README.md

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

People often serve the front-end React app from the same host and port as their backend implementation.<br>
For example, a production setup might look like this after the app is deployed:

```
/             - static server returns index.html with React app
/todos        - static server returns index.html with React app
/api/todos    - server handles any /api/* requests using the backend implementation
```

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.

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:

```js
  "proxy": "http://localhost:4000",
```

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.

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

```
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.
```

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`.

The `proxy` option supports HTTP, HTTPS and WebSocket connections.<br>
If the `proxy` option is **not** flexible enough for you, alternatively you can:

* [Configure the proxy yourself](#configuring-the-proxy-manually)
* Enable CORS on your server ([here’s how to do it for Express](http://enable-cors.org/server_expressjs.html)).
* Use [environment variables](#adding-custom-environment-variables) to inject the right server host and port into your app.

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

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).

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:

>Invalid Host header

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

```
HOST=mypublicdevhost.com
```

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

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:**

```
# NOTE: THIS IS DANGEROUS!
# It exposes your machine to attacks from the websites you visit.
DANGEROUSLY_DISABLE_HOST_CHECK=true
```

We don’t recommend this approach.

### Configuring the Proxy Manually

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

If the `proxy` option is **not** flexible enough for you, you can specify an object in the following form (in `package.json`).<br>
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.
```js
{
  // ...
  "proxy": {
    "/api": {
      "target": "<url>",
      "ws": true
      // ...
    }
  }
  // ...
}
```

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

If you need to specify multiple proxies, you may do so by specifying additional entries.
Matches are regular expressions, so that you can use a regexp to match multiple paths.
```js
{
  // ...
  "proxy": {
    // Matches any request starting with /api
    "/api": {
      "target": "<url_1>",
      "ws": true
      // ...
    },
    // Matches any request starting with /foo
    "/foo": {
      "target": "<url_2>",
      "ssl": true,
      "pathRewrite": {
        "^/foo": "/foo/beta"
      }
      // ...
    },
    // Matches /bar/abc.html but not /bar/sub/def.html
    "/bar/[^/]*[.]html": {
      "target": "<url_3>",
      // ...
    },
    // Matches /baz/abc.html and /baz/sub/def.html
    "/baz/.*/.*[.]html": {
      "target": "<url_4>"
      // ...
    }
  }
  // ...
}
```

### Configuring a WebSocket Proxy

When setting up a WebSocket proxy, there are a some extra considerations to be aware of.

If you’re using a WebSocket engine like [Socket.io](https://socket.io/), you must have a Socket.io server running that you can use as the proxy target. Socket.io will not work with a standard WebSocket server. Specifically, don't expect Socket.io to work with [the websocket.org echo test](http://websocket.org/echo.html).

There’s some good documentation available for [setting up a Socket.io server](https://socket.io/docs/).

Standard WebSockets **will** work with a standard WebSocket server as well as the websocket.org echo test. You can use libraries like [ws](https://github.com/websockets/ws) for the server, with [native WebSockets in the browser](https://developer.mozilla.org/en-US/docs/Web/API/WebSocket).

Either way, you can proxy WebSocket requests manually in `package.json`:

```js
{
  // ...
  "proxy": {
    "/socket": {
      // Your compatible WebSocket server
      "target": "ws://<socket_url>",
      // Tell http-proxy-middleware that this is a WebSocket proxy.
      // Also allows you to proxy WebSocket requests without an additional HTTP request
      // https://github.com/chimurai/http-proxy-middleware#external-websocket-upgrade
      "ws": true
      // ...
    }
  }
  // ...
}
```

## Using HTTPS in Development

>Note: this feature is available with `react-scripts@0.4.0` and higher.

You may require the dev server to serve pages over HTTPS. One particular case where this could be useful is when using [the "proxy" feature](#proxying-api-requests-in-development) to proxy requests to an API server when that API server is itself serving HTTPS.

To do this, set the `HTTPS` environment variable to `true`, then start the dev server as usual with `npm start`:

#### Windows (cmd.exe)

```cmd
set HTTPS=true&&npm start
```

(Note: the lack of whitespace is intentional.)

#### Linux, macOS (Bash)

```bash
HTTPS=true npm start
```

Note that the server will use a self-signed certificate, so your web browser will almost definitely display a warning upon accessing the page.

## Generating Dynamic `<meta>` Tags on the Server

Since Create React App doesn’t support server rendering, you might be wondering how to make `<meta>` tags dynamic and reflect the current URL. To solve this, we recommend to add placeholders into the HTML, like this:

```html
<!doctype html>
<html lang="en">
  <head>
    <meta property="og:title" content="__OG_TITLE__">
    <meta property="og:description" content="__OG_DESCRIPTION__">
```

Then, on the server, regardless of the backend you use, you can read `index.html` into memory and replace `__OG_TITLE__`, `__OG_DESCRIPTION__`, and any other placeholders with values depending on the current URL. Just make sure to sanitize and escape the interpolated values so that they are safe to embed into HTML!

