Commit e49738a2 authored by Eugen Rochko's avatar Eugen Rochko

Initial commit

# See for more about ignoring files.
# dependencies
# testing
# production
# misc
This project was bootstrapped with [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](
## Table of Contents
- [Updating to New Releases](#updating-to-new-releases)
- [Sending Feedback](#sending-feedback)
- [Folder Structure](#folder-structure)
- [Available Scripts](#available-scripts)
- [npm start](#npm-start)
- [npm test](#npm-test)
- [npm run build](#npm-run-build)
- [npm run eject](#npm-run-eject)
- [Supported Language Features and Polyfills](#supported-language-features-and-polyfills)
- [Syntax Highlighting in the Editor](#syntax-highlighting-in-the-editor)
- [Displaying Lint Output in the Editor](#displaying-lint-output-in-the-editor)
- [Debugging in the Editor](#debugging-in-the-editor)
- [Changing the Page `<title>`](#changing-the-page-title)
- [Installing a Dependency](#installing-a-dependency)
- [Importing a Component](#importing-a-component)
- [Adding a Stylesheet](#adding-a-stylesheet)
- [Post-Processing CSS](#post-processing-css)
- [Adding a CSS Preprocessor (Sass, Less etc.)](#adding-a-css-preprocessor-sass-less-etc)
- [Adding Images and Fonts](#adding-images-and-fonts)
- [Using the `public` Folder](#using-the-public-folder)
- [Changing the HTML](#changing-the-html)
- [Adding Assets Outside of the Module System](#adding-assets-outside-of-the-module-system)
- [When to Use the `public` Folder](#when-to-use-the-public-folder)
- [Using Global Variables](#using-global-variables)
- [Adding Bootstrap](#adding-bootstrap)
- [Using a Custom Theme](#using-a-custom-theme)
- [Adding Flow](#adding-flow)
- [Adding 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)
- [Integrating with an API Backend](#integrating-with-an-api-backend)
- [Node](#node)
- [Ruby on Rails](#ruby-on-rails)
- [Proxying API Requests in Development](#proxying-api-requests-in-development)
- [Using HTTPS in Development](#using-https-in-development)
- [Generating Dynamic `<meta>` Tags on the Server](#generating-dynamic-meta-tags-on-the-server)
- [Pre-Rendering into Static HTML Files](#pre-rendering-into-static-html-files)
- [Injecting Data from the Server into the Page](#injecting-data-from-the-server-into-the-page)
- [Running Tests](#running-tests)
- [Filename Conventions](#filename-conventions)
- [Command Line Interface](#command-line-interface)
- [Version Control Integration](#version-control-integration)
- [Writing Tests](#writing-tests)
- [Testing Components](#testing-components)
- [Using Third Party Assertion Libraries](#using-third-party-assertion-libraries)
- [Initializing Test Environment](#initializing-test-environment)
- [Focusing and Excluding Tests](#focusing-and-excluding-tests)
- [Coverage Reporting](#coverage-reporting)
- [Continuous Integration](#continuous-integration)
- [Disabling jsdom](#disabling-jsdom)
- [Snapshot Testing](#snapshot-testing)
- [Editor Integration](#editor-integration)
- [Developing Components in Isolation](#developing-components-in-isolation)
- [Making a Progressive Web App](#making-a-progressive-web-app)
- [Deployment](#deployment)
- [Static Server](#static-server)
- [Other Solutions](#other-solutions)
- [Serving Apps with Client-Side Routing](#serving-apps-with-client-side-routing)
- [Building for Relative Paths](#building-for-relative-paths)
- [Azure](#azure)
- [Firebase](#firebase)
- [GitHub Pages](#github-pages)
- [Heroku](#heroku)
- [Modulus](#modulus)
- [Netlify](#netlify)
- [Now](#now)
- [S3 and CloudFront](#s3-and-cloudfront)
- [Surge](#surge)
- [Advanced Configuration](#advanced-configuration)
- [Troubleshooting](#troubleshooting)
- [`npm start` doesn’t detect changes](#npm-start-doesnt-detect-changes)
- [`npm test` hangs on macOS Sierra](#npm-test-hangs-on-macos-sierra)
- [`npm run build` silently fails](#npm-run-build-silently-fails)
- [`npm run build` fails on Heroku](#npm-run-build-fails-on-heroku)
- [Something Missing?](#something-missing)
## Updating to New Releases
Create React App is divided into two packages:
* `create-react-app` is a global command-line utility that you use to create new projects.
