      // First, run the linter.
      // It's important to do this before Babel processes the JS.
      {
        test: /\.(js|jsx|mjs)$/,
        enforce: 'pre',
        use: [
          {
            options: {
              formatter: eslintFormatter,
              eslintPath: require.resolve('eslint'),
              baseConfig: {
                extends: [require.resolve('eslint-config-react-app')],
              },
              // @remove-on-eject-begin
              ignore: false,
              useEslintrc: false,
              // @remove-on-eject-end
            },
            loader: require.resolve('eslint-loader'),
          },
        ],
        include: paths.srcPaths,
        exclude: [/[/\\\\]node_modules[/\\\\]/],
      },
      {
        // "oneOf" will traverse all following loaders until one will
        // match the requirements. When no loader matches it will fall
        // back to the "file" loader at the end of the loader list.
        oneOf: [
          // "url" loader works like "file" loader except that it embeds assets
          // smaller than specified limit in bytes as data URLs to avoid requests.
          // A missing `test` is equivalent to a match.
          {
            test: [/\.bmp$/, /\.gif$/, /\.jpe?g$/, /\.png$/],
            loader: require.resolve('url-loader'),
            options: {
              limit: 10000,
              name: 'static/media/[name].[hash:8].[ext]',
            },
          },
          // Process application JS with Babel.
          // The preset includes JSX, Flow, and some ESnext features.
          {
            test: /\.(js|jsx|mjs)$/,
            include: paths.srcPaths,
            exclude: [/[/\\\\]node_modules[/\\\\]/],
            use: [
              // This loader parallelizes code compilation, it is optional but
              // improves compile time on larger projects
              require.resolve('thread-loader'),
              {
                loader: require.resolve('babel-loader'),
                options: {
                  // @remove-on-eject-begin
                  babelrc: false,
                  // @remove-on-eject-end
                  presets: [
                    [
                      require.resolve('babel-preset-react-app'),
                      {
                        targets: {
                          node: 'current',
                        },
                      },
                    ],
                  ],
                  plugins: [
                    require.resolve('@7rulnik/react-loadable/babel'),
                    [
                      require.resolve('babel-plugin-named-asset-import'),
                      {
                        loaderMap: {
                          svg: {
                            ReactComponent: 'svgr/webpack![path]',
                          },
                        },
                      },
                    ],
                  ],
                  // This is a feature of `babel-loader` for webpack (not Babel itself).
                  // It enables caching results in ./node_modules/.cache/babel-loader/
                  // directory for faster rebuilds.
                  cacheDirectory: true,
                  highlightCode: true,
                },
              },
            ],
          },
          // Process any JS outside of the app with Babel.
          // Unlike the application JS, we only compile the standard ES features.
          {
            test: /\.js$/,
            use: [
              // This loader parallelizes code compilation, it is optional but
              // improves compile time on larger projects
              require.resolve('thread-loader'),
              {
                loader: require.resolve('babel-loader'),
                options: {
                  babelrc: false,
                  compact: false,
                  presets: [
                    [
                      require.resolve('babel-preset-react-app/dependencies'),
                      {
                        targets: {
                          node: 'current',
                        },
                      },
                    ],
                  ],
                  plugins: [require.resolve('@7rulnik/react-loadable/babel')],
                  cacheDirectory: true,
                  highlightCode: true,
                },
              },
            ],
          },