If you use a Node server, you can even share the route matching logic between the client and the server. However duplicating it also works fine in simple cases.

## Pre-Rendering into Static HTML Files

If you’re hosting your `build` with a static hosting provider you can use [react-snapshot](https://www.npmjs.com/package/react-snapshot) or [react-snap](https://github.com/stereobooster/react-snap) to generate HTML pages for each route, or relative link, in your application. These pages will then seamlessly become active, or “hydrated”, when the JavaScript bundle has loaded.

There are also opportunities to use this outside of static hosting, to take the pressure off the server when generating and caching routes.

The primary benefit of pre-rendering is that you get the core content of each page _with_ the HTML payload—regardless of whether or not your JavaScript bundle successfully downloads. It also increases the likelihood that each route of your application will be picked up by search engines.

You can read more about [zero-configuration pre-rendering (also called snapshotting) here](https://medium.com/superhighfives/an-almost-static-stack-6df0a2791319).

## Injecting Data from the Server into the Page

Similarly to the previous section, you can leave some placeholders in the HTML that inject global variables, for example:

```js
<!doctype html>
<html lang="en">
  <head>
    <script>
      window.SERVER_DATA = __SERVER_DATA__;
    </script>
```

Then, on the server, you can replace `__SERVER_DATA__` with a JSON of real data right before sending the response. The client code can then read `window.SERVER_DATA` to use it. **Make sure to [sanitize the JSON before sending it to the client](https://medium.com/node-security/the-most-common-xss-vulnerability-in-react-js-applications-2bdffbcc1fa0) as it makes your app vulnerable to XSS attacks.**

## Running Tests

>Note: this feature is available with `react-scripts@0.3.0` and higher.<br>
>[Read the migration guide to learn how to enable it in older projects!](https://github.com/facebookincubator/create-react-app/blob/master/CHANGELOG.md#migrating-from-023-to-030)

Create React App uses [Jest](https://facebook.github.io/jest/) as its test runner. To prepare for this integration, we did a [major revamp](https://facebook.github.io/jest/blog/2016/09/01/jest-15.html) of Jest so if you heard bad things about it years ago, give it another try.

Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.

While Jest provides browser globals such as `window` thanks to [jsdom](https://github.com/tmpvar/jsdom), they are only approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.

We recommend that you use a separate tool for browser end-to-end tests if you need them. They are beyond the scope of Create React App.

### Filename Conventions

Jest will look for test files with any of the following popular naming conventions:

* Files with `.js` suffix in `__tests__` folders.
* Files with `.test.js` suffix.
* Files with `.spec.js` suffix.

The `.test.js` / `.spec.js` files (or the `__tests__` folders) can be located at any depth under the `src` top level folder.

We recommend to put the test files (or `__tests__` folders) next to the code they are testing so that relative imports appear shorter. For example, if `App.test.js` and `App.js` are in the same folder, the test just needs to `import App from './App'` instead of a long relative path. Colocation also helps find tests more quickly in larger projects.

### Command Line Interface

When you run `npm test`, Jest will launch in the watch mode. Every time you save a file, it will re-run the tests, just like `npm start` recompiles the code.

The watcher includes an interactive command-line interface with the ability to run all tests, or focus on a search pattern. It is designed this way so that you can keep it open and enjoy fast re-runs. You can learn the commands from the “Watch Usage” note that the watcher prints after every run:

![Jest watch mode](http://facebook.github.io/jest/img/blog/15-watch.gif)

### Version Control Integration

By default, when you run `npm test`, Jest will only run the tests related to files changed since the last commit. This is an optimization designed to make your tests run fast regardless of how many tests you have. However it assumes that you don’t often commit the code that doesn’t pass the tests.

Jest will always explicitly mention that it only ran tests related to the files changed since the last commit. You can also press `a` in the watch mode to force Jest to run all tests.

Jest will always run all tests on a [continuous integration](#continuous-integration) server or if the project is not inside a Git or Mercurial repository.

### Writing Tests

To create tests, add `it()` (or `test()`) blocks with the name of the test and its code. You may optionally wrap them in `describe()` blocks for logical grouping but this is neither required nor recommended.

