1 This project was bootstrapped with [Create React App](https://github.com/facebookincubator/create-react-app).

     2 

     3 Below you will find some information on how to perform common tasks.<br>

     4 You can find the most recent version of this guide [here](https://github.com/facebookincubator/create-react-app/blob/master/packages/react-scripts/template/README.md).

     5 

     6 ## Table of Contents

     7 

     8 - [Updating to New Releases](#updating-to-new-releases)

     9 - [Sending Feedback](#sending-feedback)

    10 - [Folder Structure](#folder-structure)

    11 - [Available Scripts](#available-scripts)

    12   - [npm start](#npm-start)

    13   - [npm test](#npm-test)

    14   - [npm run build](#npm-run-build)

    15   - [npm run eject](#npm-run-eject)

    16 - [Supported Browsers](#supported-browsers)

    17 - [Supported Language Features and Polyfills](#supported-language-features-and-polyfills)

    18 - [Syntax Highlighting in the Editor](#syntax-highlighting-in-the-editor)

    19 - [Displaying Lint Output in the Editor](#displaying-lint-output-in-the-editor)

    20 - [Debugging in the Editor](#debugging-in-the-editor)

    21 - [Formatting Code Automatically](#formatting-code-automatically)

    22 - [Changing the Page `<title>`](#changing-the-page-title)

    23 - [Installing a Dependency](#installing-a-dependency)

    24 - [Importing a Component](#importing-a-component)

    25 - [Code Splitting](#code-splitting)

    26 - [Adding a Stylesheet](#adding-a-stylesheet)

    27 - [Post-Processing CSS](#post-processing-css)

    28 - [Adding a CSS Preprocessor (Sass, Less etc.)](#adding-a-css-preprocessor-sass-less-etc)

    29 - [Adding Images, Fonts, and Files](#adding-images-fonts-and-files)

    30 - [Using the `public` Folder](#using-the-public-folder)

    31   - [Changing the HTML](#changing-the-html)

    32   - [Adding Assets Outside of the Module System](#adding-assets-outside-of-the-module-system)

    33   - [When to Use the `public` Folder](#when-to-use-the-public-folder)

    34 - [Using Global Variables](#using-global-variables)

    35 - [Adding Bootstrap](#adding-bootstrap)

    36   - [Using a Custom Theme](#using-a-custom-theme)

    37 - [Adding Flow](#adding-flow)

    38 - [Adding a Router](#adding-a-router)

    39 - [Adding Custom Environment Variables](#adding-custom-environment-variables)

    40   - [Referencing Environment Variables in the HTML](#referencing-environment-variables-in-the-html)

    41   - [Adding Temporary Environment Variables In Your Shell](#adding-temporary-environment-variables-in-your-shell)

    42   - [Adding Development Environment Variables In `.env`](#adding-development-environment-variables-in-env)

    43 - [Can I Use Decorators?](#can-i-use-decorators)

    44 - [Fetching Data with AJAX Requests](#fetching-data-with-ajax-requests)

    45 - [Integrating with an API Backend](#integrating-with-an-api-backend)

    46   - [Node](#node)

    47   - [Ruby on Rails](#ruby-on-rails)

    48 - [Proxying API Requests in Development](#proxying-api-requests-in-development)

    49   - ["Invalid Host Header" Errors After Configuring Proxy](#invalid-host-header-errors-after-configuring-proxy)

    50   - [Configuring the Proxy Manually](#configuring-the-proxy-manually)

    51   - [Configuring a WebSocket Proxy](#configuring-a-websocket-proxy)

    52 - [Using HTTPS in Development](#using-https-in-development)

    53 - [Generating Dynamic `<meta>` Tags on the Server](#generating-dynamic-meta-tags-on-the-server)

    54 - [Pre-Rendering into Static HTML Files](#pre-rendering-into-static-html-files)

    55 - [Injecting Data from the Server into the Page](#injecting-data-from-the-server-into-the-page)

    56 - [Running Tests](#running-tests)

    57   - [Filename Conventions](#filename-conventions)

    58   - [Command Line Interface](#command-line-interface)

    59   - [Version Control Integration](#version-control-integration)

    60   - [Writing Tests](#writing-tests)

    61   - [Testing Components](#testing-components)

    62   - [Using Third Party Assertion Libraries](#using-third-party-assertion-libraries)

    63   - [Initializing Test Environment](#initializing-test-environment)

    64   - [Focusing and Excluding Tests](#focusing-and-excluding-tests)

    65   - [Coverage Reporting](#coverage-reporting)

    66   - [Continuous Integration](#continuous-integration)

    67   - [Disabling jsdom](#disabling-jsdom)

    68   - [Snapshot Testing](#snapshot-testing)

    69   - [Editor Integration](#editor-integration)

    70 - [Debugging Tests](#debugging-tests)

    71   - [Debugging Tests in Chrome](#debugging-tests-in-chrome)

    72   - [Debugging Tests in Visual Studio Code](#debugging-tests-in-visual-studio-code)

    73 - [Developing Components in Isolation](#developing-components-in-isolation)

    74   - [Getting Started with Storybook](#getting-started-with-storybook)

    75   - [Getting Started with Styleguidist](#getting-started-with-styleguidist)

    76 - [Publishing Components to npm](#publishing-components-to-npm)

    77 - [Making a Progressive Web App](#making-a-progressive-web-app)

    78   - [Opting Out of Caching](#opting-out-of-caching)

    79   - [Offline-First Considerations](#offline-first-considerations)

    80   - [Progressive Web App Metadata](#progressive-web-app-metadata)

    81 - [Analyzing the Bundle Size](#analyzing-the-bundle-size)

    82 - [Deployment](#deployment)

    83   - [Static Server](#static-server)

    84   - [Other Solutions](#other-solutions)

    85   - [Serving Apps with Client-Side Routing](#serving-apps-with-client-side-routing)

    86   - [Building for Relative Paths](#building-for-relative-paths)

    87   - [Azure](#azure)

    88   - [Firebase](#firebase)

    89   - [GitHub Pages](#github-pages)

    90   - [Heroku](#heroku)

    91   - [Netlify](#netlify)

    92   - [Now](#now)

    93   - [S3 and CloudFront](#s3-and-cloudfront)

    94   - [Surge](#surge)

    95 - [Advanced Configuration](#advanced-configuration)

    96 - [Troubleshooting](#troubleshooting)

    97   - [`npm start` doesn’t detect changes](#npm-start-doesnt-detect-changes)

    98   - [`npm test` hangs on macOS Sierra](#npm-test-hangs-on-macos-sierra)

    99   - [`npm run build` exits too early](#npm-run-build-exits-too-early)

   100   - [`npm run build` fails on Heroku](#npm-run-build-fails-on-heroku)

   101   - [`npm run build` fails to minify](#npm-run-build-fails-to-minify)

   102   - [Moment.js locales are missing](#momentjs-locales-are-missing)

   103 - [Alternatives to Ejecting](#alternatives-to-ejecting)

   104 - [Something Missing?](#something-missing)

   105 

   106 ## Updating to New Releases

   107 

   108 Create React App is divided into two packages:

   109 

   110 * `create-react-app` is a global command-line utility that you use to create new projects.