* `react-scripts` is a development dependency in the generated projects (including this one).
You almost never need to update `create-react-app` itself: it delegates all the setup to `react-scripts`.
When you run `create-react-app`, it always creates the project with the latest version of `react-scripts` so you’ll get all the new features and improvements in newly created apps automatically.
To update an existing project to a new version of `react-scripts`, [open the changelog](, find the version you’re currently on (check `package.json` in this folder if you’re not sure), and apply the migration instructions for the newer versions.
In most cases bumping the `react-scripts` version in `package.json` and running `npm install` in this folder should be enough, but it’s good to consult the [changelog]( for potential breaking changes.
We commit to keeping the breaking changes minimal so you can upgrade `react-scripts` painlessly.
## Sending Feedback
We are always open to [your feedback](
## Folder Structure
After creation, your project should look like this:
For the project to build, **these files must exist with exact filenames**:
* `public/index.html` is the page template;
* `src/index.js` is the JavaScript entry point.
You can delete or rename the other files.
You may create subdirectories inside `src`. For faster rebuilds, only files inside `src` are processed by Webpack.<br>
You need to **put any JS and CSS files inside `src`**, or Webpack won’t see them.
Only files inside `public` can be used from `public/index.html`.<br>
Read instructions below for using assets from JavaScript and HTML.
You can, however, create more top-level directories.<br>
They will not be included in the production build so you can use them for things like documentation.
## Available Scripts
In the project directory, you can run:
### `npm start`
Runs the app in the development mode.<br>
Open [http://localhost:3000](http://localhost:3000) to view it in the browser.
The page will reload if you make edits.<br>
You will also see any lint errors in the console.
### `npm test`
Launches the test runner in the interactive watch mode.<br>
See the section about [running tests](#running-tests) for more information.
### `npm run build`
Builds the app for production to the `build` folder.<br>
It correctly bundles React in production mode and optimizes the build for the best performance.
The build is minified and the filenames include the hashes.<br>
Your app is ready to be deployed!
See the section about [deployment](#deployment) for more information.
### `npm run eject`
**Note: this is a one-way operation. Once you `eject`, you can’t go back!**
If you aren’t satisfied with the build tool and configuration choices, you can `eject` at any time. This command will remove the single build dependency from your project.
Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except `eject` will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.
You don’t have to ever use `eject`. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.
## Supported Language Features and Polyfills
This project supports a superset of the latest JavaScript standard.<br>
In addition to [ES6]( syntax features, it also supports:
* [Exponentiation Operator]( (ES2016).
* [Async/await]( (ES2017).
* [Object Rest/Spread Properties]( (stage 3 proposal).
* [Class Fields and Static Properties]( (stage 2 proposal).
* [JSX]( and [Flow]( syntax.
Learn more about [different proposal stages](
While we recommend to use experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide [codemods]( if any of these proposals change in the future.
Note that **the project only includes a few ES6 [polyfills](**:
* [`Object.assign()`]( via [`object-assign`](
* [`Promise`]( via [`promise`](
* [`fetch()`]( via [`whatwg-fetch`](
If you use any other ES6+ features that need **runtime support** (such as `Array.from()` or `Symbol`), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.
## Syntax Highlighting in the Editor
To configure the syntax highlighting in your favorite text editor, head to the [relevant Babel documentation page]( and follow the instructions. Some of the most popular editors are covered.
## Displaying Lint Output in the Editor
>Note: this feature is available with `react-scripts@0.2.0` and higher.