Jest provides a built-in `expect()` global function for making assertions. A basic test could look like this:

```js
import sum from './sum';

it('sums numbers', () => {
  expect(sum(1, 2)).toEqual(3);
  expect(sum(2, 2)).toEqual(4);
});
```

All `expect()` matchers supported by Jest are [extensively documented here](https://facebook.github.io/jest/docs/en/expect.html#content).<br>
You can also use [`jest.fn()` and `expect(fn).toBeCalled()`](https://facebook.github.io/jest/docs/en/expect.html#tohavebeencalled) to create “spies” or mock functions.

### Testing Components

There is a broad spectrum of component testing techniques. They range from a “smoke test” verifying that a component renders without throwing, to shallow rendering and testing some of the output, to full rendering and testing component lifecycle and state changes.

Different projects choose different testing tradeoffs based on how often components change, and how much logic they contain. If you haven’t decided on a testing strategy yet, we recommend that you start with creating simple smoke tests for your components:

```js
import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

it('renders without crashing', () => {
  const div = document.createElement('div');
  ReactDOM.render(<App />, div);
});
```

This test mounts a component and makes sure that it didn’t throw during rendering. Tests like this provide a lot of value with very little effort so they are great as a starting point, and this is the test you will find in `src/App.test.js`.

When you encounter bugs caused by changing components, you will gain a deeper insight into which parts of them are worth testing in your application. This might be a good time to introduce more specific tests asserting specific expected output or behavior.

If you’d like to test components in isolation from the child components they render, we recommend using [`shallow()` rendering API](http://airbnb.io/enzyme/docs/api/shallow.html) from [Enzyme](http://airbnb.io/enzyme/). To install it, run:

```sh
npm install --save enzyme enzyme-adapter-react-16 react-test-renderer
```

Alternatively you may use `yarn`:

```sh
yarn add enzyme enzyme-adapter-react-16 react-test-renderer
```

As of Enzyme 3, you will need to install Enzyme along with an Adapter corresponding to the version of React you are using. (The examples above use the adapter for React 16.)

The adapter will also need to be configured in your [global setup file](#initializing-test-environment):

#### `src/setupTests.js`
```js
import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() });
```

Now you can write a smoke test with it:

```js
import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders without crashing', () => {
  shallow(<App />);
});
```

Unlike the previous smoke test using `ReactDOM.render()`, this test only renders `<App>` and doesn’t go deeper. For example, even if `<App>` itself renders a `<Button>` that throws, this test will pass. Shallow rendering is great for isolated unit tests, but you may still want to create some full rendering tests to ensure the components integrate correctly. Enzyme supports [full rendering with `mount()`](http://airbnb.io/enzyme/docs/api/mount.html), and you can also use it for testing state changes and component lifecycle.

You can read the [Enzyme documentation](http://airbnb.io/enzyme/) for more testing techniques. Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-in `expect()` and `jest.fn()` for spies.

Here is an example from Enzyme documentation that asserts specific output, rewritten to use Jest matchers:

```js
import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders welcome message', () => {
  const wrapper = shallow(<App />);
  const welcome = <h2>Welcome to React</h2>;
  // expect(wrapper.contains(welcome)).to.equal(true);
  expect(wrapper.contains(welcome)).toEqual(true);
});
```

All Jest matchers are [extensively documented here](http://facebook.github.io/jest/docs/en/expect.html).<br>
Nevertheless you can use a third-party assertion library like [Chai](http://chaijs.com/) if you want to, as described below.

Additionally, you might find [jest-enzyme](https://github.com/blainekasten/enzyme-matchers) helpful to simplify your tests with readable matchers. The above `contains` code can be written simpler with jest-enzyme.

```js
expect(wrapper).toContainReact(welcome)
```

To enable this, install `jest-enzyme`:

```sh
npm install --save jest-enzyme
```

Alternatively you may use `yarn`:

```sh
yarn add jest-enzyme
```

Import it in [`src/setupTests.js`](#initializing-test-environment) to make its matchers available in every test:

```js
import 'jest-enzyme';
```

### Using Third Party Assertion Libraries

We recommend that you use `expect()` for assertions and `jest.fn()` for spies. If you are having issues with them please [file those against Jest](https://github.com/facebook/jest/issues/new), and we’ll fix them. We intend to keep making them better for React, supporting, for example, [pretty-printing React elements as JSX](https://github.com/facebook/jest/pull/1566).

However, if you are used to other libraries, such as [Chai](http://chaijs.com/) and [Sinon](http://sinonjs.org/), or if you have existing code using them that you’d like to port over, you can import them normally like this:

```js
import sinon from 'sinon';
import { expect } from 'chai';
```

and then use them in your tests like you normally do.

### Initializing Test Environment

>Note: this feature is available with `react-scripts@0.4.0` and higher.

If your app uses a browser API that you need to mock in your tests or if you just need a global setup before running your tests, add a `src/setupTests.js` to your project. It will be automatically executed before running your tests.