   111 * `react-scripts` is a development dependency in the generated projects (including this one).

   112 

   113 You almost never need to update `create-react-app` itself: it delegates all the setup to `react-scripts`.

   114 

   115 When you run `create-react-app`, it always creates the project with the latest version of `react-scripts` so you’ll get all the new features and improvements in newly created apps automatically.

   116 

   117 To update an existing project to a new version of `react-scripts`, [open the changelog](https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md), find the version you’re currently on (check `package.json` in this folder if you’re not sure), and apply the migration instructions for the newer versions.

   118 

   119 In most cases bumping the `react-scripts` version in `package.json` and running `npm install` in this folder should be enough, but it’s good to consult the [changelog](https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md) for potential breaking changes.

   120 

   121 We commit to keeping the breaking changes minimal so you can upgrade `react-scripts` painlessly.

   122 

   123 ## Sending Feedback

   124 

   125 We are always open to [your feedback](https://github.com/facebookincubator/create-react-app/issues).

   126 

   127 ## Folder Structure

   128 

   129 After creation, your project should look like this:

   130 

   131 ```

   132 my-app/

   133   README.md

   134   node_modules/

   135   package.json

   136   public/

   137     index.html

   138     favicon.ico

   139   src/

   140     App.css

   141     App.js

   142     App.test.js

   143     index.css

   144     index.js

   145     logo.svg

   146 ```

   147 

   148 For the project to build, **these files must exist with exact filenames**:

   149 

   150 * `public/index.html` is the page template;

   151 * `src/index.js` is the JavaScript entry point.

   152 

   153 You can delete or rename the other files.

   154 

   155 You may create subdirectories inside `src`. For faster rebuilds, only files inside `src` are processed by Webpack.<br>

   156 You need to **put any JS and CSS files inside `src`**, otherwise Webpack won’t see them.

   157 

   158 Only files inside `public` can be used from `public/index.html`.<br>

   159 Read instructions below for using assets from JavaScript and HTML.

   160 

   161 You can, however, create more top-level directories.<br>

   162 They will not be included in the production build so you can use them for things like documentation.

   163 

   164 ## Available Scripts

   165 

   166 In the project directory, you can run:

   167 

   168 ### `npm start`

   169 

   170 Runs the app in the development mode.<br>

   171 Open [http://localhost:3000](http://localhost:3000) to view it in the browser.

   172 

   173 The page will reload if you make edits.<br>

   174 You will also see any lint errors in the console.

   175 

   176 ### `npm test`

   177 

   178 Launches the test runner in the interactive watch mode.<br>

   179 See the section about [running tests](#running-tests) for more information.

   180 

   181 ### `npm run build`

   182 

   183 Builds the app for production to the `build` folder.<br>

   184 It correctly bundles React in production mode and optimizes the build for the best performance.

   185 

   186 The build is minified and the filenames include the hashes.<br>

   187 Your app is ready to be deployed!

   188 

   189 See the section about [deployment](#deployment) for more information.

   190 

   191 ### `npm run eject`

   192 

   193 **Note: this is a one-way operation. Once you `eject`, you can’t go back!**

   194 

   195 If you aren’t satisfied with the build tool and configuration choices, you can `eject` at any time. This command will remove the single build dependency from your project.

   196 

   197 Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except `eject` will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.

   198 

   199 You don’t have to ever use `eject`. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.

   200 

   201 ## Supported Browsers

   202 

   203 By default, the generated project uses the latest version of React.

   204 

   205 You can refer [to the React documentation](https://reactjs.org/docs/react-dom.html#browser-support) for more information about supported browsers.

   206 

   207 ## Supported Language Features and Polyfills

   208 

   209 This project supports a superset of the latest JavaScript standard.<br>

   210 In addition to [ES6](https://github.com/lukehoban/es6features) syntax features, it also supports:

   211 

   212 * [Exponentiation Operator](https://github.com/rwaldron/exponentiation-operator) (ES2016).

   213 * [Async/await](https://github.com/tc39/ecmascript-asyncawait) (ES2017).

   214 * [Object Rest/Spread Properties](https://github.com/sebmarkbage/ecmascript-rest-spread) (stage 3 proposal).

   215 * [Dynamic import()](https://github.com/tc39/proposal-dynamic-import) (stage 3 proposal)

   216 * [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (part of stage 3 proposal).

   217 * [JSX](https://facebook.github.io/react/docs/introducing-jsx.html) and [Flow](https://flowtype.org/) syntax.

   218 

   219 Learn more about [different proposal stages](https://babeljs.io/docs/plugins/#presets-stage-x-experimental-presets-).

   220 

   221 While we recommend using experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide [codemods](https://medium.com/@cpojer/effective-javascript-codemods-5a6686bb46fb) if any of these proposals change in the future.

   222 

   223 Note that **the project only includes a few ES6 [polyfills](https://en.wikipedia.org/wiki/Polyfill)**:

   224 

   225 * [`Object.assign()`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) via [`object-assign`](https://github.com/sindresorhus/object-assign).

   226 * [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) via [`promise`](https://github.com/then/promise).

   227 * [`fetch()`](https://developer.mozilla.org/en/docs/Web/API/Fetch_API) via [`whatwg-fetch`](https://github.com/github/fetch).

   228 

   229 If you use any other ES6+ features that need **runtime support** (such as `Array.from()` or `Symbol`), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.

   230 

   231 Also note that using some newer syntax features like `for...of` or `[...nonArrayValue]` causes Babel to emit code that depends on ES6 runtime features and might not work without a polyfill. When in doubt, use [Babel REPL](https://babeljs.io/repl/) to see what any specific syntax compiles down to.

   232 

   233 ## Syntax Highlighting in the Editor

   234 

   235 To configure the syntax highlighting in your favorite text editor, head to the [relevant Babel documentation page](https://babeljs.io/docs/editors) and follow the instructions. Some of the most popular editors are covered.

   236 

   237 ## Displaying Lint Output in the Editor

   238 

   239 >Note: this feature is available with `react-scripts@0.2.0` and higher.<br>

   240 >It also only works with npm 3 or higher.

   241 

   242 Some editors, including Sublime Text, Atom, and Visual Studio Code, provide plugins for ESLint.

   243 

   244 They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.

   245 

   246 You would need to install an ESLint plugin for your editor first. Then, add a file called `.eslintrc` to the project root:

   247 

   248 ```js

   249 {

   250   "extends": "react-app"

   251 }

   252 ```

   253 

   254 Now your editor should report the linting warnings.

   255 

   256 Note that even if you edit your `.eslintrc` file further, these changes will **only affect the editor integration**. They won’t affect the terminal and in-browser lint output. This is because Create React App intentionally provides a minimal set of rules that find common mistakes.