Some editors, including Sublime Text, Atom, and Visual Studio Code, provide plugins for ESLint.
They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.
You would need to install an ESLint plugin for your editor first.
>**A note for Atom `linter-eslint` users**
>If you are using the Atom `linter-eslint` plugin, make sure that **Use global ESLint installation** option is checked:
><img src="" width="300">
>**For Visual Studio Code users**
>VS Code ESLint plugin automatically detects Create React App's configuration file. So you do not need to create `eslintrc.json` at the root directory, except when you want to add your own rules. In that case, you should include CRA's config by adding this line:
// ...
"extends": "react-app"
Then add this block to the `package.json` file of your project:
// ...
"eslintConfig": {
"extends": "react-app"
Finally, you will need to install some packages *globally*:
npm install -g eslint-config-react-app@0.3.0 eslint@3.8.1 babel-eslint@7.0.0 eslint-plugin-react@6.4.1 eslint-plugin-import@2.0.1 eslint-plugin-jsx-a11y@4.0.0 eslint-plugin-flowtype@2.21.0
We recognize that this is suboptimal, but it is currently required due to the way we hide the ESLint dependency. The ESLint team is already [working on a solution to this]( so this may become unnecessary in a couple of months.
## Debugging in the Editor
**This feature is currently only supported by [Visual Studio Code]( editor.**
Visual Studio Code supports live-editing and debugging out of the box with Create React App. This enables you as a developer to write and debug your React code without leaving the editor, and most importantly it enables you to have a continuous development workflow, where context switching is minimal, as you don’t have to switch between tools.
You would need to have the latest version of [VS Code]( and VS Code [Chrome Debugger Extension]( installed.
Then add the block below to your `launch.json` file and put it inside the `.vscode` folder in your app’s root directory.
"version": "0.2.0",
"configurations": [{
"name": "Chrome",
"type": "chrome",
"request": "launch",
"url": "http://localhost:3000",
"webRoot": "${workspaceRoot}/src",
"userDataDir": "${workspaceRoot}/.vscode/chrome",
"sourceMapPathOverrides": {
"webpack:///src/*": "${webRoot}/*"
Start your app by running `npm start`, and start debugging in VS Code by pressing `F5` or by clicking the green debug icon. You can now write code, set breakpoints, make changes to the code, and debug your newly modified code—all from your editor.
## Changing the Page `<title>`
You can find the source HTML file in the `public` folder of the generated project. You may edit the `<title>` tag in it to change the title from “React App” to anything else.
Note that normally you wouldn’t edit files in the `public` folder very often. For example, [adding a stylesheet](#adding-a-stylesheet) is done without touching the HTML.
If you need to dynamically update the page title based on the content, you can use the browser [`document.title`]( API. For more complex scenarios when you want to change the title from React components, you can use [React Helmet](, a third party library.
If you use a custom server for your app in production and want to modify the title before it gets sent to the browser, you can follow advice in [this section](#generating-dynamic-meta-tags-on-the-server). Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is covered [here](#pre-rendering-into-static-html-files).
## Installing a Dependency
The generated project includes React and ReactDOM as dependencies. It also includes a set of scripts used by Create React App as a development dependency. You may install other dependencies (for example, React Router) with `npm`:
npm install --save <library-name>
## Importing a Component
This project setup supports ES6 modules thanks to Babel.<br>
While you can still use `require()` and `module.exports`, we encourage you to use [`import` and `export`]( instead.
For example:
### `Button.js`
import React, { Component } from 'react';
class Button extends Component {
render() {
// ...
export default Button; // Don’t forget to use export default!
### `DangerButton.js`
import React, { Component } from 'react';
import Button from './Button'; // Import a component from another file
class DangerButton extends Component {
render() {
return <Button color="red" />;
export default DangerButton;
Be aware of the [difference between default and named exports]( It is a common source of mistakes.
We suggest that you stick to using default imports and exports when a module only exports a single thing (for example, a component). That’s what you get when you use `export default Button` and `import Button from './Button'`.
