README.md

- It is an experimental proposal and is subject to change.
- The current specification version is not officially supported by Babel.
- If the specification changes, we won’t be able to write a codemod because we don’t use them internally at Facebook.

However in many cases you can rewrite decorator-based code without decorators just as fine.<br>
Please refer to these two threads for reference:

- [#214](https://github.com/facebook/create-react-app/issues/214)
- [#411](https://github.com/facebook/create-react-app/issues/411)

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

## Fetching Data with AJAX Requests

React doesn't prescribe a specific approach to data fetching, but people commonly use either a library like [axios](https://github.com/axios/axios) or the [`fetch()` API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) provided by the browser.

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

A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises [here](https://www.promisejs.org/) and [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise). Both axios and `fetch()` use Promises under the hood. You can also use the [`async / await`](https://davidwalsh.name/async-await) syntax to reduce the callback nesting.

Make sure the [`fetch()` API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) and [Promises](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) are available in your target audience's browsers.
For example, support in Internet Explorer requires a [polyfill](https://github.com/facebook/create-react-app/blob/next/packages/react-app-polyfill/README.md).

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

## Integrating with an API Backend

These tutorials will help you to integrate your app with an API backend running on another port,
using `fetch()` to access it.

### Node

Check out [this tutorial](https://www.fullstackreact.com/articles/using-create-react-app-with-a-server/).
You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo).

### Ruby on Rails

Check out [this tutorial](https://www.fullstackreact.com/articles/how-to-get-create-react-app-to-work-with-your-rails-api/).
You can find the companion GitHub repository [here](https://github.com/fullstackreact/food-lookup-demo-rails).

### API Platform (PHP and Symfony)

[API Platform](https://api-platform.com) is a framework designed to build API-driven projects.
It allows to create hypermedia and GraphQL APIs in minutes.
It is shipped with an official Progressive Web App generator as well as a dynamic administration interface, both built for Create React App.
Check out [this tutorial](https://api-platform.com/docs/distribution).

## Proxying API Requests in Development

> 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/facebook/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@2.0.0` and higher.

If the `proxy` option is **not** flexible enough for you, you can get direct access to the Express app instance and hook up your own proxy middleware.

First, install `http-proxy-middleware` using npm or Yarn:

```bash
$ npm install http-proxy-middleware --save
$ # or
$ yarn add http-proxy-middleware
```

Next, create `src/setupProxy.js` and place the following contents in it:

```js
const proxy = require('http-proxy-middleware');

module.exports = function(app) {
  // ...
};
```

You can now register proxies as you wish! Here's an example using the above `http-proxy-middleware`:

```js
const proxy = require('http-proxy-middleware');

module.exports = function(app) {
  app.use(proxy('/api', { target: 'http://localhost:5000/' }));
};
```

## 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.)

#### Windows (Powershell)

```Powershell
($env:HTTPS = $true) -and (npm start)
```

#### 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/facebook/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() });
```

> Note: Keep in mind that if you decide to "eject" before creating `src/setupTests.js`, the resulting `package.json` file won't contain any reference to it. [Read here](#initializing-test-environment) to learn how to add this after ejecting.

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)).toBe(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 more simply 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;
```

> Note: Keep in mind that if you decide to "eject" before creating `src/setupTests.js`, the resulting `package.json` file won't contain any reference to it, so you should manually create the property `setupTestFrameworkScriptFile` in the configuration for Jest, something like the following:

> ```js
> "jest": {
>   // ...
>   "setupTestFrameworkScriptFile": "<rootDir>/src/setupTests.js"
>  }
> ```

### 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 overridden 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:
  - 8
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.)

##### Windows (Powershell)

```Powershell
($env:CI = $true) -and (npm test)
```

```Powershell
($env:CI = $true) -and (npm run build)
```

##### 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/facebook/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

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

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

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"
  }
```

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": "${workspaceRoot}/node_modules/.bin/react-scripts",
      "args": [
        "test",
        "--runInBand",
        "--no-cache"
      ],
      "cwd": "${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:

- [Learn Storybook (tutorial)](https://learnstorybook.com)
- [Documentation](https://storybook.js.org/basics/introduction/)
- [GitHub Repo](https://github.com/storybooks/storybook)
- [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)

## Publishing Components to npm

Create React App doesn't provide any built-in functionality to publish a component to npm. If you're ready to extract a component from your project so other people can use it, we recommend moving it to a separate directory outside of your project and then using a tool like [nwb](https://github.com/insin/nwb#react-components-and-libraries) to prepare it for publishing.

## Making a Progressive Web App

The production build has all the tools necessary to generate a first-class
[Progressive Web App](https://developers.google.com/web/progressive-web-apps/),
but **the offline/cache-first behavior is opt-in only**. By default,
the build process will generate a service worker file, but it will not be
registered, so it will not take control of your production web app.

In order to opt-in to the offline-first behavior, developers should look for the
following in their [`src/index.js`](src/index.js) file:

```js
// If you want your app to work offline and load faster, you can change
// unregister() to register() below. Note this comes with some pitfalls.
// Learn more about service workers: http://bit.ly/CRA-PWA
serviceWorker.unregister();
```

As the comment states, switching `serviceWorker.unregister()` to
`serviceWorker.register()` will opt you in to using the service worker.

### Why Opt-in?

Offline-first 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. This eliminates the need for the app store.

The [`workbox-webpack-plugin`](https://developers.google.com/web/tools/workbox/modules/workbox-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
[navigation requests](https://developers.google.com/web/fundamentals/primers/service-workers/high-performance-loading#first_what_are_navigation_requests)
for your HTML, ensuring that your web app is consistently fast, even on a slow
or unreliable network.

### Offline-First Considerations

If you do decide to opt-in to service worker registration, please take the
following into account:

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 supported](https://jakearchibald.github.io/isserviceworkerready/#moar)
   in older 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. 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.

### 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.

Progressive web apps that have been added to the homescreen will load faster and
work offline when there's an active service worker. That being said, the
metadata from the web app manifest will still be used regardless of whether or
not you opt-in to service worker registration.

## 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",
```

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 when you've [opted-in](#why-opt-in),
a [service worker](https://developers.google.com/web/fundamentals/primers/service-workers/) 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.

### Customizing Environment Variables for Arbitrary Build Environments

You can create an arbitrary build environment by creating a custom `.env` file and loading it using [env-cmd](https://www.npmjs.com/package/env-cmd).

For example, to create a build environment for a staging environment:

1. Create a file called `.env.staging`