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---
id: user-guide
title: User guide
sidebar_label: User guide
---
This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).
Below you will find some information on how to perform common tasks.<br>
You can find the most recent version of this guide [here](https://github.com/facebook/create-react-app/blob/master/packages/react-scripts/template/README.md).
## Table of Contents
- [Adding Bootstrap](#adding-bootstrap)
- [Using a Custom Theme](#using-a-custom-theme)
- [Adding Flow](#adding-flow)
- [Adding Relay](#adding-relay)
- [Adding a Router](#adding-a-router)
- [Adding Custom Environment Variables](#adding-custom-environment-variables)
- [Referencing Environment Variables in the HTML](#referencing-environment-variables-in-the-html)
- [Adding Temporary Environment Variables In Your Shell](#adding-temporary-environment-variables-in-your-shell)
- [Adding Development Environment Variables In `.env`](#adding-development-environment-variables-in-env)
- [Can I Use Decorators?](#can-i-use-decorators)
- [Fetching Data with AJAX Requests](#fetching-data-with-ajax-requests)
- [Integrating with an API Backend](#integrating-with-an-api-backend)
- [Node](#node)
- [Ruby on Rails](#ruby-on-rails)
- [Using HTTPS in Development](#using-https-in-development)
- [Generating Dynamic `<meta>` Tags on the Server](#generating-dynamic-meta-tags-on-the-server)
- [Pre-Rendering into Static HTML Files](#pre-rendering-into-static-html-files)
- [Injecting Data from the Server into the Page](#injecting-data-from-the-server-into-the-page)
- [Making a Progressive Web App](#making-a-progressive-web-app)
- [Why Opt-in?](#why-opt-in)
- [Offline-First Considerations](#offline-first-considerations)
- [Progressive Web App Metadata](#progressive-web-app-metadata)
- [Advanced Configuration](#advanced-configuration)
- [Alternatives to Ejecting](#alternatives-to-ejecting)
## Adding Bootstrap
You don’t have to use [reactstrap](https://reactstrap.github.io/) together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:
Install reactstrap and Bootstrap from npm. reactstrap does not include Bootstrap CSS so this needs to be installed as well:
```sh
npm install --save reactstrap bootstrap@4
```
Alternatively you may use `yarn`:
```sh
yarn add bootstrap@4 reactstrap
```
Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your `src/index.js` file:
```js
import 'bootstrap/dist/css/bootstrap.css';
// Put any other imports below so that CSS from your
// components takes precedence over default styles.
```
Import required reactstrap components within `src/App.js` file or your custom component files:
```js
import { Button } from 'reactstrap';
```
Now you are ready to use the imported reactstrap components within your component hierarchy defined in the render method. Here is an example [`App.js`](https://gist.githubusercontent.com/zx6658/d9f128cd57ca69e583ea2b5fea074238/raw/a56701c142d0c622eb6c20a457fbc01d708cb485/App.js) redone using reactstrap.
### Using a Custom Theme
> Note: this feature is available with `react-scripts@2.0.0` and higher.
Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br>
As of `react-scripts@2.0.0` you can import `.scss` files. This makes it possible to use a package's built-in Sass variables for global style preferences.
To customize Bootstrap, create a file called `src/custom.scss` (or similar) and import the Bootstrap source stylesheet. Add any overrides _before_ the imported file(s). You can reference [Bootstrap's documentation](http://getbootstrap.com/docs/4.1/getting-started/theming/#css-variables) for the names of the available variables.
```scss
// Override default variables before the import
$body-bg: #000;
// Import Bootstrap and its default variables
@import '~bootstrap/scss/bootstrap.scss';
```
> **Note:** You must prefix imports from `node_modules` with `~` as displayed above.
Finally, import the newly created `.scss` file instead of the default Bootstrap `.css` in the beginning of your `src/index.js` file, for example:
```javascript
import './custom.scss';
```
## Adding Flow
Flow is a static type checker that helps you write code with fewer bugs. Check out this [introduction to using static types in JavaScript](https://medium.com/@preethikasireddy/why-use-static-types-in-javascript-part-1-8382da1e0adb) if you are new to this concept.
Recent versions of [Flow](https://flow.org/) work with Create React App projects out of the box.
To add Flow to a Create React App project, follow these steps:
1. Run `npm install --save flow-bin` (or `yarn add flow-bin`).
2. Add `"flow": "flow"` to the `scripts` section of your `package.json`.
3. Run `npm run flow init` (or `yarn flow init`) to create a [`.flowconfig` file](https://flow.org/en/docs/config/) in the root directory.
4. Add `// @flow` to any files you want to type check (for example, to `src/App.js`).
Now you can run `npm run flow` (or `yarn flow`) to check the files for type errors.
You can optionally use an IDE like [Nuclide](https://nuclide.io/docs/languages/flow/) for a better integrated experience.
