From 8e1629578f32d6ad676a80af6435cfdb0d0bbf37 Mon Sep 17 00:00:00 2001
From: Dan Abramov <dan.abramov@gmail.com>
Date: Thu, 4 Aug 2016 20:56:52 +0100
Subject: [PATCH] Document configuration and build process (#362)

---
 config/babel.dev.js           | 27 +++++++++-----
 config/babel.prod.js          | 30 ++++++++++------
 config/polyfills.js           |  1 +
 config/webpack.config.dev.js  | 67 ++++++++++++++++++++++++++++++++++-
 config/webpack.config.prod.js | 64 ++++++++++++++++++++++++++++++---
 scripts/build.js              |  8 ++++-
 scripts/start.js              | 46 ++++++++++++++++++------
 7 files changed, 205 insertions(+), 38 deletions(-)

diff --git a/config/babel.dev.js b/config/babel.dev.js
index 64545127..04af8606 100644
--- a/config/babel.dev.js
+++ b/config/babel.dev.js
@@ -8,22 +8,31 @@
  */
 
 module.exports = {
+  // Don't try to find .babelrc because we want to force this configuration.
   babelrc: false,
+  // This is a feature of `babel-loader` for webpack (not Babel itself).
+  // It enables caching results in OS temporary directory for faster rebuilds.
   cacheDirectory: true,
   presets: [
-    'babel-preset-es2015',
-    'babel-preset-es2016',
-    'babel-preset-react'
-  ].map(require.resolve),
+    // let, const, destructuring, classes, modules
+    require.resolve('babel-preset-es2015'),
+    // exponentiation
+    require.resolve('babel-preset-es2016'),
+    // JSX, Flow
+    require.resolve('babel-preset-react')
+  ],
   plugins: [
-    'babel-plugin-syntax-trailing-function-commas',
-    'babel-plugin-transform-class-properties',
-    'babel-plugin-transform-object-rest-spread'
-  ].map(require.resolve).concat([
+    // function x(a, b, c,) { }
+    require.resolve('babel-plugin-syntax-trailing-function-commas'),
+    // class { handleClick = () => { } }
+    require.resolve('babel-plugin-transform-class-properties'),
+    // { ...todo, completed: true }
+    require.resolve('babel-plugin-transform-object-rest-spread'),
+    // Polyfills the runtime needed for generators
     [require.resolve('babel-plugin-transform-runtime'), {
       helpers: false,
       polyfill: false,
       regenerator: true
     }]
-  ])
+  ]
 };
diff --git a/config/babel.prod.js b/config/babel.prod.js
index f5446012..ae5ea038 100644
--- a/config/babel.prod.js
+++ b/config/babel.prod.js
@@ -8,22 +8,30 @@
  */
 
 module.exports = {
+  // Don't try to find .babelrc because we want to force this configuration.
   babelrc: false,
   presets: [
-    'babel-preset-es2015',
-    'babel-preset-es2016',
-    'babel-preset-react'
-  ].map(require.resolve),
+    // let, const, destructuring, classes, modules
+    require.resolve('babel-preset-es2015'),
+    // exponentiation
+    require.resolve('babel-preset-es2016'),
+    // JSX, Flow
+    require.resolve('babel-preset-react')
+  ],
   plugins: [
-    'babel-plugin-syntax-trailing-function-commas',
-    'babel-plugin-transform-class-properties',
-    'babel-plugin-transform-object-rest-spread',
-    'babel-plugin-transform-react-constant-elements',
-  ].map(require.resolve).concat([
+    // function x(a, b, c,) { }
+    require.resolve('babel-plugin-syntax-trailing-function-commas'),
+    // class { handleClick = () => { } }
+    require.resolve('babel-plugin-transform-class-properties'),
+    // { ...todo, completed: true }
+    require.resolve('babel-plugin-transform-object-rest-spread'),
+    // Polyfills the runtime needed for generators
     [require.resolve('babel-plugin-transform-runtime'), {
       helpers: false,
       polyfill: false,
       regenerator: true
-    }]
-  ])
+    }],
+    // Optimization: hoist JSX that never changes out of render()
+    require.resolve('babel-plugin-transform-react-constant-elements')
+  ],
 };
diff --git a/config/polyfills.js b/config/polyfills.js
index 0a0bd950..1f71e4ac 100644
--- a/config/polyfills.js
+++ b/config/polyfills.js
@@ -6,4 +6,5 @@ if (typeof Promise === 'undefined') {
   window.Promise = require('promise/lib/es6-extensions.js');
 }
 
