README.md

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

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

## Table of Contents

- [Updating to New Releases](#updating-to-new-releases)
- [Sending Feedback](#sending-feedback)
- [Folder Structure](#folder-structure)
- [Available Scripts](#available-scripts)
  - [npm start](#npm-start)
  - [npm test](#npm-test)
  - [npm run build](#npm-run-build)
  - [npm run eject](#npm-run-eject)
- [Supported Browsers](#supported-browsers)
- [Supported Language Features and Polyfills](#supported-language-features-and-polyfills)
- [Syntax Highlighting in the Editor](#syntax-highlighting-in-the-editor)
- [Displaying Lint Output in the Editor](#displaying-lint-output-in-the-editor)
- [Debugging in the Editor](#debugging-in-the-editor)
- [Formatting Code Automatically](#formatting-code-automatically)
- [Changing the Page `<title>`](#changing-the-page-title)
- [Installing a Dependency](#installing-a-dependency)
- [Importing a Component](#importing-a-component)
- [Code Splitting](#code-splitting)
- [Adding a Stylesheet](#adding-a-stylesheet)
- [Adding a CSS Modules stylesheet](#adding-a-css-modules-stylesheet)
- [Post-Processing CSS](#post-processing-css)
- [Adding a CSS Preprocessor (Sass, Less etc.)](#adding-a-css-preprocessor-sass-less-etc)
- [Adding Images, Fonts, and Files](#adding-images-fonts-and-files)
- [Using the `public` Folder](#using-the-public-folder)
  - [Changing the HTML](#changing-the-html)
  - [Adding Assets Outside of the Module System](#adding-assets-outside-of-the-module-system)
  - [When to Use the `public` Folder](#when-to-use-the-public-folder)
- [Using Global Variables](#using-global-variables)
- [Adding Bootstrap](#adding-bootstrap)
  - [Using a Custom Theme](#using-a-custom-theme)
- [Adding Flow](#adding-flow)
- [Adding a Router](#adding-a-router)
- [Adding Custom Environment Variables](#adding-custom-environment-variables)
  - [Referencing Environment Variables in the HTML](#referencing-environment-variables-in-the-html)
  - [Adding Temporary Environment Variables In Your Shell](#adding-temporary-environment-variables-in-your-shell)
  - [Adding Development Environment Variables In `.env`](#adding-development-environment-variables-in-env)
- [Can I Use Decorators?](#can-i-use-decorators)
- [Fetching Data with AJAX Requests](#fetching-data-with-ajax-requests)
- [Integrating with an API Backend](#integrating-with-an-api-backend)
  - [Node](#node)
  - [Ruby on Rails](#ruby-on-rails)
- [Proxying API Requests in Development](#proxying-api-requests-in-development)
  - ["Invalid Host Header" Errors After Configuring Proxy](#invalid-host-header-errors-after-configuring-proxy)
  - [Configuring the Proxy Manually](#configuring-the-proxy-manually)
  - [Configuring a WebSocket Proxy](#configuring-a-websocket-proxy)
- [Using HTTPS in Development](#using-https-in-development)
- [Generating Dynamic `<meta>` Tags on the Server](#generating-dynamic-meta-tags-on-the-server)
- [Pre-Rendering into Static HTML Files](#pre-rendering-into-static-html-files)
- [Injecting Data from the Server into the Page](#injecting-data-from-the-server-into-the-page)
- [Running Tests](#running-tests)
  - [Filename Conventions](#filename-conventions)
  - [Command Line Interface](#command-line-interface)
  - [Version Control Integration](#version-control-integration)
  - [Writing Tests](#writing-tests)
  - [Testing Components](#testing-components)
  - [Using Third Party Assertion Libraries](#using-third-party-assertion-libraries)
  - [Initializing Test Environment](#initializing-test-environment)
  - [Focusing and Excluding Tests](#focusing-and-excluding-tests)
  - [Coverage Reporting](#coverage-reporting)
  - [Continuous Integration](#continuous-integration)
  - [Disabling jsdom](#disabling-jsdom)
  - [Snapshot Testing](#snapshot-testing)
  - [Editor Integration](#editor-integration)
- [Debugging Tests](#debugging-tests)
  - [Debugging Tests in Chrome](#debugging-tests-in-chrome)
  - [Debugging Tests in Visual Studio Code](#debugging-tests-in-visual-studio-code)
- [Developing Components in Isolation](#developing-components-in-isolation)
  - [Getting Started with Storybook](#getting-started-with-storybook)
  - [Getting Started with Styleguidist](#getting-started-with-styleguidist)
- [Publishing Components to npm](#publishing-components-to-npm)
- [Making a Progressive Web App](#making-a-progressive-web-app)
  - [Opting Out of Caching](#opting-out-of-caching)
  - [Offline-First Considerations](#offline-first-considerations)
  - [Progressive Web App Metadata](#progressive-web-app-metadata)
- [Analyzing the Bundle Size](#analyzing-the-bundle-size)
- [Deployment](#deployment)
  - [Static Server](#static-server)
  - [Other Solutions](#other-solutions)
  - [Serving Apps with Client-Side Routing](#serving-apps-with-client-side-routing)
    - [Service Worker Considerations](#service-worker-considerations)
  - [Building for Relative Paths](#building-for-relative-paths)
  - [Azure](#azure)
  - [Firebase](#firebase)
  - [GitHub Pages](#github-pages)
  - [Heroku](#heroku)
  - [Netlify](#netlify)
  - [Now](#now)
  - [S3 and CloudFront](#s3-and-cloudfront)
  - [Surge](#surge)
- [Advanced Configuration](#advanced-configuration)
- [Troubleshooting](#troubleshooting)
  - [`npm start` doesn’t detect changes](#npm-start-doesnt-detect-changes)
  - [`npm test` hangs on macOS Sierra](#npm-test-hangs-on-macos-sierra)
  - [`npm run build` exits too early](#npm-run-build-exits-too-early)
  - [`npm run build` fails on Heroku](#npm-run-build-fails-on-heroku)
  - [`npm run build` fails to minify](#npm-run-build-fails-to-minify)
  - [Moment.js locales are missing](#momentjs-locales-are-missing)
- [Alternatives to Ejecting](#alternatives-to-ejecting)
- [Something Missing?](#something-missing)

