This project was bootstrapped with Create React App.

Below you will find some information on how to perform common tasks.

You can find the most recent version of this guide here.

Table of Contents

- Updating to New Releases
- Sending Feedback
- Folder Structure
- Available Scripts
- npm start
- npm test
- npm run build
- npm run eject
- Supported Browsers
- Supported Language Features and Polyfills
- Syntax Highlighting in the Editor
- Displaying Lint Output in the Editor
- Debugging in the Editor
- Formatting Code Automatically
- Changing the Page
`

Note that the caveats from the above section apply:

* Apart from a few built-in variables (NODE_ENV and PUBLIC_URL), variable names must start with REACT_APP_ to work.
* The environment variables are injected at build time. If you need to inject them at runtime, follow this approach instead.

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

#### Windows (cmd.exe)

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

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

#### Windows (Powershell)

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

#### Linux, macOS (Bash)

`bash
REACT_APP_SECRET_CODE=abcdef npm start
`

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

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

`
REACT_APP_SECRET_CODE=abcdef
`
>Note: You must create custom environment variables beginning with REACT_APP_. Any other variables except NODE_ENV will be ignored to avoid accidentally exposing a private key on the machine that could have the same name. Changing any environment variables will require you to restart the development server if it is running.

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

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

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

* .env: Default.
* .env.local: Local overrides. This file is loaded for all environments except test.
* .env.development, .env.test, .env.production: Environment-specific settings.
* .env.development.local, .env.test.local, .env.production.local: Local overrides of environment-specific settings.

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

* npm start: .env.development.local, .env.development, .env.local, .env
* npm run build: .env.production.local, .env.production, .env.local, .env
* npm test: .env.test.local, .env.test, .env (note .env.local is missing)

These variables will act as the defaults if the machine does not explicitly set them.

Please refer to the dotenv documentation for more details.

>Note: If you are defining environment variables for development, your CI and/or hosting platform will most likely need
these defined as well. Consult their documentation how to do this. For example, see the documentation for Travis CI or Heroku.

#### Expanding Environment Variables In .env

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

Expand variables already on your machine for use in your .env file (using dotenv-expand).

For example, to get the environment variable npm_package_version:

`
REACT_APP_VERSION=$npm_package_version


also works:

REACT_APP_VERSION=${npm_package_version}

`

Or expand variables local to the current .env file:

`
DOMAIN=www.example.com
REACT_APP_FOO=$DOMAIN/foo
REACT_APP_BAR=$DOMAIN/bar
`

Can I Use Decorators?

Many popular libraries use decorators in their documentation.

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

* 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.

Please refer to these two threads for reference:

* #214
* #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 or the fetch() API provided by the browser. Conveniently, Create React App includes a polyfill for fetch() so you can use it without worrying about the browser support.

The global fetch function allows to easily makes AJAX requests. It takes in a URL as an input and returns a Promise that resolves to a Response object. You can find more information about fetch here.

This project also includes a Promise polyfill which provides a full implementation of Promises/A+. A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises here and here. Both axios and fetch() use Promises under the hood. You can also use the async / await syntax to reduce the callback nesting.

You can learn more about making AJAX requests from React components in the FAQ entry on the React website.

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.

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Check out this tutorial.
You can find the companion GitHub repository here.

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Check out this tutorial.
You can find the companion GitHub repository here.

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.

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 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.

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

* Configure the proxy yourself
* Enable CORS on your server (here’s how to do it for Express).
* Use environment variables to inject the right server host and port into your app.

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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 and this issue.

This shouldn’t affect you when developing on localhost, but if you develop remotely like described here, 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.

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>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).

You may also specify any configuration value http-proxy-middleware or http-proxy supports.
`js
{
// ...
"proxy": {
"/api": {
"target": "",
"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": "",
"ws": true
// ...
},
// Matches any request starting with /foo
"/foo": {
"target": "",
"ssl": true,
"pathRewrite": {
"^/foo": "/foo/beta"
}
// ...
},
// Matches /bar/abc.html but not /bar/sub/def.html
"/bar/[^/]*[.]html": {
"target": "",
// ...
},
// Matches /baz/abc.html and /baz/sub/def.html
"/baz/./.[.]html": {
"target": ""
// ...
}
}
// ...
}
`

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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, 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.

There’s some good documentation available for setting up a Socket.io server.

Standard WebSockets will work with a standard WebSocket server as well as the websocket.org echo test. You can use libraries like ws for the server, with native WebSockets in the browser.

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

`js
{
// ...
"proxy": {
"/socket": {
// Your compatible WebSocket server
"target": "ws://",
// 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 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
`

#### Windows (Powershell)

`Powershell
($env:HTTPS = $true) -and (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 Tags on the Server

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

`html





`

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 or 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.

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




`

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 as it makes your app vulnerable to XSS attacks.

Running Tests

>Note: this feature is available with react-scripts@0.3.0 and higher.

>Read the migration guide to learn how to enable it in older projects!

Create React App uses Jest as its test runner. To prepare for this integration, we did a major revamp 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, 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.

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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.

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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

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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 server or if the project is not inside a Git or Mercurial repository.

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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.

You can also use jest.fn() and expect(fn).toBeCalled()` to create “spies” or mock functions.

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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 compo