+// fetch() polyfill for making API calls.
 require('whatwg-fetch');
diff --git a/config/webpack.config.dev.js b/config/webpack.config.dev.js
index 26e60af3..7893f09c 100644
--- a/config/webpack.config.dev.js
+++ b/config/webpack.config.dev.js
@@ -15,22 +15,56 @@ var CaseSensitivePathsPlugin = require('case-sensitive-paths-webpack-plugin');
 var WatchMissingNodeModulesPlugin = require('../scripts/utils/WatchMissingNodeModulesPlugin');
 var paths = require('./paths');
 
+// 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 = {
+  // This makes the bundle appear split into separate modules in the devtools.
+  // We don't use source maps here because they can be confusing:
+  // https://github.com/facebookincubator/create-react-app/issues/343#issuecomment-237241875
+  // You may want 'cheap-module-source-map' instead if you prefer source maps.
   devtool: 'eval',
+  // 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: [
+    // Include WebpackDevServer client. It connects to WebpackDevServer via
+    // sockets and waits for recompile notifications. When WebpackDevServer
+    // recompiles, it sends a message to the client by socket. If only CSS
+    // was changed, the app reload just the CSS. Otherwise, it will refresh.
+    // The "?/" bit at the end tells the client to look for the socket at
+    // the root path, i.e. /sockjs-node/. Otherwise visiting a client-side
+    // route like /todos/42 would make it wrongly request /todos/42/sockjs-node.
+    // The socket server is a part of WebpackDevServer which we are using.
+    // The /sockjs-node/ path I'm referring to is hardcoded in WebpackDevServer.
     require.resolve('webpack-dev-server/client') + '?/',
+    // Include Webpack hot module replacement runtime. Webpack is pretty
+    // low-level so we need to put all the pieces together. The runtime listens
+    // to the events received by the client above, and applies updates (such as
+    // new CSS) to the running application.
     require.resolve('webpack/hot/dev-server'),
+    // We ship a few polyfills by default.
     require.resolve('./polyfills'),
+    // Finally, this is your app's code:
     path.join(paths.appSrc, 'index')
+    // 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: {
     // Next line is not used in dev but WebpackDevServer crashes without it:
     path: paths.appBuild,
+    // 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: 'static/js/bundle.js',
+    // In development, we always serve from the root. This makes config easier.
     publicPath: '/'
   },
   resolve: {
+    // These are the reasonable defaults supported by the Node ecosystem.
     extensions: ['.js', '.json', ''],
     alias: {
       // This `alias` section can be safely removed after ejection.
@@ -44,11 +78,16 @@ module.exports = {
       'babel-runtime/regenerator': require.resolve('babel-runtime/regenerator')
     }
   },
+  // Resolve loaders (webpack plugins for CSS, images, transpilation) from the
+  // directory of `react-scripts` itself rather than the project directory.
+  // You can remove this after ejecting.
   resolveLoader: {
     root: paths.ownNodeModules,
     moduleTemplates: ['*-loader']
   },
   module: {
+    // First, run the linter.
+    // It's important to do this before Babel processes the JS.
     preLoaders: [
       {
         test: /\.js$/,
@@ -57,22 +96,33 @@ module.exports = {
       }
     ],
     loaders: [
+      // Process JS with Babel.
       {
         test: /\.js$/,
         include: paths.appSrc,
         loader: 'babel',
         query: require('./babel.dev')
       },
+      // "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.
       {
         test: /\.css$/,
         include: [paths.appSrc, paths.appNodeModules],
         loader: 'style!css!postcss'
       },
+      // JSON is not enabled by default in Webpack but both Node and Browserify
+      // allow it implicitly so we also enable it.
       {
         test: /\.json$/,
         include: [paths.appSrc, paths.appNodeModules],
         loader: 'json'
       },
+      // "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.
       {
         test: /\.(jpg|png|gif|eot|svg|ttf|woff|woff2)(\?.*)?$/,
         include: [paths.appSrc, paths.appNodeModules],
@@ -81,6 +131,8 @@ module.exports = {
           name: 'static/media/[name].[ext]'
         }
       },
+      // "url" loader works just like "file" loader but it also embeds
+      // assets smaller than specified size as data URLs to avoid requests.
       {
         test: /\.(mp4|webm)(\?.*)?$/,
         include: [paths.appSrc, paths.appNodeModules],
@@ -92,10 +144,12 @@ module.exports = {
       }
     ]
   },
+  // Point ESLint to our predefined config.
   eslint: {
     configFile: path.join(__dirname, 'eslint.js'),
     useEslintrc: false
   },
+  // We use PostCSS for autoprefixing only.
   postcss: function() {
     return [
       autoprefixer({
@@ -103,21 +157,32 @@ module.exports = {
           '>1%',
           'last 4 versions',
           'Firefox ESR',
-          'not ie < 9',
+          'not ie < 9', // React doesn't support IE8 anyway
         ]
       }),
     ];
   },
   plugins: [
+    // Generates an `index.html` file with the <script> injected.
     new HtmlWebpackPlugin({
       inject: true,
       template: paths.appHtml,
       favicon: paths.appFavicon,
     }),
+    // Makes the environment available to the JS code, for example:
+    // if (process.env.NODE_ENV === 'development') { ... }. See `env.js`.
     new webpack.DefinePlugin({ 'process.env.NODE_ENV': '"development"' }),
     // Note: only CSS is currently hot reloaded
+    // This is necessary to emit hot updates (currently CSS only):
     new 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/facebookincubator/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/facebookincubator/create-react-app/issues/186
     new WatchMissingNodeModulesPlugin(paths.appNodeModules)
   ]
 };
diff --git a/config/webpack.config.prod.js b/config/webpack.config.prod.js
index 3dd18cf1..443fc58b 100644
--- a/config/webpack.config.prod.js
+++ b/config/webpack.config.prod.js
@@ -15,27 +15,45 @@ var ExtractTextPlugin = require('extract-text-webpack-plugin');
 var url = require('url');
 var paths = require('./paths');
 