## Updating to New Releases

Create React App is divided into two packages:

* `create-react-app` is a global command-line utility that you use to create new projects.
* `react-scripts` is a development dependency in the generated projects (including this one).

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

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.

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.

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.

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

## Sending Feedback

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

## Folder Structure

After creation, your project should look like this:

```
my-app/
  README.md
  node_modules/
  package.json
  public/
    index.html
    favicon.ico
  src/
    App.css
    App.js
    App.test.js
    index.css
    index.js
    logo.svg
```

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

* `public/index.html` is the page template;
* `src/index.js` is the JavaScript entry point.

You can delete or rename the other files.

You may create subdirectories inside `src`. For faster rebuilds, only files inside `src` are processed by Webpack.<br>
You need to **put any JS and CSS files inside `src`**, otherwise Webpack won’t see them.

Only files inside `public` can be used from `public/index.html`.<br>
Read instructions below for using assets from JavaScript and HTML.

You can, however, create more top-level directories.<br>
They will not be included in the production build so you can use them for things like documentation.

## Available Scripts

In the project directory, you can run:

### `npm start`

Runs the app in the development mode.<br>
Open [http://localhost:3000](http://localhost:3000) to view it in the browser.

The page will reload if you make edits.<br>
You will also see any lint errors in the console.

### `npm test`

Launches the test runner in the interactive watch mode.<br>
See the section about [running tests](#running-tests) for more information.

### `npm run build`

Builds the app for production to the `build` folder.<br>
It correctly bundles React in production mode and optimizes the build for the best performance.

The build is minified and the filenames include the hashes.<br>
Your app is ready to be deployed!

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

### `npm run eject`

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

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.

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.

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.