   257 

   258 If you want to enforce a coding style for your project, consider using [Prettier](https://github.com/jlongster/prettier) instead of ESLint style rules.

   259 

   260 ## Debugging in the Editor

   261 

   262 **This feature is currently only supported by [Visual Studio Code](https://code.visualstudio.com) and [WebStorm](https://www.jetbrains.com/webstorm/).**

   263 

   264 Visual Studio Code and WebStorm support debugging out of the box with Create React App. This enables you as a developer to write and debug your React code without leaving the editor, and most importantly it enables you to have a continuous development workflow, where context switching is minimal, as you don’t have to switch between tools.

   265 

   266 ### Visual Studio Code

   267 

   268 You would need to have the latest version of [VS Code](https://code.visualstudio.com) and VS Code [Chrome Debugger Extension](https://marketplace.visualstudio.com/items?itemName=msjsdiag.debugger-for-chrome) installed.

   269 

   270 Then add the block below to your `launch.json` file and put it inside the `.vscode` folder in your app’s root directory.

   271 

   272 ```json

   273 {

   274   "version": "0.2.0",

   275   "configurations": [{

   276     "name": "Chrome",

   277     "type": "chrome",

   278     "request": "launch",

   279     "url": "http://localhost:3000",

   280     "webRoot": "${workspaceRoot}/src",

   281     "sourceMapPathOverrides": {

   282       "webpack:///src/*": "${webRoot}/*"

   283     }

   284   }]

   285 }

   286 ```

   287 >Note: the URL may be different if you've made adjustments via the [HOST or PORT environment variables](#advanced-configuration).

   288 

   289 Start your app by running `npm start`, and start debugging in VS Code by pressing `F5` or by clicking the green debug icon. You can now write code, set breakpoints, make changes to the code, and debug your newly modified code—all from your editor.

   290 

   291 Having problems with VS Code Debugging? Please see their [troubleshooting guide](https://github.com/Microsoft/vscode-chrome-debug/blob/master/README.md#troubleshooting).

   292 

   293 ### WebStorm

   294 

   295 You would need to have [WebStorm](https://www.jetbrains.com/webstorm/) and [JetBrains IDE Support](https://chrome.google.com/webstore/detail/jetbrains-ide-support/hmhgeddbohgjknpmjagkdomcpobmllji) Chrome extension installed.

   296 

   297 In the WebStorm menu `Run` select `Edit Configurations...`. Then click `+` and select `JavaScript Debug`. Paste `http://localhost:3000` into the URL field and save the configuration.

   298 

   299 >Note: the URL may be different if you've made adjustments via the [HOST or PORT environment variables](#advanced-configuration).

   300 

   301 Start your app by running `npm start`, then press `^D` on macOS or `F9` on Windows and Linux or click the green debug icon to start debugging in WebStorm.

   302 

   303 The same way you can debug your application in IntelliJ IDEA Ultimate, PhpStorm, PyCharm Pro, and RubyMine. 

   304 

   305 ## Formatting Code Automatically

   306 

   307 Prettier is an opinionated code formatter with support for JavaScript, CSS and JSON. With Prettier you can format the code you write automatically to ensure a code style within your project. See the [Prettier's GitHub page](https://github.com/prettier/prettier) for more information, and look at this [page to see it in action](https://prettier.github.io/prettier/).

   308 

   309 To format our code whenever we make a commit in git, we need to install the following dependencies:

   310 

   311 ```sh

   312 npm install --save husky lint-staged prettier

   313 ```

   314 

   315 Alternatively you may use `yarn`:

   316 

   317 ```sh

   318 yarn add husky lint-staged prettier

   319 ```

   320 

   321 * `husky` makes it easy to use githooks as if they are npm scripts.

   322 * `lint-staged` allows us to run scripts on staged files in git. See this [blog post about lint-staged to learn more about it](https://medium.com/@okonetchnikov/make-linting-great-again-f3890e1ad6b8).

   323 * `prettier` is the JavaScript formatter we will run before commits.

   324 

   325 Now we can make sure every file is formatted correctly by adding a few lines to the `package.json` in the project root.

   326 

   327 Add the following line to `scripts` section:

   328 

   329 ```diff

   330   "scripts": {

   331 +   "precommit": "lint-staged",

   332     "start": "react-scripts start",

   333     "build": "react-scripts build",

   334 ```

   335 

   336 Next we add a 'lint-staged' field to the `package.json`, for example:

   337 

   338 ```diff

   339   "dependencies": {

   340     // ...

   341   },

   342 + "lint-staged": {

   343 +   "src/**/*.{js,jsx,json,css}": [

   344 +     "prettier --single-quote --write",

   345 +     "git add"

   346 +   ]

   347 + },

   348   "scripts": {

   349 ```

   350 

   351 Now, whenever you make a commit, Prettier will format the changed files automatically. You can also run `./node_modules/.bin/prettier --single-quote --write "src/**/*.{js,jsx,json,css}"` to format your entire project for the first time.

   352 

   353 Next you might want to integrate Prettier in your favorite editor. Read the section on [Editor Integration](https://prettier.io/docs/en/editors.html) on the Prettier GitHub page.

   354 

   355 ## Changing the Page `<title>`

   356 

   357 You can find the source HTML file in the `public` folder of the generated project. You may edit the `<title>` tag in it to change the title from “React App” to anything else.

   358 

   359 Note that normally you wouldn’t edit files in the `public` folder very often. For example, [adding a stylesheet](#adding-a-stylesheet) is done without touching the HTML.

   360 

   361 If you need to dynamically update the page title based on the content, you can use the browser [`document.title`](https://developer.mozilla.org/en-US/docs/Web/API/Document/title) API. For more complex scenarios when you want to change the title from React components, you can use [React Helmet](https://github.com/nfl/react-helmet), a third party library.

   362 

   363 If you use a custom server for your app in production and want to modify the title before it gets sent to the browser, you can follow advice in [this section](#generating-dynamic-meta-tags-on-the-server). Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is covered [here](#pre-rendering-into-static-html-files).

   364 

   365 ## Installing a Dependency

   366 

   367 The generated project includes React and ReactDOM as dependencies. It also includes a set of scripts used by Create React App as a development dependency. You may install other dependencies (for example, React Router) with `npm`:

   368 

   369 ```sh

   370 npm install --save react-router

   371 ```

   372 

   373 Alternatively you may use `yarn`:

   374 

   375 ```sh

   376 yarn add react-router

   377 ```

   378 

   379 This works for any library, not just `react-router`.

   380 

   381 ## Importing a Component

   382 

   383 This project setup supports ES6 modules thanks to Babel.<br>

   384 While you can still use `require()` and `module.exports`, we encourage you to use [`import` and `export`](http://exploringjs.com/es6/ch_modules.html) instead.