For example:

#### `src/setupTests.js`
```js
const localStorageMock = {
  getItem: jest.fn(),
  setItem: jest.fn(),
  clear: jest.fn()
};
global.localStorage = localStorageMock
```

### Focusing and Excluding Tests

You can replace `it()` with `xit()` to temporarily exclude a test from being executed.<br>
Similarly, `fit()` lets you focus on a specific test without running any other tests.

### Coverage Reporting

Jest has an integrated coverage reporter that works well with ES6 and requires no configuration.<br>
Run `npm test -- --coverage` (note extra `--` in the middle) to include a coverage report like this:

![coverage report](http://i.imgur.com/5bFhnTS.png)

Note that tests run much slower with coverage so it is recommended to run it separately from your normal workflow.

#### Configuration

The default Jest coverage configuration can be overriden by adding any of the following supported keys to a Jest config in your package.json.

Supported overrides:
 - [`collectCoverageFrom`](https://facebook.github.io/jest/docs/en/configuration.html#collectcoveragefrom-array)
 - [`coverageReporters`](https://facebook.github.io/jest/docs/en/configuration.html#coveragereporters-array-string)
 - [`coverageThreshold`](https://facebook.github.io/jest/docs/en/configuration.html#coveragethreshold-object)
 - [`snapshotSerializers`](https://facebook.github.io/jest/docs/en/configuration.html#snapshotserializers-array-string)

Example package.json:

```json
{
  "name": "your-package",
  "jest": {
    "collectCoverageFrom" : [
      "src/**/*.{js,jsx}",
      "!<rootDir>/node_modules/",
      "!<rootDir>/path/to/dir/"
    ],
    "coverageThreshold": {
      "global": {
        "branches": 90,
        "functions": 90,
        "lines": 90,
        "statements": 90
      }
    },
    "coverageReporters": ["text"],
    "snapshotSerializers": ["my-serializer-module"]
  }
}
```

### Continuous Integration

By default `npm test` runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called `CI`.

When creating a build of your application with `npm run build` linter warnings are not checked by default. Like `npm test`, you can force the build to perform a linter warning check by setting the environment variable `CI`. If any warnings are encountered then the build fails.

Popular CI servers already set the environment variable `CI` by default but you can do this yourself too:

### On CI servers
#### Travis CI

1. Following the [Travis Getting started](https://docs.travis-ci.com/user/getting-started/) guide for syncing your GitHub repository with Travis.  You may need to initialize some settings manually in your [profile](https://travis-ci.org/profile) page.
1. Add a `.travis.yml` file to your git repository.
```
language: node_js
node_js:
  - 6
cache:
  directories:
    - node_modules
script:
  - npm run build
  - npm test
```
1. Trigger your first build with a git push.
1. [Customize your Travis CI Build](https://docs.travis-ci.com/user/customizing-the-build/) if needed.

#### CircleCI

Follow [this article](https://medium.com/@knowbody/circleci-and-zeits-now-sh-c9b7eebcd3c1) to set up CircleCI with a Create React App project.

### On your own environment
##### Windows (cmd.exe)

```cmd
set CI=true&&npm test
```

```cmd
set CI=true&&npm run build
```

(Note: the lack of whitespace is intentional.)

##### Linux, macOS (Bash)

```bash
CI=true npm test
```

```bash
CI=true npm run build
```

The test command will force Jest to run tests once instead of launching the watcher.

>  If you find yourself doing this often in development, please [file an issue](https://github.com/facebookincubator/create-react-app/issues/new) to tell us about your use case because we want to make watcher the best experience and are open to changing how it works to accommodate more workflows.

The build command will check for linter warnings and fail if any are found.

### Disabling jsdom

By default, the `package.json` of the generated project looks like this:

```js
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test --env=jsdom"
```

If you know that none of your tests depend on [jsdom](https://github.com/tmpvar/jsdom), you can safely remove `--env=jsdom`, and your tests will run faster:

```diff
  "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
-   "test": "react-scripts test --env=jsdom"
+   "test": "react-scripts test"
```

To help you make up your mind, here is a list of APIs that **need jsdom**:

* Any browser globals like `window` and `document`
* [`ReactDOM.render()`](https://facebook.github.io/react/docs/top-level-api.html#reactdom.render)
* [`TestUtils.renderIntoDocument()`](https://facebook.github.io/react/docs/test-utils.html#renderintodocument) ([a shortcut](https://github.com/facebook/react/blob/34761cf9a252964abfaab6faf74d473ad95d1f21/src/test/ReactTestUtils.js#L83-L91) for the above)
* [`mount()`](http://airbnb.io/enzyme/docs/api/mount.html) in [Enzyme](http://airbnb.io/enzyme/index.html)

In contrast, **jsdom is not needed** for the following APIs:

* [`TestUtils.createRenderer()`](https://facebook.github.io/react/docs/test-utils.html#shallow-rendering) (shallow rendering)
* [`shallow()`](http://airbnb.io/enzyme/docs/api/shallow.html) in [Enzyme](http://airbnb.io/enzyme/index.html)

Finally, jsdom is also not needed for [snapshot testing](http://facebook.github.io/jest/blog/2016/07/27/jest-14.html).