## Supported Browsers

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

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

## Supported Language Features and Polyfills

This project supports a superset of the latest JavaScript standard.<br>
In addition to [ES6](https://github.com/lukehoban/es6features) syntax features, it also supports:

* [Exponentiation Operator](https://github.com/rwaldron/exponentiation-operator) (ES2016).
* [Async/await](https://github.com/tc39/ecmascript-asyncawait) (ES2017).
* [Object Rest/Spread Properties](https://github.com/sebmarkbage/ecmascript-rest-spread) (stage 3 proposal).
* [Dynamic import()](https://github.com/tc39/proposal-dynamic-import) (stage 3 proposal)
* [Class Fields and Static Properties](https://github.com/tc39/proposal-class-public-fields) (part of stage 3 proposal).
* [JSX](https://facebook.github.io/react/docs/introducing-jsx.html) and [Flow](https://flowtype.org/) syntax.

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

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.

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

* [`Object.assign()`](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign) via [`object-assign`](https://github.com/sindresorhus/object-assign).
* [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) via [`promise`](https://github.com/then/promise).
* [`fetch()`](https://developer.mozilla.org/en/docs/Web/API/Fetch_API) via [`whatwg-fetch`](https://github.com/github/fetch).

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.

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.

## Syntax Highlighting in the Editor

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.

## Displaying Lint Output in the Editor

>Note: this feature is available with `react-scripts@0.2.0` and higher.<br>
>It also only works with npm 3 or higher.

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

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.

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

```js
{
  "extends": "react-app"
}
```

Now your editor should report the linting warnings.

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.

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

## Debugging in the Editor

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

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.

### Visual Studio Code

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.

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

```json
{
  "version": "0.2.0",
  "configurations": [{
    "name": "Chrome",
    "type": "chrome",
    "request": "launch",
    "url": "http://localhost:3000",
    "webRoot": "${workspaceRoot}/src",
    "sourceMapPathOverrides": {
      "webpack:///src/*": "${webRoot}/*"
    }
  }]
}
```
>Note: the URL may be different if you've made adjustments via the [HOST or PORT environment variables](#advanced-configuration).

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.

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

### WebStorm

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.

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.

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

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.

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

## Formatting Code Automatically

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/).

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

```sh
npm install --save husky lint-staged prettier
```

Alternatively you may use `yarn`:

```sh
yarn add husky lint-staged prettier
```

* `husky` makes it easy to use githooks as if they are npm scripts.
* `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).
* `prettier` is the JavaScript formatter we will run before commits.

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

Add the following line to `scripts` section:

```diff
  "scripts": {
+   "precommit": "lint-staged",
    "start": "react-scripts start",
    "build": "react-scripts build",
```

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

```diff
  "dependencies": {
    // ...
  },
+ "lint-staged": {
+   "src/**/*.{js,jsx,json,css}": [
+     "prettier --single-quote --write",
+     "git add"
+   ]
+ },
  "scripts": {
```

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}"` to format your entire project for the first time.

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.

## Changing the Page `<title>`

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.

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.

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.

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).

## Installing a Dependency

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`:

```sh
npm install --save react-router
```

Alternatively you may use `yarn`:

```sh
yarn add react-router
```

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

## Importing a Component

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

For example:

### `Button.js`

```js
import React, { Component } from 'react';

class Button extends Component {
  render() {
    // ...
  }
}

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

### `DangerButton.js`


```js
import React, { Component } from 'react';
import Button from './Button'; // Import a component from another file

class DangerButton extends Component {
  render() {
    return <Button color="red" />;
  }
}

export default DangerButton;
```

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.

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'`.

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.

Learn more about ES6 modules:

* [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)
* [Exploring ES6: Modules](http://exploringjs.com/es6/ch_modules.html)
* [Understanding ES6: Modules](https://leanpub.com/understandinges6/read#leanpub-auto-encapsulating-code-with-modules)

## Code Splitting

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.

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.

Here is an example:

### `moduleA.js`

```js
const moduleA = 'Hello';

export { moduleA };
```
### `App.js`

```js
import React, { Component } from 'react';

class App extends Component {
  handleClick = () => {
    import('./moduleA')
      .then(({ moduleA }) => {
        // Use moduleA
      })
      .catch(err => {
        // Handle failure
      });
  };

  render() {
    return (
      <div>
        <button onClick={this.handleClick}>Load</button>
      </div>
    );
  }
}

export default App;
```

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

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

### With React Router

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).

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

## Adding a Stylesheet

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**:

### `Button.css`