   385 

   386 For example:

   387 

   388 ### `Button.js`

   389 

   390 ```js

   391 import React, { Component } from 'react';

   392 

   393 class Button extends Component {

   394   render() {

   395     // ...

   396   }

   397 }

   398 

   399 export default Button; // Don’t forget to use export default!

   400 ```

   401 

   402 ### `DangerButton.js`

   403 

   404 

   405 ```js

   406 import React, { Component } from 'react';

   407 import Button from './Button'; // Import a component from another file

   408 

   409 class DangerButton extends Component {

   410   render() {

   411     return <Button color="red" />;

   412   }

   413 }

   414 

   415 export default DangerButton;

   416 ```

   417 

   418 Be aware of the [difference between default and named exports](http://stackoverflow.com/questions/36795819/react-native-es-6-when-should-i-use-curly-braces-for-import/36796281#36796281). It is a common source of mistakes.

   419 

   420 We suggest that you stick to using default imports and exports when a module only exports a single thing (for example, a component). That’s what you get when you use `export default Button` and `import Button from './Button'`.

   421 

   422 Named exports are useful for utility modules that export several functions. A module may have at most one default export and as many named exports as you like.

   423 

   424 Learn more about ES6 modules:

   425 

   426 * [When to use the curly braces?](http://stackoverflow.com/questions/36795819/react-native-es-6-when-should-i-use-curly-braces-for-import/36796281#36796281)

   427 * [Exploring ES6: Modules](http://exploringjs.com/es6/ch_modules.html)

   428 * [Understanding ES6: Modules](https://leanpub.com/understandinges6/read#leanpub-auto-encapsulating-code-with-modules)

   429 

   430 ## Code Splitting

   431 

   432 Instead of downloading the entire app before users can use it, code splitting allows you to split your code into small chunks which you can then load on demand.

   433 

   434 This project setup supports code splitting via [dynamic `import()`](http://2ality.com/2017/01/import-operator.html#loading-code-on-demand). Its [proposal](https://github.com/tc39/proposal-dynamic-import) is in stage 3. The `import()` function-like form takes the module name as an argument and returns a [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) which always resolves to the namespace object of the module.

   435 

   436 Here is an example:

   437 

   438 ### `moduleA.js`

   439 

   440 ```js

   441 const moduleA = 'Hello';

   442 

   443 export { moduleA };

   444 ```

   445 ### `App.js`

   446 

   447 ```js

   448 import React, { Component } from 'react';

   449 

   450 class App extends Component {

   451   handleClick = () => {

   452     import('./moduleA')

   453       .then(({ moduleA }) => {

   454         // Use moduleA

   455       })

   456       .catch(err => {

   457         // Handle failure

   458       });

   459   };

   460 

   461   render() {

   462     return (

   463       <div>

   464         <button onClick={this.handleClick}>Load</button>

   465       </div>

   466     );

   467   }

   468 }

   469 

   470 export default App;

   471 ```

   472 

   473 This will make `moduleA.js` and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.

   474 

   475 You can also use it with `async` / `await` syntax if you prefer it.

   476 

   477 ### With React Router

   478 

   479 If you are using React Router check out [this tutorial](http://serverless-stack.com/chapters/code-splitting-in-create-react-app.html) on how to use code splitting with it. You can find the companion GitHub repository [here](https://github.com/AnomalyInnovations/serverless-stack-demo-client/tree/code-splitting-in-create-react-app).

   480 

   481 Also check out the [Code Splitting](https://reactjs.org/docs/code-splitting.html) section in React documentation.

   482 

   483 ## Adding a Stylesheet

   484 

   485 This project setup uses [Webpack](https://webpack.js.org/) for handling all assets. Webpack offers a custom way of “extending” the concept of `import` beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need to **import the CSS from the JavaScript file**:

   486 

   487 ### `Button.css`

   488 

   489 ```css

   490 .Button {

   491   padding: 20px;

   492 }

   493 ```

   494 

   495 ### `Button.js`

   496 

   497 ```js

   498 import React, { Component } from 'react';

   499 import './Button.css'; // Tell Webpack that Button.js uses these styles

   500 

   501 class Button extends Component {

   502   render() {

   503     // You can use them as regular CSS styles

   504     return <div className="Button" />;

   505   }

   506 }

   507 ```

   508 

   509 **This is not required for React** but many people find this feature convenient. You can read about the benefits of this approach [here](https://medium.com/seek-blog/block-element-modifying-your-javascript-components-d7f99fcab52b). However you should be aware that this makes your code less portable to other build tools and environments than Webpack.

   510 

   511 In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified `.css` file in the build output.

   512 

   513 If you are concerned about using Webpack-specific semantics, you can put all your CSS right into `src/index.css`. It would still be imported from `src/index.js`, but you could always remove that import if you later migrate to a different build tool.

   514 

   515 ## Post-Processing CSS

   516 

   517 This project setup minifies your CSS and adds vendor prefixes to it automatically through [Autoprefixer](https://github.com/postcss/autoprefixer) so you don’t need to worry about it.

   518 

   519 For example, this:

   520 

   521 ```css

   522 .App {

   523   display: flex;

   524   flex-direction: row;

   525   align-items: center;

   526 }

   527 ```

   528 

   529 becomes this:

   530 

   531 ```css

   532 .App {

   533   display: -webkit-box;

   534   display: -ms-flexbox;

   535   display: flex;

   536   -webkit-box-orient: horizontal;

   537   -webkit-box-direction: normal;

   538       -ms-flex-direction: row;

   539           flex-direction: row;

   540   -webkit-box-align: center;

   541       -ms-flex-align: center;

   542           align-items: center;

   543 }

   544 ```

   545 

   546 If you need to disable autoprefixing for some reason, [follow this section](https://github.com/postcss/autoprefixer#disabling).

   547 

   548 ## Adding a CSS Preprocessor (Sass, Less etc.)

   549 

   550 Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a `.Button` CSS class in `<AcceptButton>` and `<RejectButton>` components, we recommend creating a `<Button>` component with its own `.Button` styles, that both `<AcceptButton>` and `<RejectButton>` can render (but [not inherit](https://facebook.github.io/react/docs/composition-vs-inheritance.html)).

   551 

   552 Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.

   553 

   554 First, let’s install the command-line interface for Sass:

   555 

   556 ```sh

   557 npm install --save node-sass-chokidar

   558 ```

   559 

   560 Alternatively you may use `yarn`:

   561 

   562 ```sh

   563 yarn add node-sass-chokidar

   564 ```

   565 

   566 Then in `package.json`, add the following lines to `scripts`:

   567 

   568 ```diff

   569    "scripts": {

   570 +    "build-css": "node-sass-chokidar src/ -o src/",

   571 +    "watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",

   572      "start": "react-scripts start",

   573      "build": "react-scripts build",

   574      "test": "react-scripts test --env=jsdom",

   575 ```

   576 

   577 >Note: To use a different preprocessor, replace `build-css` and `watch-css` commands according to your preprocessor’s documentation.