+// We use "homepage" field to infer "public path" at which the app is served.
+// Webpack needs to know it to put the right <script> hrefs into HTML even in
+// single-page apps that may serve index.html for nested URLs like /todos/42.
+// We can't use a relative path in HTML because we don't want to load something
+// like /todos/42/static/js/bundle.7289d.js. We have to know the root.
 var homepagePath = require(paths.appPackageJson).homepage;
 var publicPath = homepagePath ? url.parse(homepagePath).pathname : '/';
 if (!publicPath.endsWith('/')) {
-  // Prevents incorrect paths in file-loader
+  // If we don't do this, file assets will get incorrect paths.
   publicPath += '/';
 }
 
+// This is the production configuration.
+// It compiles slowly and is focused on producing a fast and minimal bundle.
+// The development configuration is different and lives in a separate file.
 module.exports = {
+  // Don't attempt to continue if there are any errors.
   bail: true,
+  // We generate sourcemaps in production. This is slow but gives good results.
+  // You can exclude the *.map files from the build during deployment.
   devtool: 'source-map',
+  // In production, we only want to load the polyfills and the app code.
   entry: [
     require.resolve('./polyfills'),
     path.join(paths.appSrc, 'index')
   ],
   output: {
+    // The build folder.
     path: paths.appBuild,
+    // Generated JS file names (with nested folders).
+    // There will be one main bundle, and one file per asynchronous chunk.
+    // We don't currently advertise code splitting but Webpack supports it.
     filename: 'static/js/[name].[chunkhash:8].js',
     chunkFilename: 'static/js/[name].[chunkhash:8].chunk.js',
+    // We inferred the "public path" (such as / or /my-project) from homepage.
     publicPath: publicPath
   },
   resolve: {
+    // These are the reasonable defaults supported by the Node ecosystem.
     extensions: ['.js', '.json', ''],
     alias: {
       // This `alias` section can be safely removed after ejection.
@@ -49,11 +67,16 @@ module.exports = {
       'babel-runtime/regenerator': require.resolve('babel-runtime/regenerator')
     }
   },
+  // Resolve loaders (webpack plugins for CSS, images, transpilation) from the
+  // directory of `react-scripts` itself rather than the project directory.
+  // You can remove this after ejecting.
   resolveLoader: {
     root: paths.ownNodeModules,
     moduleTemplates: ['*-loader']
   },
   module: {
+    // First, run the linter.
+    // It's important to do this before Babel processes the JS.
     preLoaders: [
       {
         test: /\.js$/,
@@ -62,26 +85,44 @@ module.exports = {
       }
     ],
     loaders: [
+      // Process JS with Babel.
       {
         test: /\.js$/,
         include: paths.appSrc,
         loader: 'babel',
         query: require('./babel.prod')
       },
+      // The notation here is somewhat confusing.
+      // "postcss" loader applies autoprefixer to our CSS.
+      // "css" loader resolves paths in CSS and adds assets as dependencies.
+      // "style" loader normally turns CSS into JS modules injecting <style>,
+      // but unlike in development configuration, we do something different.
+      // `ExtractTextPlugin` first applies the "postcss" and "css" loaders
+      // (second argument), then grabs the result CSS and puts it into a
+      // separate file in our build process. This way we actually ship
+      // a single CSS file in production instead of JS code injecting <style>
+      // tags. If you use code splitting, however, any async bundles will still
+      // use the "style" loader inside the async code so CSS from them won't be