```css
.Button {
  padding: 20px;
}
```

### `Button.js`

```js
import React, { Component } from 'react';
import './Button.css'; // Tell Webpack that Button.js uses these styles

class Button extends Component {
  render() {
    // You can use them as regular CSS styles
    return <div className="Button" />;
  }
}
```

**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-ui-engineering/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.

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.

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.

## Adding a CSS Modules stylesheet

This project supports [CSS Modules](https://github.com/css-modules/css-modules) alongside regular stylesheets using the **[name].module.css** file naming convention. CSS Modules allows the scoping of CSS by automatically creating a unique classname of the format **[dir]\_\_[filename]___[classname]**.

An advantage of this is the ability to repeat the same classname within many CSS files without worrying about a clash.

### `Button.module.css`

```css
.button {
  padding: 20px;
}
```

### `another-stylesheet.css`

```css
.button {
  color: green;
}
```

### `Button.js`

```js
import React, { Component } from 'react';
import './another-stylesheet.css'; // Import regular stylesheet
import styles from './Button.module.css'; // Import css modules stylesheet as styles

class Button extends Component {
  render() {
    // You can use them as regular CSS styles
    return <div className={styles.button} />;
  }
}
```
### `exported HTML`
No clashes from other `.button` classnames

```html
<div class="src__Button-module___button"></div>
```

**This is an optional feature.** Regular html stylesheets and js imported stylesheets are fully supported. CSS Modules are only added when explictly named as a css module stylesheet using the extension `.module.css`.

## Post-Processing CSS

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.

For example, this:

```css
.App {
  display: flex;
  flex-direction: row;
  align-items: center;
}
```

becomes this:

```css
.App {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
      -ms-flex-direction: row;
          flex-direction: row;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
}
```

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

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

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)).

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.

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

```sh
npm install --save node-sass-chokidar
```

Alternatively you may use `yarn`:

```sh
yarn add node-sass-chokidar
```

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

```diff
   "scripts": {
+    "build-css": "node-sass-chokidar src/ -o src/",
+    "watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test --env=jsdom",
```

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

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.

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.

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

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

This will allow you to do imports like

```scss
@import 'styles/_colors.scss'; // assuming a styles directory under src/
@import 'nprogress/nprogress'; // importing a css file from the nprogress node module
```

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.

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:

```sh
npm install --save npm-run-all
```

Alternatively you may use `yarn`:

```sh
yarn add npm-run-all
```

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

```diff
   "scripts": {
     "build-css": "node-sass-chokidar src/ -o src/",
     "watch-css": "npm run build-css && node-sass-chokidar src/ -o src/ --watch --recursive",
-    "start": "react-scripts start",
-    "build": "react-scripts build",
+    "start-js": "react-scripts start",
+    "start": "npm-run-all -p watch-css start-js",
+    "build-js": "react-scripts build",
+    "build": "npm-run-all build-css build-js",
     "test": "react-scripts test --env=jsdom",
     "eject": "react-scripts eject"
   }
```

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

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

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

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

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

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

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

## Adding Images, Fonts, and Files

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

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.

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).

Here is an example:

```js
import React from 'react';
import logo from './logo.png'; // Tell Webpack this JS file uses this image

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

function Header() {
  // Import result is the URL of your image
  return <img src={logo} alt="Logo" />;
}

export default Header;
```

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

This works in CSS too:

```css
.Logo {
  background-image: url(./logo.png);
}
```

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.

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

**It is not required for React** but many people enjoy it (and React Native uses a similar mechanism for images).<br>
An alternative way of handling static assets is described in the next section.

## Using the `public` Folder

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

### Changing the HTML

The `public` folder contains the HTML file so you can tweak it, for example, to [set the page title](#changing-the-page-title).
The `<script>` tag with the compiled code will be added to it automatically during the build process.

### Adding Assets Outside of the Module System

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

Note that we normally encourage you to `import` assets in JavaScript files instead.
For example, see the sections on [adding a stylesheet](#adding-a-stylesheet) and [adding images and fonts](#adding-images-fonts-and-files).
This mechanism provides a number of benefits:

* Scripts and stylesheets get minified and bundled together to avoid extra network requests.
* Missing files cause compilation errors instead of 404 errors for your users.
* Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.

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

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`.

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

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

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.

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.

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