   578 

   579 Now you can rename `src/App.css` to `src/App.scss` and run `npm run watch-css`. The watcher will find every Sass file in `src` subdirectories, and create a corresponding CSS file next to it, in our case overwriting `src/App.css`. Since `src/App.js` still imports `src/App.css`, the styles become a part of your application. You can now edit `src/App.scss`, and `src/App.css` will be regenerated.

   580 

   581 To share variables between Sass files, you can use Sass imports. For example, `src/App.scss` and other component style files could include `@import "./shared.scss";` with variable definitions.

   582 

   583 To enable importing files without using relative paths, you can add the  `--include-path` option to the command in `package.json`.

   584 

   585 ```

   586 "build-css": "node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/",

   587 "watch-css": "npm run build-css && node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/ --watch --recursive",

   588 ```

   589 

   590 This will allow you to do imports like

   591 

   592 ```scss

   593 @import 'styles/_colors.scss'; // assuming a styles directory under src/

   594 @import 'nprogress/nprogress'; // importing a css file from the nprogress node module

   595 ```

   596 

   597 At this point you might want to remove all CSS files from the source control, and add `src/**/*.css` to your `.gitignore` file. It is generally a good practice to keep the build products outside of the source control.

   598 

   599 As a final step, you may find it convenient to run `watch-css` automatically with `npm start`, and run `build-css` as a part of `npm run build`. You can use the `&&` operator to execute two scripts sequentially. However, there is no cross-platform way to run two scripts in parallel, so we will install a package for this:

   600 

   601 ```sh

   602 npm install --save npm-run-all

   603 ```

   604 

   605 Alternatively you may use `yarn`:

   606 

   607 ```sh

   608 yarn add npm-run-all

   609 ```

   610 

   611 Then we can change `start` and `build` scripts to include the CSS preprocessor commands:

   612 

   613 ```diff

   614    "scripts": {

   615      "build-css": "node-sass-chokidar src/ -o src/",

   616      "watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",

   617 -    "start": "react-scripts start",

   618 -    "build": "react-scripts build",

   619 +    "start-js": "react-scripts start",

   620 +    "start": "npm-run-all -p watch-css start-js",

   621 +    "build-js": "react-scripts build",

   622 +    "build": "npm-run-all build-css build-js",

   623      "test": "react-scripts test --env=jsdom",

   624      "eject": "react-scripts eject"

   625    }

   626 ```

   627 

   628 Now running `npm start` and `npm run build` also builds Sass files.

   629 

   630 **Why `node-sass-chokidar`?**

   631 

   632 `node-sass` has been reported as having the following issues:

   633 

   634 - `node-sass --watch` has been reported to have *performance issues* in certain conditions when used in a virtual machine or with docker.

   635 

   636 - Infinite styles compiling [#1939](https://github.com/facebookincubator/create-react-app/issues/1939)

   637 

   638 - `node-sass` has been reported as having issues with detecting new files in a directory [#1891](https://github.com/sass/node-sass/issues/1891)

   639 

   640  `node-sass-chokidar` is used here as it addresses these issues.

   641 

   642 ## Adding Images, Fonts, and Files

   643 

   644 With Webpack, using static assets like images and fonts works similarly to CSS.

   645 

   646 You can **`import` a file right in a JavaScript module**. This tells Webpack to include that file in the bundle. Unlike CSS imports, importing a file gives you a string value. This value is the final path you can reference in your code, e.g. as the `src` attribute of an image or the `href` of a link to a PDF.

   647 

   648 To reduce the number of requests to the server, importing images that are less than 10,000 bytes returns a [data URI](https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URIs) instead of a path. This applies to the following file extensions: bmp, gif, jpg, jpeg, and png. SVG files are excluded due to [#1153](https://github.com/facebookincubator/create-react-app/issues/1153).

   649 

   650 Here is an example:

   651 

   652 ```js

   653 import React from 'react';

   654 import logo from './logo.png'; // Tell Webpack this JS file uses this image

   655 

   656 console.log(logo); // /logo.84287d09.png

   657 

   658 function Header() {

   659   // Import result is the URL of your image

   660   return <img src={logo} alt="Logo" />;

   661 }

   662 

   663 export default Header;

   664 ```

   665 

   666 This ensures that when the project is built, Webpack will correctly move the images into the build folder, and provide us with correct paths.

   667 

   668 This works in CSS too:

   669 

   670 ```css

   671 .Logo {

   672   background-image: url(./logo.png);

   673 }

   674 ```

   675 

   676 Webpack finds all relative module references in CSS (they start with `./`) and replaces them with the final paths from the compiled bundle. If you make a typo or accidentally delete an important file, you will see a compilation error, just like when you import a non-existent JavaScript module. The final filenames in the compiled bundle are generated by Webpack from content hashes. If the file content changes in the future, Webpack will give it a different name in production so you don’t need to worry about long-term caching of assets.

   677 

   678 Please be advised that this is also a custom feature of Webpack.

   679 

   680 **It is not required for React** but many people enjoy it (and React Native uses a similar mechanism for images).<br>

   681 An alternative way of handling static assets is described in the next section.

   682 

   683 ## Using the `public` Folder

   684 

   685 >Note: this feature is available with `react-scripts@0.5.0` and higher.

   686 

   687 ### Changing the HTML

   688 

   689 The `public` folder contains the HTML file so you can tweak it, for example, to [set the page title](#changing-the-page-title).

   690 The `<script>` tag with the compiled code will be added to it automatically during the build process.

   691 

   692 ### Adding Assets Outside of the Module System

   693 

   694 You can also add other assets to the `public` folder.

   695 

   696 Note that we normally encourage you to `import` assets in JavaScript files instead.

   697 For example, see the sections on [adding a stylesheet](#adding-a-stylesheet) and [adding images and fonts](#adding-images-fonts-and-files).

   698 This mechanism provides a number of benefits:

   699 

   700 * Scripts and stylesheets get minified and bundled together to avoid extra network requests.

   701 * Missing files cause compilation errors instead of 404 errors for your users.

   702 * Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.

   703 

   704 However there is an **escape hatch** that you can use to add an asset outside of the module system.

   705 

   706 If you put a file into the `public` folder, it will **not** be processed by Webpack. Instead it will be copied into the build folder untouched.   To reference assets in the `public` folder, you need to use a special variable called `PUBLIC_URL`.

   707 

   708 Inside `index.html`, you can use it like this:

   709 

   710 ```html

   711 <link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">

   712 ```

   713 

   714 Only files inside the `public` folder will be accessible by `%PUBLIC_URL%` prefix. If you need to use a file from `src` or `node_modules`, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.

   715 

   716 When you run `npm run build`, Create React App will substitute `%PUBLIC_URL%` with a correct absolute path so your project works even if you use client-side routing or host it at a non-root URL.