+      // in the main CSS file.
       {
         test: /\.css$/,
         include: [paths.appSrc, paths.appNodeModules],
-        // Disable autoprefixer in css-loader itself:
+        // "?-autoprefixer" disables autoprefixer in css-loader itself:
         // https://github.com/webpack/css-loader/issues/281
         // We already have it thanks to postcss.
         loader: ExtractTextPlugin.extract('style', 'css?-autoprefixer!postcss')
+        // Note: this won't work without `new ExtractTextPlugin()` in `plugins`.
       },
       {
+        // JSON is not enabled by default in Webpack but both Node and Browserify
+        // allow it implicitly so we also enable it.
         test: /\.json$/,
         include: [paths.appSrc, paths.appNodeModules],
         loader: 'json'
       },
       {
+        // "file" loader makes sure those assets end up in the `build` folder.
+        // When you `import` an asset, you get its filename.
         test: /\.(jpg|png|gif|eot|svg|ttf|woff|woff2)(\?.*)?$/,
         include: [paths.appSrc, paths.appNodeModules],
         loader: 'file',
@@ -89,6 +130,8 @@ module.exports = {
           name: 'static/media/[name].[hash:8].[ext]'
         }
       },
+      // "url" loader works just like "file" loader but it also embeds
+      // assets smaller than specified size as data URLs to avoid requests.
       {
         test: /\.(mp4|webm)(\?.*)?$/,
         include: [paths.appSrc, paths.appNodeModules],
@@ -100,12 +143,14 @@ module.exports = {
       }
     ]
   },
+  // Point ESLint to our predefined config.
   eslint: {
     // TODO: consider separate config for production,
-    // e.g. to enable no-console and no-debugger only in prod.
+    // e.g. to enable no-console and no-debugger only in production.
     configFile: path.join(__dirname, 'eslint.js'),
     useEslintrc: false
   },
+  // We use PostCSS for autoprefixing only.
   postcss: function() {
     return [
       autoprefixer({
@@ -113,12 +158,13 @@ module.exports = {
           '>1%',
           'last 4 versions',
           'Firefox ESR',
-          'not ie < 9',
+          'not ie < 9', // React doesn't support IE8 anyway
         ]
       }),
     ];
   },
   plugins: [
+    // Generates an `index.html` file with the <script> injected.
     new HtmlWebpackPlugin({
       inject: true,
       template: paths.appHtml,
@@ -136,12 +182,19 @@ module.exports = {
         minifyURLs: true
       }
     }),
+    // Makes the environment available to the JS code, for example:
+    // if (process.env.NODE_ENV === 'production') { ... }. See `env.js`.
+    // It is absolutely essential that NODE_ENV was set to production here.
+    // Otherwise React will be compiled in the very slow development mode.
     new webpack.DefinePlugin({ 'process.env.NODE_ENV': '"production"' }),
+    // This helps ensure the builds are consistent if source hasn't changed:
     new webpack.optimize.OccurrenceOrderPlugin(),
+    // Try to dedupe duplicated modules, if any:
     new webpack.optimize.DedupePlugin(),
+    // Minify the code.
     new webpack.optimize.UglifyJsPlugin({
       compress: {
-        screw_ie8: true,
+        screw_ie8: true, // React doesn't support IE8
         warnings: false
       },
       mangle: {
@@ -152,6 +205,7 @@ module.exports = {
         screw_ie8: true
       }
     }),
+    // Note: this won't work without ExtractTextPlugin.extract(..) in `loaders`.
     new ExtractTextPlugin('static/css/[name].[contenthash:8].css')
   ]
 };
diff --git a/scripts/build.js b/scripts/build.js
index 36affa9e..1f3079f2 100644
--- a/scripts/build.js
+++ b/scripts/build.js
@@ -7,6 +7,7 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  */
 