```js
render() {
  // Note: this is an escape hatch and should be used sparingly!
  // Normally we recommend using `import` for getting asset URLs
  // as described in “Adding Images and Fonts” above this section.
  return <img src={process.env.PUBLIC_URL + '/img/logo.png'} />;
}
```

Keep in mind the downsides of this approach:

* None of the files in `public` folder get post-processed or minified.
* Missing files will not be called at compilation time, and will cause 404 errors for your users.
* Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.

### When to Use the `public` Folder

Normally we recommend importing [stylesheets](#adding-a-stylesheet), [images, and fonts](#adding-images-fonts-and-files) from JavaScript.
The `public` folder is useful as a workaround for a number of less common cases:

* 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).
* You have thousands of images and need to dynamically reference their paths.
* You want to include a small script like [`pace.js`](http://github.hubspot.com/pace/docs/welcome/) outside of the bundled code.
* Some library may be incompatible with Webpack and you have no other option but to include it as a `<script>` tag.

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

## Using Global Variables

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.

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

```js
const $ = window.$;
```

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

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

## Adding Bootstrap

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:

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

```sh
npm install --save react-bootstrap bootstrap@3
```

Alternatively you may use `yarn`:

```sh
yarn add react-bootstrap bootstrap@3
```

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

```js
import 'bootstrap/dist/css/bootstrap.css';
import 'bootstrap/dist/css/bootstrap-theme.css';
// Put any other imports below so that CSS from your
// components takes precedence over default styles.
```

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

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

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.

### Using a Custom Theme

Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br>
We suggest the following approach:

* Create a new package that depends on the package you wish to customize, e.g. Bootstrap.
* Add the necessary build steps to tweak the theme, and publish your package on npm.
* Install your own theme npm package as a dependency of your app.

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

## Adding Flow

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.

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

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

1. Run `npm install --save flow-bin` (or `yarn add flow-bin`).
2. Add `"flow": "flow"` to the `scripts` section of your `package.json`.
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.
4. Add `// @flow` to any files you want to type check (for example, to `src/App.js`).

Now you can run `npm run flow` (or `yarn flow`) to check the files for type errors.
You can optionally use an IDE like [Nuclide](https://nuclide.io/docs/languages/flow/) for a better integrated experience.
In the future we plan to integrate it into Create React App even more closely.

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

## Adding a Router

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

To add it, run:

```sh
npm install --save react-router-dom
```

Alternatively you may use `yarn`:

```sh
yarn add react-router-dom
```

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.

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.

## Adding Custom Environment Variables

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

Your project can consume variables declared in your environment as if they were declared locally in your JS files. By
default you will have `NODE_ENV` defined for you, and any other environment variables starting with
`REACT_APP_`.

**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.

>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.

These environment variables will be defined for you on `process.env`. For example, having an environment
variable named `REACT_APP_SECRET_CODE` will be exposed in your JS as `process.env.REACT_APP_SECRET_CODE`.

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.

These environment variables can be useful for displaying information conditionally based on where the project is
deployed or consuming sensitive data that lives outside of version control.

First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined
in the environment inside a `<form>`:

```jsx
render() {
  return (
    <div>
      <small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
      <form>
        <input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
      </form>
    </div>
  );
}
```

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.

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`:

```html
<div>
  <small>You are running this application in <b>development</b> mode.</small>
  <form>
    <input type="hidden" value="abcdef" />
  </form>
</div>
```

The above form is looking for a variable called `REACT_APP_SECRET_CODE` from the environment. In order to consume this
value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in
a `.env` file. Both of these ways are described in the next few sections.

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

```js
if (process.env.NODE_ENV !== 'production') {
  analytics.disable();
}
```

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

### Referencing Environment Variables in the HTML

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

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

```html
<title>%REACT_APP_WEBSITE_NAME%</title>
```

Note that the caveats from the above section apply:

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

### Adding Temporary Environment Variables In Your Shell

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

#### Windows (cmd.exe)

```cmd
set "REACT_APP_SECRET_CODE=abcdef" && npm start
```

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

#### Windows (Powershell)

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

#### Linux, macOS (Bash)

```bash
REACT_APP_SECRET_CODE=abcdef npm start
```

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

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

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

```