   717 

   718 In JavaScript code, you can use `process.env.PUBLIC_URL` for similar purposes:

   719 

   720 ```js

   721 render() {

   722   // Note: this is an escape hatch and should be used sparingly!

   723   // Normally we recommend using `import` for getting asset URLs

   724   // as described in “Adding Images and Fonts” above this section.

   725   return <img src={process.env.PUBLIC_URL + '/img/logo.png'} />;

   726 }

   727 ```

   728 

   729 Keep in mind the downsides of this approach:

   730 

   731 * None of the files in `public` folder get post-processed or minified.

   732 * Missing files will not be called at compilation time, and will cause 404 errors for your users.

   733 * Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.

   734 

   735 ### When to Use the `public` Folder

   736 

   737 Normally we recommend importing [stylesheets](#adding-a-stylesheet), [images, and fonts](#adding-images-fonts-and-files) from JavaScript.

   738 The `public` folder is useful as a workaround for a number of less common cases:

   739 

   740 * You need a file with a specific name in the build output, such as [`manifest.webmanifest`](https://developer.mozilla.org/en-US/docs/Web/Manifest).

   741 * You have thousands of images and need to dynamically reference their paths.

   742 * You want to include a small script like [`pace.js`](http://github.hubspot.com/pace/docs/welcome/) outside of the bundled code.

   743 * Some library may be incompatible with Webpack and you have no other option but to include it as a `<script>` tag.

   744 

   745 Note that if you add a `<script>` that declares global variables, you also need to read the next section on using them.

   746 

   747 ## Using Global Variables

   748 

   749 When you include a script in the HTML file that defines global variables and try to use one of these variables in the code, the linter will complain because it cannot see the definition of the variable.

   750 

   751 You can avoid this by reading the global variable explicitly from the `window` object, for example:

   752 

   753 ```js

   754 const $ = window.$;

   755 ```

   756 

   757 This makes it obvious you are using a global variable intentionally rather than because of a typo.

   758 

   759 Alternatively, you can force the linter to ignore any line by adding `// eslint-disable-line` after it.

   760 

   761 ## Adding Bootstrap

   762 

   763 You don’t have to use [React Bootstrap](https://react-bootstrap.github.io) together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:

   764 

   765 Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:

   766 

   767 ```sh

   768 npm install --save react-bootstrap bootstrap@3

   769 ```

   770 

   771 Alternatively you may use `yarn`:

   772 

   773 ```sh

   774 yarn add react-bootstrap bootstrap@3

   775 ```

   776 

   777 Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your ```src/index.js``` file:

   778 

   779 ```js

   780 import 'bootstrap/dist/css/bootstrap.css';

   781 import 'bootstrap/dist/css/bootstrap-theme.css';

   782 // Put any other imports below so that CSS from your

   783 // components takes precedence over default styles.

   784 ```

   785 

   786 Import required React Bootstrap components within ```src/App.js``` file or your custom component files:

   787 

   788 ```js

   789 import { Navbar, Jumbotron, Button } from 'react-bootstrap';

   790 ```

   791 

   792 Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example [`App.js`](https://gist.githubusercontent.com/gaearon/85d8c067f6af1e56277c82d19fd4da7b/raw/6158dd991b67284e9fc8d70b9d973efe87659d72/App.js) redone using React Bootstrap.

   793 

   794 ### Using a Custom Theme

   795 

   796 Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br>

   797 We suggest the following approach:

   798 

   799 * Create a new package that depends on the package you wish to customize, e.g. Bootstrap.

   800 * Add the necessary build steps to tweak the theme, and publish your package on npm.

   801 * Install your own theme npm package as a dependency of your app.

   802 

   803 Here is an example of adding a [customized Bootstrap](https://medium.com/@tacomanator/customizing-create-react-app-aa9ffb88165) that follows these steps.

   804 

   805 ## Adding Flow

   806 

   807 Flow is a static type checker that helps you write code with fewer bugs. Check out this [introduction to using static types in JavaScript](https://medium.com/@preethikasireddy/why-use-static-types-in-javascript-part-1-8382da1e0adb) if you are new to this concept.

   808 

   809 Recent versions of [Flow](http://flowtype.org/) work with Create React App projects out of the box.

   810 

   811 To add Flow to a Create React App project, follow these steps:

   812 

   813 1. Run `npm install --save flow-bin` (or `yarn add flow-bin`).

   814 2. Add `"flow": "flow"` to the `scripts` section of your `package.json`.

   815 3. Run `npm run flow init` (or `yarn flow init`) to create a [`.flowconfig` file](https://flowtype.org/docs/advanced-configuration.html) in the root directory.

   816 4. Add `// @flow` to any files you want to type check (for example, to `src/App.js`).

   817 

   818 Now you can run `npm run flow` (or `yarn flow`) to check the files for type errors.

   819 You can optionally use an IDE like [Nuclide](https://nuclide.io/docs/languages/flow/) for a better integrated experience.

   820 In the future we plan to integrate it into Create React App even more closely.

   821 

   822 To learn more about Flow, check out [its documentation](https://flowtype.org/).

   823 

   824 ## Adding a Router

   825 

   826 Create React App doesn't prescribe a specific routing solution, but [React Router](https://reacttraining.com/react-router/) is the most popular one.

   827 

   828 To add it, run:

   829 

   830 ```sh

   831 npm install --save react-router-dom

   832 ```

   833 

   834 Alternatively you may use `yarn`:

   835 

   836 ```sh

   837 yarn add react-router-dom

   838 ```

   839 

   840 To try it, delete all the code in `src/App.js` and replace it with any of the examples on its website. The [Basic Example](https://reacttraining.com/react-router/web/example/basic) is a good place to get started.

   841 

   842 Note that [you may need to configure your production server to support client-side routing](#serving-apps-with-client-side-routing) before deploying your app.

   843 

   844 ## Adding Custom Environment Variables

   845 

   846 >Note: this feature is available with `react-scripts@0.2.3` and higher.

   847 

   848 Your project can consume variables declared in your environment as if they were declared locally in your JS files. By

   849 default you will have `NODE_ENV` defined for you, and any other environment variables starting with

   850 `REACT_APP_`.

   851 

   852 **The environment variables are embedded during the build time**. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, just like [described here](#injecting-data-from-the-server-into-the-page). Alternatively you can rebuild the app on the server anytime you change them.

   853 

   854 >Note: You must create custom environment variables beginning with `REACT_APP_`. Any other variables except `NODE_ENV` will be ignored to avoid accidentally [exposing a private key on the machine that could have the same name](https://github.com/facebookincubator/create-react-app/issues/865#issuecomment-252199527). Changing any environment variables will require you to restart the development server if it is running.

   855 

   856 These environment variables will be defined for you on `process.env`. For example, having an environment

   857 variable named `REACT_APP_SECRET_CODE` will be exposed in your JS as `process.env.REACT_APP_SECRET_CODE`.