          // "postcss" loader applies autoprefixer to our CSS.
          // "css" loader resolves paths in CSS and adds assets as dependencies.
          // "style" loader turns CSS into JS modules that inject <style> tags.
          // In production, we use a plugin to extract that CSS to a file, but
          // in development "style" loader enables hot editing of CSS.
          // By default we support CSS Modules with the extension .module.css
          {
            test: /\.css$/,
            exclude: /\.module\.css$/,
            use: [
              // NOTE: we don't need require.resolve('style-loader') on the server
              {
                loader: require.resolve('css-loader'),
                options: {
                  importLoaders: 1,
                },
              },
              {
                loader: require.resolve('postcss-loader'),
                options: postCSSLoaderOptions,
              },
            ],
          },
          // Adds support for CSS Modules (https://github.com/css-modules/css-modules)
          // using the extension .module.css
          {
            test: /\.module\.css$/,
            use: [
              // NOTE: we don't need require.resolve('style-loader') on the server
              {
                loader: require.resolve('css-loader'),
                options: {
                  importLoaders: 1,
                  modules: true,
                  localIdentName: '[path]__[name]___[local]',
                },
              },
              {
                loader: require.resolve('postcss-loader'),
                options: postCSSLoaderOptions,
              },
            ],
          },
          // The GraphQL loader preprocesses GraphQL queries in .graphql files.
          {
            test: /\.(graphql)$/,
            loader: 'graphql-tag/loader',
          },
          // "file" loader makes sure those assets get served by WebpackDevServer.
          // When you `import` an asset, you get its (virtual) filename.
          // In production, they would get copied to the `build` folder.
          // This loader doesn't use a "test" so it will catch all modules
          // that fall through the other loaders.
          {
            // Exclude `js` files to keep "css" loader working as it injects
            // its runtime that would otherwise be processed through "file" loader.
            // Also exclude `html` and `json` extensions so they get processed
            // by webpacks internal loaders.
            exclude: [/\.(js|jsx|mjs)$/, /\.html$/, /\.json$/],
            loader: require.resolve('file-loader'),
            options: {
              name: 'static/media/[name].[hash:8].[ext]',
            },
          },
        ],
      },
      // ** STOP ** Are you adding a new loader?
      // Make sure to add the new loader(s) before the "file" loader.
    ],
  },
  plugins: [
    // NOTE: exclude HtmlWebpackPlugin
    // NOTE: exclude InterpolateHtmlPlugin
    // Add module names to factory functions so they appear in browser profiler.
    new webpack.NamedModulesPlugin(),
    // Makes some environment variables available to the JS code, for example:
    // if (process.env.NODE_ENV === 'development') { ... }. See `./env.js`.
    new webpack.DefinePlugin(env.stringified),
    // NOTE: exclude webpack.HotModuleReplacementPlugin
    // Watcher doesn't work well if you mistype casing in a path so we use
    // a plugin that prints an error when you attempt to do this.
    // See https://github.com/facebook/create-react-app/issues/240
    new CaseSensitivePathsPlugin(),
    // If you require a missing module and then `npm install` it, you still have
    // to restart the development server for Webpack to discover it. This plugin
    // makes the discovery automatic so you don't have to restart.
    // See https://github.com/facebook/create-react-app/issues/186
    new WatchMissingNodeModulesPlugin(paths.appNodeModules),
    // Moment.js is an extremely popular library that bundles large locale files
    // by default due to how Webpack interprets its code. This is a practical
    // solution that requires the user to opt into importing specific locales.
    // https://github.com/jmblog/how-to-optimize-momentjs-with-webpack
    // You can remove this if you don't use Moment.js:
    // https://github.com/bitinn/node-fetch/issues/41#issuecomment-167368854
    new webpack.IgnorePlugin(/^\.\/locale$/, /moment$/),
  ],