+// Do this as the first thing so that any code reading it knows the right env.
 process.env.NODE_ENV = 'production';
 
 var chalk = require('chalk');
@@ -21,12 +22,16 @@ var paths = require('../config/paths');
 var recursive = require('recursive-readdir');
 var stripAnsi = require('strip-ansi');
 
+// Input: /User/dan/app/build/static/js/main.82be8.js
+// Output: /static/js/main.js
 function removeFileNameHash(fileName) {
   return fileName
     .replace(paths.appBuild, '')
     .replace(/\/?(.*)(\.\w+)(\.js|\.css)/, (match, p1, p2, p3) => p1 + p3);
 }
 
+// Input: 1024, 2048
+// Output: "(+1 KB)"
 function getDifferenceLabel(currentSize, previousSize) {
   var FIFTY_KILOBYTES = 1024 * 50;
   var difference = currentSize - previousSize;
@@ -62,6 +67,7 @@ recursive(paths.appBuild, (err, fileNames) => {
   build(previousSizeMap);
 });
 
+// Print a detailed summary of build files.
 function printFileSizes(stats, previousSizeMap) {
   var assets = stats.toJson().assets
     .filter(asset => /\.(js|css)$/.test(asset.name))
@@ -78,7 +84,6 @@ function printFileSizes(stats, previousSizeMap) {
       };
     });
   assets.sort((a, b) => b.size - a.size);
-
   var longestSizeLabelLength = Math.max.apply(null,
     assets.map(a => stripAnsi(a.sizeLabel).length)
   );
@@ -96,6 +101,7 @@ function printFileSizes(stats, previousSizeMap) {
   });
 }
 
+// Create the production build and print the deployment instructions.
 function build(previousSizeMap) {
   console.log('Creating an optimized production build...');
   webpack(config).run((err, stats) => {
diff --git a/scripts/start.js b/scripts/start.js
index dd09ab51..a5d2bd9c 100644
--- a/scripts/start.js
+++ b/scripts/start.js
@@ -19,7 +19,7 @@ var detect = require('detect-port');
 var prompt = require('./utils/prompt');
 var config = require('../config/webpack.config.dev');
 
-// Tools like Cloud9 rely on this
+// Tools like Cloud9 rely on this.
 var DEFAULT_PORT = process.env.PORT || 3000;
 var compiler;
 