   858 

   859 There is also a special built-in environment variable called `NODE_ENV`. You can read it from `process.env.NODE_ENV`. When you run `npm start`, it is always equal to `'development'`, when you run `npm test` it is always equal to `'test'`, and when you run `npm run build` to make a production bundle, it is always equal to `'production'`. **You cannot override `NODE_ENV` manually.** This prevents developers from accidentally deploying a slow development build to production.

   860 

   861 These environment variables can be useful for displaying information conditionally based on where the project is

   862 deployed or consuming sensitive data that lives outside of version control.

   863 

   864 First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined

   865 in the environment inside a `<form>`:

   866 

   867 ```jsx

   868 render() {

   869   return (

   870     <div>

   871       <small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>

   872       <form>

   873         <input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />

   874       </form>

   875     </div>

   876   );

   877 }

   878 ```

   879 

   880 During the build, `process.env.REACT_APP_SECRET_CODE` will be replaced with the current value of the `REACT_APP_SECRET_CODE` environment variable. Remember that the `NODE_ENV` variable will be set for you automatically.

   881 

   882 When you load the app in the browser and inspect the `<input>`, you will see its value set to `abcdef`, and the bold text will show the environment provided when using `npm start`:

   883 

   884 ```html

   885 <div>

   886   <small>You are running this application in <b>development</b> mode.</small>

   887   <form>

   888     <input type="hidden" value="abcdef" />

   889   </form>

   890 </div>

   891 ```

   892 

   893 The above form is looking for a variable called `REACT_APP_SECRET_CODE` from the environment. In order to consume this

   894 value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in

   895 a `.env` file. Both of these ways are described in the next few sections.

   896 

   897 Having access to the `NODE_ENV` is also useful for performing actions conditionally:

   898 

   899 ```js

   900 if (process.env.NODE_ENV !== 'production') {

   901   analytics.disable();

   902 }

   903 ```

   904 

   905 When you compile the app with `npm run build`, the minification step will strip out this condition, and the resulting bundle will be smaller.

   906 

   907 ### Referencing Environment Variables in the HTML

   908 

   909 >Note: this feature is available with `react-scripts@0.9.0` and higher.

   910 

   911 You can also access the environment variables starting with `REACT_APP_` in the `public/index.html`. For example:

   912 

   913 ```html

   914 <title>%REACT_APP_WEBSITE_NAME%</title>

   915 ```

   916 

   917 Note that the caveats from the above section apply:

   918 

   919 * Apart from a few built-in variables (`NODE_ENV` and `PUBLIC_URL`), variable names must start with `REACT_APP_` to work.

   920 * The environment variables are injected at build time. If you need to inject them at runtime, [follow this approach instead](#generating-dynamic-meta-tags-on-the-server).

   921 

   922 ### Adding Temporary Environment Variables In Your Shell

   923 

   924 Defining environment variables can vary between OSes. It’s also important to know that this manner is temporary for the

   925 life of the shell session.

   926 

   927 #### Windows (cmd.exe)

   928 

   929 ```cmd

   930 set "REACT_APP_SECRET_CODE=abcdef" && npm start

   931 ```

   932 

   933 (Note: Quotes around the variable assignment are required to avoid a trailing whitespace.)

   934 

   935 #### Windows (Powershell)

   936 

   937 ```Powershell

   938 ($env:REACT_APP_SECRET_CODE = "abcdef") -and (npm start)

   939 ```

   940 

   941 #### Linux, macOS (Bash)

   942 

   943 ```bash

   944 REACT_APP_SECRET_CODE=abcdef npm start

   945 ```

   946 

   947 ### Adding Development Environment Variables In `.env`

   948 

   949 >Note: this feature is available with `react-scripts@0.5.0` and higher.

   950 

   951 To define permanent environment variables, create a file called `.env` in the root of your project:

   952 

   953 ```

   954 REACT_APP_SECRET_CODE=abcdef

   955 ```

   956 >Note: You must create custom environment variables beginning with `REACT_APP_`. Any other variables except `NODE_ENV` will be ignored to avoid [accidentally exposing a private key on the machine that could have the same name](https://github.com/facebookincubator/create-react-app/issues/865#issuecomment-252199527). Changing any environment variables will require you to restart the development server if it is running.

   957 

   958 `.env` files **should be** checked into source control (with the exclusion of `.env*.local`).

   959 

   960 #### What other `.env` files can be used?

   961 

   962 >Note: this feature is **available with `react-scripts@1.0.0` and higher**.

   963 

   964 * `.env`: Default.

   965 * `.env.local`: Local overrides. **This file is loaded for all environments except test.**

   966 * `.env.development`, `.env.test`, `.env.production`: Environment-specific settings.

   967 * `.env.development.local`, `.env.test.local`, `.env.production.local`: Local overrides of environment-specific settings.

   968 

   969 Files on the left have more priority than files on the right:

   970 

   971 * `npm start`: `.env.development.local`, `.env.development`, `.env.local`, `.env`

   972 * `npm run build`: `.env.production.local`, `.env.production`, `.env.local`, `.env`

   973 * `npm test`: `.env.test.local`, `.env.test`, `.env` (note `.env.local` is missing)

   974 

   975 These variables will act as the defaults if the machine does not explicitly set them.<br>

   976 Please refer to the [dotenv documentation](https://github.com/motdotla/dotenv) for more details.

   977 

   978 >Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need

   979 these defined as well. Consult their documentation how to do this. For example, see the documentation for [Travis CI](https://docs.travis-ci.com/user/environment-variables/) or [Heroku](https://devcenter.heroku.com/articles/config-vars).

   980 

   981 #### Expanding Environment Variables In `.env`

   982 

   983 >Note: this feature is available with `react-scripts@1.1.0` and higher.

   984 

   985 Expand variables already on your machine for use in your `.env` file (using [dotenv-expand](https://github.com/motdotla/dotenv-expand)).

   986 

   987 For example, to get the environment variable `npm_package_version`:

   988 

   989 ```

   990 REACT_APP_VERSION=$npm_package_version

   991 # also works:

   992 # REACT_APP_VERSION=${npm_package_version}

   993 ```

   994 

   995 Or expand variables local to the current `.env` file:

   996 

   997 ```

   998 DOMAIN=www.example.com

   999 REACT_APP_FOO=$DOMAIN/foo

  1000 REACT_APP_BAR=$DOMAIN/bar

  1001 ```

  1002 

  1003 ## Can I Use Decorators?

  1004 

  1005 Many popular libraries use [decorators](https://medium.com/google-developers/exploring-es7-decorators-76ecb65fb841) in their documentation.<br>

  1006 Create React App doesn’t support decorator syntax at the moment because:

  1007 

  1008 * It is an experimental proposal and is subject to change.

  1009 * The current specification version is not officially supported by Babel.

  1010 * If the specification changes, we won’t be able to write a codemod because we don’t use them internally at Facebook.