Named exports are useful for utility modules that export several functions. A module may have at most one default export and as many named exports as you like.
Learn more about ES6 modules:
* [When to use the curly braces?](
* [Exploring ES6: Modules](
* [Understanding ES6: Modules](
## Adding a Stylesheet
This project setup uses [Webpack]( for handling all assets. Webpack offers a custom way of “extending” the concept of `import` beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need to **import the CSS from the JavaScript file**:
### `Button.css`
.Button {
padding: 20px;
### `Button.js`
import React, { Component } from 'react';
import './Button.css'; // Tell Webpack that Button.js uses these styles
class Button extends Component {
render() {
// You can use them as regular CSS styles
return <div className="Button" />;
**This is not required for React** but many people find this feature convenient. You can read about the benefits of this approach [here]( However you should be aware that this makes your code less portable to other build tools and environments than Webpack.
In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified `.css` file in the build output.
If you are concerned about using Webpack-specific semantics, you can put all your CSS right into `src/index.css`. It would still be imported from `src/index.js`, but you could always remove that import if you later migrate to a different build tool.
## Post-Processing CSS
This project setup minifies your CSS and adds vendor prefixes to it automatically through [Autoprefixer]( so you don’t need to worry about it.
For example, this:
.App {
display: flex;
flex-direction: row;
align-items: center;
becomes this:
.App {
display: -webkit-box;
display: -ms-flexbox;
display: flex;
-webkit-box-orient: horizontal;
-webkit-box-direction: normal;
-ms-flex-direction: row;
flex-direction: row;
-webkit-box-align: center;
-ms-flex-align: center;
align-items: center;
If you need to disable autoprefixing for some reason, [follow this section](
## Adding a CSS Preprocessor (Sass, Less etc.)
Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a `.Button` CSS class in `<AcceptButton>` and `<RejectButton>` components, we recommend creating a `<Button>` component with its own `.Button` styles, that both `<AcceptButton>` and `<RejectButton>` can render (but [not inherit](
Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.
First, let’s install the command-line interface for Sass:
npm install node-sass --save-dev
Then in `package.json`, add the following lines to `scripts`:
"scripts": {
+ "build-css": "node-sass src/ -o src/",
+ "watch-css": "npm run build-css && node-sass src/ -o src/ --watch --recursive",
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test --env=jsdom",
>Note: To use a different preprocessor, replace `build-css` and `watch-css` commands according to your preprocessor’s documentation.
Now you can rename `src/App.css` to `src/App.scss` and run `npm run watch-css`. The watcher will find every Sass file in `src` subdirectories, and create a corresponding CSS file next to it, in our case overwriting `src/App.css`. Since `src/App.js` still imports `src/App.css`, the styles become a part of your application. You can now edit `src/App.scss`, and `src/App.css` will be regenerated.
To share variables between Sass files, you can use Sass imports. For example, `src/App.scss` and other component style files could include `@import "./shared.scss";` with variable definitions.
At this point you might want to remove all CSS files from the source control, and add `src/**/*.css` to your `.gitignore` file. It is generally a good practice to keep the build products outside of the source control.
As a final step, you may find it convenient to run `watch-css` automatically with `npm start`, and run `build-css` as a part of `npm run build`. You can use the `&&` operator to execute two scripts sequentially. However, there is no cross-platform way to run two scripts in parallel, so we will install a package for this:
npm install --save-dev npm-run-all
Then we can change `start` and `build` scripts to include the CSS preprocessor commands:
"scripts": {
"build-css": "node-sass src/ -o src/",
"watch-css": "npm run build-css && node-sass src/ -o src/ --watch --recursive",
- "start": "react-scripts start",
- "build": "react-scripts build",
+ "start-js": "react-scripts start",
+ "start": "npm-run-all -p watch-css start-js",
+ "build": "npm run build-css && react-scripts build",
"test": "react-scripts test --env=jsdom",
"eject": "react-scripts eject"
Now running `npm start` and `npm run build` also builds Sass files. Note that `node-sass` seems to have an [issue recognizing newly created files on some systems]( so you might need to restart the watcher when you create a file until it’s resolved.