@@ -37,15 +37,13 @@ if (isSmokeTest) {
   };
 }
 
+// Some custom utilities to prettify Webpack output.
+// This is a little hacky.
+// It would be easier if webpack provided a rich error object.
 var friendlySyntaxErrorLabel = 'Syntax error:';
-
 function isLikelyASyntaxError(message) {
   return message.indexOf(friendlySyntaxErrorLabel) !== -1;
 }
-
-// This is a little hacky.
-// It would be easier if webpack provided a rich error object.
-
 function formatMessage(message) {
   return message
     // Make some common errors shorter:
@@ -66,17 +64,27 @@ function formatMessage(message) {
 }
 
 function clearConsole() {
+  // This seems to work best on Windows and other systems.
+  // The intention is to clear the output so you can focus on most recent build.
   process.stdout.write('\x1bc');
 }
 
 function setupCompiler(port) {
+  // "Compiler" is a low-level interface to Webpack.
+  // It lets us listen to some events and provide our own custom messages.
   compiler = webpack(config, handleCompile);
 
+  // "invalid" event fires when you have changed a file, and Webpack is
+  // recompiling a bundle. WebpackDevServer takes care to pause serving the
+  // bundle, so if you refresh, it'll wait instead of serving the old one.
+  // "invalid" is short for "bundle invalidated", it doesn't imply any errors.
   compiler.plugin('invalid', function() {
     clearConsole();
     console.log('Compiling...');
   });
 
+  // "done" event fires when Webpack has finished recompiling the bundle.
+  // Whether or not you have warnings or errors, you will get this event.
   compiler.plugin('done', function(stats) {
     clearConsole();
     var hasErrors = stats.hasErrors();
@@ -91,10 +99,12 @@ function setupCompiler(port) {
       console.log('Note that the development build is not optimized.');
       console.log('To create a production build, use ' + chalk.cyan('npm run build') + '.');
       console.log();
-
       return;
     }
 
+    // We have switched off the default Webpack output in WebpackDevServer
+    // options so we are going to "massage" the warnings and errors and present
+    // them in a readable focused way.
     var json = stats.toJson();
     var formattedErrors = json.errors.map(message =>
       'Error in ' + formatMessage(message)
@@ -102,7 +112,6 @@ function setupCompiler(port) {
     var formattedWarnings = json.warnings.map(message =>
       'Warning in ' + formatMessage(message)
     );
-
     if (hasErrors) {
       console.log(chalk.red('Failed to compile.'));
       console.log();
@@ -119,7 +128,6 @@ function setupCompiler(port) {
       // If errors exist, ignore warnings.
       return;
     }
-
     if (hasWarnings) {
       console.log(chalk.yellow('Compiled with warnings.'));
       console.log();
@@ -127,7 +135,7 @@ function setupCompiler(port) {
         console.log(message);
         console.log();
       });
-
+      // Teach some ESLint tricks.
       console.log('You may use special comments to disable some warnings.');
       console.log('Use ' + chalk.yellow('// eslint-disable-next-line') + ' to ignore the next line.');
       console.log('Use ' + chalk.yellow('/* eslint-disable */') + ' to ignore all warnings in a file.');
@@ -156,11 +164,25 @@ function openBrowser(port) {
 }
 
 function runDevServer(port) {
+  // Launch WebpackDevServer.
   new WebpackDevServer(compiler, {
+    // When an unrecognized URL is requested (e.g. localhost:3000/todos),
+    // assume that this is a single-page app, and serve index.html.
     historyApiFallback: true,
-    hot: true, // Note: only CSS is currently hot reloaded
+    // 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.plugin` calls above.
     quiet: true,
+    // Reportedly, this avoids CPU overload on some systems.
+    // https://github.com/facebookincubator/create-react-app/issues/293
     watchOptions: {
       ignored: /node_modules/
     }
@@ -181,6 +203,8 @@ function run(port) {
   runDevServer(port);
 }
 
+// We attempt to use the default port but if it is busy, we offer the user to
+// run on a different port. `detect()` Promise resolves to the next free port.
 detect(DEFAULT_PORT).then(port => {
   if (port === DEFAULT_PORT) {
     run(port);