### Snapshot Testing

Snapshot testing is a feature of Jest that automatically generates text snapshots of your components and saves them on the disk so if the UI output changes, you get notified without manually writing any assertions on the component output. [Read more about snapshot testing.](http://facebook.github.io/jest/blog/2016/07/27/jest-14.html)

### Editor Integration

If you use [Visual Studio Code](https://code.visualstudio.com), there is a [Jest extension](https://github.com/orta/vscode-jest) which works with Create React App out of the box. This provides a lot of IDE-like features while using a text editor: showing the status of a test run with potential fail messages inline, starting and stopping the watcher automatically, and offering one-click snapshot updates.

![VS Code Jest Preview](https://cloud.githubusercontent.com/assets/49038/20795349/a032308a-b7c8-11e6-9b34-7eeac781003f.png)

## Debugging Tests

There are various ways to setup a debugger for your Jest tests. We cover debugging in Chrome and [Visual Studio Code](https://code.visualstudio.com/).

>Note: debugging tests requires Node 8 or higher.

### Debugging Tests in Chrome

Add the following to the `scripts` section in your project's `package.json`
```json
"scripts": {
    "test:debug": "react-scripts --inspect-brk test --runInBand --env=jsdom"
  }
```
Place `debugger;` statements in any test and run:
```bash
$ npm run test:debug
```

This will start running your Jest tests, but pause before executing to allow a debugger to attach to the process.

Open the following in Chrome
```
about:inspect
```

After opening that link, the Chrome Developer Tools will be displayed. Select `inspect` on your process and a breakpoint will be set at the first line of the react script (this is done simply to give you time to open the developer tools and to prevent Jest from executing before you have time to do so). Click the button that looks like a "play" button in the upper right hand side of the screen to continue execution. When Jest executes the test that contains the debugger statement, execution will pause and you can examine the current scope and call stack.

>Note: the --runInBand cli option makes sure Jest runs test in the same process rather than spawning processes for individual tests. Normally Jest parallelizes test runs across processes but it is hard to debug many processes at the same time.

### Debugging Tests in Visual Studio Code

Debugging Jest tests is supported out of the box for [Visual Studio Code](https://code.visualstudio.com).

Use the following [`launch.json`](https://code.visualstudio.com/docs/editor/debugging#_launch-configurations) configuration file:
```
{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Debug CRA Tests",
      "type": "node",
      "request": "launch",
      "runtimeExecutable": "${workspaceRoot}/node_modules/.bin/react-scripts",      
      "args": [
        "test",
        "--runInBand",
        "--no-cache",
        "--env=jsdom"
      ],
      "cwd": "${workspaceRoot}",
      "protocol": "inspector",
      "console": "integratedTerminal",
      "internalConsoleOptions": "neverOpen"
    }
  ]
}
```

## Developing Components in Isolation

Usually, in an app, you have a lot of UI components, and each of them has many different states.
For an example, a simple button component could have following states:

* In a regular state, with a text label.
* In the disabled mode.
* In a loading state.

Usually, it’s hard to see these states without running a sample app or some examples.

Create React App doesn’t include any tools for this by default, but you can easily add [Storybook for React](https://storybook.js.org) ([source](https://github.com/storybooks/storybook)) or [React Styleguidist](https://react-styleguidist.js.org/) ([source](https://github.com/styleguidist/react-styleguidist)) to your project. **These are third-party tools that let you develop components and see all their states in isolation from your app**.

![Storybook for React Demo](http://i.imgur.com/7CIAWpB.gif)

You can also deploy your Storybook or style guide as a static app. This way, everyone in your team can view and review different states of UI components without starting a backend server or creating an account in your app.

### Getting Started with Storybook

Storybook is a development environment for React UI components. It allows you to browse a component library, view the different states of each component, and interactively develop and test components.

First, install the following npm package globally:

```sh
npm install -g @storybook/cli
```

Then, run the following command inside your app’s directory:

```sh
getstorybook
```

After that, follow the instructions on the screen.