  // Some libraries import Node modules but don't use them in the browser.
  // Tell Webpack to provide empty mocks for them so importing them works.
  node: {
    dgram: 'empty',
    fs: 'empty',
    net: 'empty',
    tls: 'empty',
    child_process: 'empty',
  },
  // Turn off performance hints during development because we don't do any
  // splitting or minification in interest of speed. These warnings become
  // cumbersome.
  performance: {
    hints: false,
  },
};

webpackDevServer.config.js

// @remove-on-eject-begin
/**
 * Copyright (c) 2015-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */
// @remove-on-eject-end
'use strict';

const errorOverlayMiddleware = require('react-dev-utils/errorOverlayMiddleware');
const noopServiceWorkerMiddleware = require('react-dev-utils/noopServiceWorkerMiddleware');
const ignoredFiles = require('react-dev-utils/ignoredFiles');
const config = require('./webpack.config.dev');
const paths = require('./paths');

const protocol = process.env.HTTPS === 'true' ? 'https' : 'http';
const host = process.env.HOST || '0.0.0.0';
const SERVER = process.env.REACT_APP_ENV === 'server'

module.exports = function(proxy, allowedHost) {
  return {
    // WebpackDevServer 2.4.3 introduced a security fix that prevents remote
    // websites from potentially accessing local content through DNS rebinding:
    // https://github.com/webpack/webpack-dev-server/issues/887
    // https://medium.com/webpack/webpack-dev-server-middleware-security-issues-1489d950874a
    // However, it made several existing use cases such as development in cloud
    // environment or subdomains in development significantly more complicated:
    // https://github.com/facebook/create-react-app/issues/2271
    // https://github.com/facebook/create-react-app/issues/2233
    // While we're investigating better solutions, for now we will take a
    // compromise. Since our WDS configuration only serves files in the `public`
    // folder we won't consider accessing them a vulnerability. However, if you
    // use the `proxy` feature, it gets more dangerous because it can expose
    // remote code execution vulnerabilities in backends like Django and Rails.
    // So we will disable the host check normally, but enable it if you have
    // specified the `proxy` setting. Finally, we let you override it if you
    // really know what you're doing with a special environment variable.
    disableHostCheck:
      !proxy || process.env.DANGEROUSLY_DISABLE_HOST_CHECK === 'true',
    // Enable gzip compression of generated files.
    compress: true,
    // Silence WebpackDevServer's own logs since they're generally not useful.
    // It will still show compile warnings and errors with this setting.
    clientLogLevel: 'none',
    // By default WebpackDevServer serves physical files from current directory
    // in addition to all the virtual build products that it serves from memory.
    // This is confusing because those files won’t automatically be available in
    // production build folder unless we copy them. However, copying the whole
    // project directory is dangerous because we may expose sensitive files.
    // Instead, we establish a convention that only files in `public` directory
    // get served. Our build script will copy `public` into the `build` folder.
    // In `index.html`, you can get URL of `public` folder with %PUBLIC_URL%:
    // <link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
    // In JavaScript code, you can access it with `process.env.PUBLIC_URL`.
    // Note that we only recommend to use `public` folder as an escape hatch
    // for files like `favicon.ico`, `manifest.json`, and libraries that are
    // for some reason broken when imported through Webpack. If you just want to
    // use an image, put it in `src` and `import` it from JavaScript instead.
    contentBase: paths.appPublic,
    // By default files from `contentBase` will not trigger a page reload.
    watchContentBase: true,
    // Enable hot reloading server. It will provide /sockjs-node/ endpoint
    // for the WebpackDevServer client so it can learn when the files were
    // updated. The WebpackDevServer client is included as an entry point
    // in the Webpack development configuration. Note that only changes
    // to CSS are currently hot reloaded. JS changes will refresh the browser.
    hot: true,
    // It is important to tell WebpackDevServer to use the same "root" path
    // as we specified in the config. In development, we always serve from /.
    publicPath: config.output.publicPath,
    // WebpackDevServer is noisy by default so we emit custom message instead
    // by listening to the compiler events with `compiler.hooks[...].tap` calls above.
    quiet: true,
    // Reportedly, this avoids CPU overload on some systems.
    // https://github.com/facebook/create-react-app/issues/293
    // src/node_modules is not ignored to support absolute imports
    // https://github.com/facebook/create-react-app/issues/1065
    watchOptions: {
      ignored: ignoredFiles(paths.appSrc),
    },
    // Enable HTTPS if the HTTPS environment variable is set to 'true'
    https: protocol === 'https',
    host: host,
    overlay: false,
    historyApiFallback: {
      // Paths with dots should still use the history fallback.
      // See https://github.com/facebook/create-react-app/issues/387.
      disableDotRule: true,
    },
    public: allowedHost,
    proxy,
    // We need to turn off serving the index.html file when doing
    // serverSideRender. Our reactApp middleware is rendering the index.html on
    // the server, which means we don't want to serve the index.html from the
    // webpack middleware anymore. We detect if we're in SSR mode and turn off
    // the index accordingly. Painfully, the options checking in
    // webpack-dev-server won't allow the index option to be anything except
    // string or undefined. So we need to set it to an empty string here, which
    // evaluates to a falsey value in the webpack-dev-middleware.
    index: SERVER ? '' : undefined,
    // We need to tell webpack-dev-middleware we're doing SSR
    serverSideRender: SERVER,
    // We need to tell webpack-dever-server to turn off serving the index.html
    // file too. Otherwise our reactApp middleware won't work as expected.
    staticOptions: SERVER ? { index: false } : undefined,
    before(app) {
      // This lets us open files from the runtime error overlay.
      app.use(errorOverlayMiddleware());
      // This service worker file is effectively a 'no-op' that will reset any
      // previous service worker registered for the same host:port combination.
      // We do this in development to avoid hitting the production cache if
      // it used the same host and port.
      // https://github.com/facebook/create-react-app/issues/2272#issuecomment-302832432
      app.use(noopServiceWorkerMiddleware());
    },
    after(app) {
      if (SERVER) {
        // This allows us to render the HTML on the server. We don't need this
        // unless we're doing SSR. For this to work, you need to define
        // appServerSideRenderMiddleware in your `config/paths.js`. Your
        // serverSideRenderMiddleware should be an express middleware function
        // that uses res.send to deliver the rendered HTML.
        // You may wish to review the webpack-dev-middleware for more
        // information on how to handle webpackStats in your middleware.
        // https://github.com/webpack/webpack-dev-middleware#server-side-rendering
        app.use(require(paths.appServerSideRenderMiddleware));
      }
    }
  };
};