In the future we plan to integrate it into Create React App even more closely.
To learn more about Flow, check out [its documentation](https://flow.org/).
## Adding Relay
Relay is a framework for building data-driven React applications powered by GraphQL. The current release candidate of Relay works with Create React App projects out of the box using Babel Macros. Simply set up your project as laid out in the [Relay documentation](https://facebook.github.io/relay/), then make sure you have a version of the babel plugin providing the macro.
To add it, run:
```sh
npm install --save --dev babel-plugin-relay@dev
```
Alternatively you may use `yarn`:
```sh
yarn upgrade babel-plugin-relay@dev
```
Then, wherever you use the `graphql` template tag, import the macro:
```js
import graphql from 'babel-plugin-relay/macro';
// instead of:
// import { graphql } from "babel-plugin-relay"
graphql`
query UserQuery {
viewer {
id
}
}
`;
```
To learn more about Relay, check out [its documentation](https://facebook.github.io/relay/).
## Adding a Router
Create React App doesn't prescribe a specific routing solution, but [React Router](https://reacttraining.com/react-router/web/) is the most popular one.
To add it, run:
```sh
npm install --save react-router-dom
```
Alternatively you may use `yarn`:
```sh
yarn add react-router-dom
```
To try it, delete all the code in `src/App.js` and replace it with any of the examples on its website. The [Basic Example](https://reacttraining.com/react-router/web/example/basic) is a good place to get started.
Note that [you may need to configure your production server to support client-side routing](#serving-apps-with-client-side-routing) before deploying your app.
## Adding Custom Environment Variables
> Note: this feature is available with `react-scripts@0.2.3` and higher.
Your project can consume variables declared in your environment as if they were declared locally in your JS files. By
default you will have `NODE_ENV` defined for you, and any other environment variables starting with
`REACT_APP_`.
**The environment variables are embedded during the build time**. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, just like [described here](#injecting-data-from-the-server-into-the-page). Alternatively you can rebuild the app on the server anytime you change them.
> Note: You must create custom environment variables beginning with `REACT_APP_`. Any other variables except `NODE_ENV` will be ignored to avoid accidentally [exposing a private key on the machine that could have the same name](https://github.com/facebook/create-react-app/issues/865#issuecomment-252199527). Changing any environment variables will require you to restart the development server if it is running.
These environment variables will be defined for you on `process.env`. For example, having an environment
variable named `REACT_APP_SECRET_CODE` will be exposed in your JS as `process.env.REACT_APP_SECRET_CODE`.
There is also a special built-in environment variable called `NODE_ENV`. You can read it from `process.env.NODE_ENV`. When you run `npm start`, it is always equal to `'development'`, when you run `npm test` it is always equal to `'test'`, and when you run `npm run build` to make a production bundle, it is always equal to `'production'`. **You cannot override `NODE_ENV` manually.** This prevents developers from accidentally deploying a slow development build to production.
These environment variables can be useful for displaying information conditionally based on where the project is
deployed or consuming sensitive data that lives outside of version control.
First, you need to have environment variables defined. For example, let’s say you wanted to consume a secret defined
in the environment inside a `<form>`:
```jsx
render() {
return (
<div>
<small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
<form>
<input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
</form>
</div>
);
}
```
During the build, `process.env.REACT_APP_SECRET_CODE` will be replaced with the current value of the `REACT_APP_SECRET_CODE` environment variable. Remember that the `NODE_ENV` variable will be set for you automatically.
When you load the app in the browser and inspect the `<input>`, you will see its value set to `abcdef`, and the bold text will show the environment provided when using `npm start`:
```html
<div>
<small>You are running this application in <b>development</b> mode.</small>
<form>
<input type="hidden" value="abcdef" />
</form>
</div>
```
The above form is looking for a variable called `REACT_APP_SECRET_CODE` from the environment. In order to consume this
value, we need to have it defined in the environment. This can be done using two ways: either in your shell or in
a `.env` file. Both of these ways are described in the next few sections.
Having access to the `NODE_ENV` is also useful for performing actions conditionally:
```js
if (process.env.NODE_ENV !== 'production') {
analytics.disable();
}
```
When you compile the app with `npm run build`, the minification step will strip out this condition, and the resulting bundle will be smaller.
### Referencing Environment Variables in the HTML
> Note: this feature is available with `react-scripts@0.9.0` and higher.
You can also access the environment variables starting with `REACT_APP_` in the `public/index.html`. For example:
```html
<title>%REACT_APP_WEBSITE_NAME%</title>
```
Note that the caveats from the above section apply:
- Apart from a few built-in variables (`NODE_ENV` and `PUBLIC_URL`), variable names must start with `REACT_APP_` to work.
- The environment variables are injected at build time. If you need to inject them at runtime, [follow this approach instead](#generating-dynamic-meta-tags-on-the-server).