Learn more about React Storybook:

* Screencast: [Getting Started with React Storybook](https://egghead.io/lessons/react-getting-started-with-react-storybook)
* [GitHub Repo](https://github.com/storybooks/storybook)
* [Documentation](https://storybook.js.org/basics/introduction/)
* [Snapshot Testing UI](https://github.com/storybooks/storybook/tree/master/addons/storyshots) with Storybook + addon/storyshot

### Getting Started with Styleguidist

Styleguidist combines a style guide, where all your components are presented on a single page with their props documentation and usage examples, with an environment for developing components in isolation, similar to Storybook. In Styleguidist you write examples in Markdown, where each code snippet is rendered as a live editable playground.

First, install Styleguidist:

```sh
npm install --save react-styleguidist
```

Alternatively you may use `yarn`:

```sh
yarn add react-styleguidist
```

Then, add these scripts to your `package.json`:

```diff
   "scripts": {
+    "styleguide": "styleguidist server",
+    "styleguide:build": "styleguidist build",
     "start": "react-scripts start",
```

Then, run the following command inside your app’s directory:

```sh
npm run styleguide
```

After that, follow the instructions on the screen.

Learn more about React Styleguidist:

* [GitHub Repo](https://github.com/styleguidist/react-styleguidist)
* [Documentation](https://react-styleguidist.js.org/docs/getting-started.html)

## Making a Progressive Web App

By default, the production build is a fully functional, offline-first
[Progressive Web App](https://developers.google.com/web/progressive-web-apps/).

Progressive Web Apps are faster and more reliable than traditional web pages, and provide an engaging mobile experience:

 * All static site assets are cached so that your page loads fast on subsequent visits, regardless of network connectivity (such as 2G or 3G). Updates are downloaded in the background.
 * Your app will work regardless of network state, even if offline. This means your users will be able to use your app at 10,000 feet and on the subway.
 * On mobile devices, your app can be added directly to the user's home screen, app icon and all. You can also re-engage users using web **push notifications**. This eliminates the need for the app store.

The [`sw-precache-webpack-plugin`](https://github.com/goldhand/sw-precache-webpack-plugin)
is integrated into production configuration,
and it will take care of generating a service worker file that will automatically
precache all of your local assets and keep them up to date as you deploy updates.
The service worker will use a [cache-first strategy](https://developers.google.com/web/fundamentals/instant-and-offline/offline-cookbook/#cache-falling-back-to-network)
for handling all requests for local assets, including the initial HTML, ensuring
that your web app is reliably fast, even on a slow or unreliable network.

### Opting Out of Caching

If you would prefer not to enable service workers prior to your initial
production deployment, then remove the call to `registerServiceWorker()`
from [`src/index.js`](src/index.js).

If you had previously enabled service workers in your production deployment and
have decided that you would like to disable them for all your existing users,
you can swap out the call to `registerServiceWorker()` in
[`src/index.js`](src/index.js) first by modifying the service worker import:
```javascript
import { unregister } from './registerServiceWorker';
```
and then call `unregister()` instead.
After the user visits a page that has `unregister()`,
the service worker will be uninstalled. Note that depending on how `/service-worker.js` is served,
it may take up to 24 hours for the cache to be invalidated.

### Offline-First Considerations

1. Service workers [require HTTPS](https://developers.google.com/web/fundamentals/getting-started/primers/service-workers#you_need_https),
although to facilitate local testing, that policy
[does not apply to `localhost`](http://stackoverflow.com/questions/34160509/options-for-testing-service-workers-via-http/34161385#34161385).
If your production web server does not support HTTPS, then the service worker
registration will fail, but the rest of your web app will remain functional.

1. Service workers are [not currently supported](https://jakearchibald.github.io/isserviceworkerready/)
in all web browsers. Service worker registration [won't be attempted](src/registerServiceWorker.js)
on browsers that lack support.

1. The service worker is only enabled in the [production environment](#deployment),
e.g. the output of `npm run build`. It's recommended that you do not enable an
offline-first service worker in a development environment, as it can lead to
frustration when previously cached assets are used and do not include the latest
changes you've made locally.

1. If you *need* to test your offline-first service worker locally, build
the application (using `npm run build`) and run a simple http server from your
build directory. After running the build script, `create-react-app` will give
instructions for one way to test your production build locally and the [deployment instructions](#deployment) have
instructions for using other methods. *Be sure to always use an
incognito window to avoid complications with your browser cache.*

1. If possible, configure your production environment to serve the generated
`service-worker.js` [with HTTP caching disabled](http://stackoverflow.com/questions/38843970/service-worker-javascript-update-frequency-every-24-hours).
If that's not possible—[GitHub Pages](#github-pages), for instance, does not
allow you to change the default 10 minute HTTP cache lifetime—then be aware
that if you visit your production site, and then revisit again before
`service-worker.js` has expired from your HTTP cache, you'll continue to get
the previously cached assets from the service worker. If you have an immediate
need to view your updated production deployment, performing a shift-refresh
will temporarily disable the service worker and retrieve all assets from the
network.