  1011 

  1012 However in many cases you can rewrite decorator-based code without decorators just as fine.<br>

  1013 Please refer to these two threads for reference:

  1014 

  1015 * [#214](https://github.com/facebookincubator/create-react-app/issues/214)

  1016 * [#411](https://github.com/facebookincubator/create-react-app/issues/411)

  1017 

  1018 Create React App will add decorator support when the specification advances to a stable stage.

  1019 

  1020 ## Fetching Data with AJAX Requests

  1021 

  1022 React doesn't prescribe a specific approach to data fetching, but people commonly use either a library like [axios](https://github.com/axios/axios) or the [`fetch()` API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) provided by the browser. Conveniently, Create React App includes a polyfill for `fetch()` so you can use it without worrying about the browser support.

  1023 

  1024 The global `fetch` function allows to easily makes AJAX requests. It takes in a URL as an input and returns a `Promise` that resolves to a `Response` object. You can find more information about `fetch` [here](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch).

  1025 

  1026 This project also includes a [Promise polyfill](https://github.com/then/promise) which provides a full implementation of Promises/A+. A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises [here](https://www.promisejs.org/) and [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise). Both axios and `fetch()` use Promises under the hood. You can also use the [`async / await`](https://davidwalsh.name/async-await) syntax to reduce the callback nesting.

  1027 

  1028 You can learn more about making AJAX requests from React components in [the FAQ entry on the React website](https://reactjs.org/docs/faq-ajax.html).

  1029 

  1030 ## Integrating with an API Backend

  1031 

  1032 These tutorials will help you to integrate your app with an API backend running on another port,

  1033 using `fetch()` to access it.

  1034 

  1035 ### Node

  1036 Check out [this tutorial](https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/).

  1037 You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo).

  1038 

  1039 ### Ruby on Rails

  1040 

  1041 Check out [this tutorial](https://www.fullstackreact.com/articles/how-to-get-create-react-app-to-work-with-your-rails-api/).

  1042 You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo-rails).

  1043 

  1044 ## Proxying API Requests in Development

  1045 

  1046 >Note: this feature is available with `react-scripts@0.2.3` and higher.

  1047 

  1048 People often serve the front-end React app from the same host and port as their backend implementation.<br>

  1049 For example, a production setup might look like this after the app is deployed:

  1050 

  1051 ```

  1052 /             - static server returns index.html with React app

  1053 /todos        - static server returns index.html with React app

  1054 /api/todos    - server handles any /api/* requests using the backend implementation

  1055 ```

  1056 

  1057 Such setup is **not** required. However, if you **do** have a setup like this, it is convenient to write requests like `fetch('/api/todos')` without worrying about redirecting them to another host or port during development.

  1058 

  1059 To tell the development server to proxy any unknown requests to your API server in development, add a `proxy` field to your `package.json`, for example:

  1060 

  1061 ```js

  1062   "proxy": "http://localhost:4000",

  1063 ```

  1064 

  1065 This way, when you `fetch('/api/todos')` in development, the development server will recognize that it’s not a static asset, and will proxy your request to `http://localhost:4000/api/todos` as a fallback. The development server will **only** attempt to send requests without `text/html` in its `Accept` header to the proxy.

  1066 

  1067 Conveniently, this avoids [CORS issues](http://stackoverflow.com/questions/21854516/understanding-ajax-cors-and-security-considerations) and error messages like this in development:

  1068 

  1069 ```

  1070 Fetch API cannot load http://localhost:4000/api/todos. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://localhost:3000' is therefore not allowed access. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

  1071 ```

  1072 

  1073 Keep in mind that `proxy` only has effect in development (with `npm start`), and it is up to you to ensure that URLs like `/api/todos` point to the right thing in production. You don’t have to use the `/api` prefix. Any unrecognized request without a `text/html` accept header will be redirected to the specified `proxy`.

  1074 

  1075 The `proxy` option supports HTTP, HTTPS and WebSocket connections.<br>

  1076 If the `proxy` option is **not** flexible enough for you, alternatively you can:

  1077 

  1078 * [Configure the proxy yourself](#configuring-the-proxy-manually)

  1079 * Enable CORS on your server ([here’s how to do it for Express](http://enable-cors.org/server_expressjs.html)).

  1080 * Use [environment variables](#adding-custom-environment-variables) to inject the right server host and port into your app.

  1081 

  1082 ### "Invalid Host Header" Errors After Configuring Proxy

  1083 

  1084 When you enable the `proxy` option, you opt into a more strict set of host checks. This is necessary because leaving the backend open to remote hosts makes your computer vulnerable to DNS rebinding attacks. The issue is explained in [this article](https://medium.com/webpack/webpack-dev-server-middleware-security-issues-1489d950874a) and [this issue](https://github.com/webpack/webpack-dev-server/issues/887).

  1085 

  1086 This shouldn’t affect you when developing on `localhost`, but if you develop remotely like [described here](https://github.com/facebookincubator/create-react-app/issues/2271), you will see this error in the browser after enabling the `proxy` option:

  1087 

  1088 >Invalid Host header

  1089 

  1090 To work around it, you can specify your public development host in a file called `.env.development` in the root of your project:

  1091 

  1092 ```

  1093 HOST=mypublicdevhost.com

  1094 ```

  1095 

  1096 If you restart the development server now and load the app from the specified host, it should work.

  1097 

  1098 If you are still having issues or if you’re using a more exotic environment like a cloud editor, you can bypass the host check completely by adding a line to `.env.development.local`. **Note that this is dangerous and exposes your machine to remote code execution from malicious websites:**

  1099 

  1100 ```

  1101 # NOTE: THIS IS DANGEROUS!

  1102 # It exposes your machine to attacks from the websites you visit.

  1103 DANGEROUSLY_DISABLE_HOST_CHECK=true

  1104 ```

  1105 

  1106 We don’t recommend this approach.

  1107 

  1108 ### Configuring the Proxy Manually

  1109 

  1110 >Note: this feature is available with `react-scripts@1.0.0` and higher.

  1111 

  1112 If the `proxy` option is **not** flexible enough for you, you can specify an object in the following form (in `package.json`).<br>

  1113 You may also specify any configuration value [`http-proxy-middleware`](https://github.com/chimurai/http-proxy-middleware#options) or [`http-proxy`](https://github.com/nodejitsu/node-http-proxy#options) supports.

  1114 ```js

  1115 {

  1116   // ...

  1117   "proxy": {

  1118     "/api": {

  1119       "target": "<url>",

  1120       "ws": true

  1121       // ...

  1122     }

  1123   }

  1124   // ...

  1125 }

  1126 ```

  1127 

  1128 All requests matching this path will be proxies, no exceptions. This includes requests for `text/html`, which the standard `proxy` option does not proxy.

  1129 

  1130 If you need to specify multiple proxies, you may do so by specifying additional entries.

  1131 Matches are regular expressions, so that you can use a regexp to match multiple paths.

  1132 ```js

  1133 {

  1134   // ...

  1135   "proxy": {

  1136     // Matches any request starting with /api