### Adding Temporary Environment Variables In Your Shell
Defining environment variables can vary between OSes. It’s also important to know that this manner is temporary for the
life of the shell session.
#### Windows (cmd.exe)
```cmd
set "REACT_APP_SECRET_CODE=abcdef" && npm start
```
(Note: Quotes around the variable assignment are required to avoid a trailing whitespace.)
#### Windows (Powershell)
```Powershell
($env:REACT_APP_SECRET_CODE = "abcdef") -and (npm start)
```
#### Linux, macOS (Bash)
```bash
REACT_APP_SECRET_CODE=abcdef npm start
```
### Adding Development Environment Variables In `.env`
> Note: this feature is available with `react-scripts@0.5.0` and higher.
To define permanent environment variables, create a file called `.env` in the root of your project:
```
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](https://github.com/facebook/create-react-app/issues/865#issuecomment-252199527). 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.<br>
Please refer to the [dotenv documentation](https://github.com/motdotla/dotenv) 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](https://docs.travis-ci.com/user/environment-variables/) or [Heroku](https://devcenter.heroku.com/articles/config-vars).
#### 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](https://github.com/motdotla/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?
Some popular libraries use [decorators](https://medium.com/google-developers/exploring-es7-decorators-76ecb65fb841) in their documentation.<br>
Create React App intentionally doesn’t support decorator syntax at the moment because:
- It is an experimental proposal and is subject to change (in fact, it has already changed once, and will change again).
- Most libraries currently support only the old version of the proposal — which will never be a standard.
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/master/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).
## 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.**
## 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.
However, they [can make debugging deployments more challenging](https://github.com/facebook/create-react-app/issues/2398) so, starting with Create React App 2, service workers are opt-in.
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.
## Advanced Configuration
You can adjust various development and production settings by setting environment variables in your shell or with [.env](#adding-development-environment-variables-in-env).
| Variable | Development | Production | Usage |
| :------------------ | :--------------------: | :----------------: | :----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| BROWSER | :white_check_mark: | :x: | By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a [browser](https://github.com/sindresorhus/opn#app) to override this behavior, or set it to `none` to disable it completely. If you need to customize the way the browser is launched, you can specify a node script instead. Any arguments passed to `npm start` will also be passed to this script, and the url where your app is served will be the last argument. Your script's file name must have the `.js` extension. |
| HOST | :white_check_mark: | :x: | By default, the development web server binds to `localhost`. You may use this variable to specify a different host. |
| PORT | :white_check_mark: | :x: | By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port. |
| HTTPS | :white_check_mark: | :x: | When set to `true`, Create React App will run the development server in `https` mode. |
| PUBLIC_URL | :x: | :white_check_mark: | Create React App assumes your application is hosted at the serving web server's root or a subpath as specified in [`package.json` (`homepage`)](#building-for-relative-paths). Normally, Create React App ignores the hostname. You may use this variable to force assets to be referenced verbatim to the url you provide (hostname included). This may be particularly useful when using a CDN to host your application. |
| CI | :large_orange_diamond: | :white_check_mark: | When set to `true`, Create React App treats warnings as failures in the build. It also makes the test runner non-watching. Most CIs set this flag by default. |
| REACT_EDITOR | :white_check_mark: | :x: | When an app crashes in development, you will see an error overlay with clickable stack trace. When you click on it, Create React App will try to determine the editor you are using based on currently running processes, and open the relevant source file. You can [send a pull request to detect your editor of choice](https://github.com/facebook/create-react-app/issues/2636). Setting this environment variable overrides the automatic detection. If you do it, make sure your systems [PATH](<https://en.wikipedia.org/wiki/PATH_(variable)>) environment variable points to your editor’s bin folder. You can also set it to `none` to disable it completely. |
| CHOKIDAR_USEPOLLING | :white_check_mark: | :x: | When set to `true`, the watcher runs in polling mode, as necessary inside a VM. Use this option if `npm start` isn't detecting changes. |
| GENERATE_SOURCEMAP | :x: | :white_check_mark: | When set to `false`, source maps are not generated for a production build. This solves OOM issues on some smaller machines. |
| NODE_PATH | :white_check_mark: | :white_check_mark: | Same as [`NODE_PATH` in Node.js](https://nodejs.org/api/modules.html#modules_loading_from_the_global_folders), but only relative folders are allowed. Can be handy for emulating a monorepo setup by setting `NODE_PATH=src`. |
## Alternatives to Ejecting
[Ejecting](#npm-run-eject) lets you customize anything, but from that point on you have to maintain the configuration and scripts yourself. This can be daunting if you have many similar projects. In such cases instead of ejecting we recommend to _fork_ `react-scripts` and any other packages you need. [This article](https://auth0.com/blog/how-to-configure-create-react-app/) dives into how to do it in depth. You can find more discussion in [this issue](https://github.com/facebook/create-react-app/issues/682).
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