1. Users aren't always familiar with offline-first web apps. It can be useful to
[let the user know](https://developers.google.com/web/fundamentals/instant-and-offline/offline-ux#inform_the_user_when_the_app_is_ready_for_offline_consumption)
when the service worker has finished populating your caches (showing a "This web
app works offline!" message) and also let them know when the service worker has
fetched the latest updates that will be available the next time they load the
page (showing a "New content is available; please refresh." message). Showing
this messages is currently left as an exercise to the developer, but as a
starting point, you can make use of the logic included in [`src/registerServiceWorker.js`](src/registerServiceWorker.js), which
demonstrates which service worker lifecycle events to listen for to detect each
scenario, and which as a default, just logs appropriate messages to the
JavaScript console.

1. By default, the generated service worker file will not intercept or cache any
cross-origin traffic, like HTTP [API requests](#integrating-with-an-api-backend),
images, or embeds loaded from a different domain. If you would like to use a
runtime caching strategy for those requests, you can [`eject`](#npm-run-eject)
and then configure the
[`runtimeCaching`](https://github.com/GoogleChrome/sw-precache#runtimecaching-arrayobject)
option in the `SWPrecacheWebpackPlugin` section of
[`webpack.config.prod.js`](../config/webpack.config.prod.js).

### Progressive Web App Metadata

The default configuration includes a web app manifest located at
[`public/manifest.json`](public/manifest.json), that you can customize with
details specific to your web application.

When a user adds a web app to their homescreen using Chrome or Firefox on
Android, the metadata in [`manifest.json`](public/manifest.json) determines what
icons, names, and branding colors to use when the web app is displayed.
[The Web App Manifest guide](https://developers.google.com/web/fundamentals/engage-and-retain/web-app-manifest/)
provides more context about what each field means, and how your customizations
will affect your users' experience.

## Analyzing the Bundle Size

[Source map explorer](https://www.npmjs.com/package/source-map-explorer) analyzes
JavaScript bundles using the source maps. This helps you understand where code
bloat is coming from.

To add Source map explorer to a Create React App project, follow these steps:

```sh
npm install --save source-map-explorer
```

Alternatively you may use `yarn`:

```sh
yarn add source-map-explorer
```

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

```diff
   "scripts": {
+    "analyze": "source-map-explorer build/static/js/main.*",
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "react-scripts test --env=jsdom",
```

Then to analyze the bundle run the production build then run the analyze
script.

```
npm run build
npm run analyze
```

## Deployment

`npm run build` creates a `build` directory with a production build of your app. Set up your favorite HTTP server so that a visitor to your site is served `index.html`, and requests to static paths like `/static/js/main.<hash>.js` are served with the contents of the `/static/js/main.<hash>.js` file.

### Static Server

For environments using [Node](https://nodejs.org/), the easiest way to handle this would be to install [serve](https://github.com/zeit/serve) and let it handle the rest:

```sh
npm install -g serve
serve -s build
```

The last command shown above will serve your static site on the port **5000**. Like many of [serve](https://github.com/zeit/serve)’s internal settings, the port can be adjusted using the `-p` or `--port` flags.

Run this command to get a full list of the options available:

```sh
serve -h
```

### Other Solutions

You don’t necessarily need a static server in order to run a Create React App project in production. It works just as fine integrated into an existing dynamic one.

Here’s a programmatic example using [Node](https://nodejs.org/) and [Express](http://expressjs.com/):

```javascript
const express = require('express');
const path = require('path');
const app = express();

app.use(express.static(path.join(__dirname, 'build')));

app.get('/', function (req, res) {
  res.sendFile(path.join(__dirname, 'build', 'index.html'));
});

app.listen(9000);
```

The choice of your server software isn’t important either. Since Create React App is completely platform-agnostic, there’s no need to explicitly use Node.

The `build` folder with static assets is the only output produced by Create React App.

However this is not quite enough if you use client-side routing. Read the next section if you want to support URLs like `/todos/42` in your single-page app.

### Serving Apps with Client-Side Routing

If you use routers that use the HTML5 [`pushState` history API](https://developer.mozilla.org/en-US/docs/Web/API/History_API#Adding_and_modifying_history_entries) under the hood (for example, [React Router](https://github.com/ReactTraining/react-router) with `browserHistory`), many static file servers will fail. For example, if you used React Router with a route for `/todos/42`, the development server will respond to `localhost:3000/todos/42` properly, but an Express serving a production build as above will not.

This is because when there is a fresh page load for a `/todos/42`, the server looks for the file `build/todos/42` and does not find it. The server needs to be configured to respond to a request to `/todos/42` by serving `index.html`. For example, we can amend our Express example above to serve `index.html` for any unknown paths:

```diff
 app.use(express.static(path.join(__dirname, 'build')));

-app.get('/', function (req, res) {
+app.get('/*', function (req, res) {
   res.sendFile(path.join(__dirname, 'build', 'index.html'));
 });
```

If you’re using [Apache HTTP Server](https://httpd.apache.org/), you need to create a `.htaccess` file in the `public` folder that looks like this:

```
    Options -MultiViews
    RewriteEngine On
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteRule ^ index.html [QSA,L]
```

It will get copied to the `build` folder when you run `npm run build`. 

If you’re using [Apache Tomcat](http://tomcat.apache.org/), you need to follow [this Stack Overflow answer](https://stackoverflow.com/a/41249464/4878474).

Now requests to `/todos/42` will be handled correctly both in development and in production.

On a production build, and in a browser that supports [service workers](https://developers.google.com/web/fundamentals/getting-started/primers/service-workers),
the service worker will automatically handle all navigation requests, like for
`/todos/42`, by serving the cached copy of your `index.html`. This
service worker navigation routing can be configured or disabled by
[`eject`ing](#npm-run-eject) and then modifying the
[`navigateFallback`](https://github.com/GoogleChrome/sw-precache#navigatefallback-string)
and [`navigateFallbackWhitelist`](https://github.com/GoogleChrome/sw-precache#navigatefallbackwhitelist-arrayregexp)
options of the `SWPreachePlugin` [configuration](../config/webpack.config.prod.js).

When users install your app to the homescreen of their device the default configuration will make a shortcut to `/index.html`. This may not work for client-side routers which expect the app to be served from `/`. Edit the web app manifest at [`public/manifest.json`](public/manifest.json) and change `start_url` to match the required URL scheme, for example:

```js
  "start_url": ".",
```

### Building for Relative Paths

By default, Create React App produces a build assuming your app is hosted at the server root.<br>
To override this, specify the `homepage` in your `package.json`, for example:

```js
  "homepage": "http://mywebsite.com/relativepath",
```

This will let Create React App correctly infer the root path to use in the generated HTML file.

**Note**: If you are using `react-router@^4`, you can root `<Link>`s using the `basename` prop on any `<Router>`.<br>
More information [here](https://reacttraining.com/react-router/web/api/BrowserRouter/basename-string).<br>
<br>
For example:
```js
<BrowserRouter basename="/calendar"/>
<Link to="/today"/> // renders <a href="/calendar/today">
```

#### Serving the Same Build from Different Paths

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

If you are not using the HTML5 `pushState` history API or not using client-side routing at all, it is unnecessary to specify the URL from which your app will be served. Instead, you can put this in your `package.json`:

```js
  "homepage": ".",
```

This will make sure that all the asset paths are relative to `index.html`. You will then be able to move your app from `http://mywebsite.com` to `http://mywebsite.com/relativepath` or even `http://mywebsite.com/relative/path` without having to rebuild it.

### [Azure](https://azure.microsoft.com/)

See [this](https://medium.com/@to_pe/deploying-create-react-app-on-microsoft-azure-c0f6686a4321) blog post on how to deploy your React app to Microsoft Azure.

### [Firebase](https://firebase.google.com/)

Install the Firebase CLI if you haven’t already by running `npm install -g firebase-tools`. Sign up for a [Firebase account](https://console.firebase.google.com/) and create a new project. Run `firebase login` and login with your previous created Firebase account.

Then run the `firebase init` command from your project’s root. You need to choose the **Hosting: Configure and deploy Firebase Hosting sites** and choose the Firebase project you created in the previous step. You will need to agree with `database.rules.json` being created, choose `build` as the public directory, and also agree to **Configure as a single-page app** by replying with `y`.

```sh
    === Project Setup

    First, let's associate this project directory with a Firebase project.
    You can create multiple project aliases by running firebase use --add,
    but for now we'll just set up a default project.

    ? What Firebase project do you want to associate as default? Example app (example-app-fd690)

    === Database Setup

    Firebase Realtime Database Rules allow you to define how your data should be
    structured and when your data can be read from and written to.

    ? What file should be used for Database Rules? database.rules.json
    ✔  Database Rules for example-app-fd690 have been downloaded to database.rules.json.
    Future modifications to database.rules.json will update Database Rules when you run
    firebase deploy.

    === Hosting Setup

    Your public directory is the folder (relative to your project directory) that