## Adding Images and Fonts
With Webpack, using static assets like images and fonts works similarly to CSS.
You can **`import` an image right in a JavaScript module**. This tells Webpack to include that image in the bundle. Unlike CSS imports, importing an image or a font gives you a string value. This value is the final image path you can reference in your code.
Here is an example:
import React from 'react';
import logo from './logo.png'; // Tell Webpack this JS file uses this image
console.log(logo); // /logo.84287d09.png
function Header() {
// Import result is the URL of your image
return <img src={logo} alt="Logo" />;
export default Header;
This ensures that when the project is built, Webpack will correctly move the images into the build folder, and provide us with correct paths.
This works in CSS too:
.Logo {
background-image: url(./logo.png);
Webpack finds all relative module references in CSS (they start with `./`) and replaces them with the final paths from the compiled bundle. If you make a typo or accidentally delete an important file, you will see a compilation error, just like when you import a non-existent JavaScript module. The final filenames in the compiled bundle are generated by Webpack from content hashes. If the file content changes in the future, Webpack will give it a different name in production so you don’t need to worry about long-term caching of assets.
Please be advised that this is also a custom feature of Webpack.
**It is not required for React** but many people enjoy it (and React Native uses a similar mechanism for images).<br>
An alternative way of handling static assets is described in the next section.
## Using the `public` Folder
>Note: this feature is available with `react-scripts@0.5.0` and higher.
### Changing the HTML
The `public` folder contains the HTML file so you can tweak it, for example, to [set the page title](#changing-the-page-title).
The `<script>` tag with the compiled code will be added to it automatically during the build process.
### Adding Assets Outside of the Module System
You can also add other assets to the `public` folder.
Note that we normally encourage you to `import` assets in JavaScript files instead.
For example, see the sections on [adding a stylesheet](#adding-a-stylesheet) and [adding images and fonts](#adding-images-and-fonts).
This mechanism provides a number of benefits:
* Scripts and stylesheets get minified and bundled together to avoid extra network requests.
* Missing files cause compilation errors instead of 404 errors for your users.
* Result filenames include content hashes so you don’t need to worry about browsers caching their old versions.
However there is an **escape hatch** that you can use to add an asset outside of the module system.
If you put a file into the `public` folder, it will **not** be processed by Webpack. Instead it will be copied into the build folder untouched. To reference assets in the `public` folder, you need to use a special variable called `PUBLIC_URL`.
Inside `index.html`, you can use it like this:
<link rel="shortcut icon" href="%PUBLIC_URL%/favicon.ico">
Only files inside the `public` folder will be accessible by `%PUBLIC_URL%` prefix. If you need to use a file from `src` or `node_modules`, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.
When you run `npm run build`, Create React App will substitute `%PUBLIC_URL%` with a correct absolute path so your project works even if you use client-side routing or host it at a non-root URL.
In JavaScript code, you can use `process.env.PUBLIC_URL` for similar purposes:
render() {
// Note: this is an escape hatch and should be used sparingly!
// Normally we recommend using `import` for getting asset URLs
// as described in “Adding Images and Fonts” above this section.
return <img src={process.env.PUBLIC_URL + '/img/logo.png'} />;
Keep in mind the downsides of this approach:
* None of the files in `public` folder get post-processed or minified.
* Missing files will not be called at compilation time, and will cause 404 errors for your users.
* Result filenames won’t include content hashes so you’ll need to add query arguments or rename them every time they change.
### When to Use the `public` Folder
Normally we recommend importing [stylesheets](#adding-a-stylesheet), [images, and fonts](#adding-images-and-fonts) from JavaScript.
The `public` folder is useful as a workaround for a number of less common cases:
* You need a file with a specific name in the build output, such as [`manifest.webmanifest`](
* You have thousands of images and need to dynamically reference their paths.
* You want to include a small script like [`pace.js`]( outside of the bundled code.
* Some library may be incompatible with Webpack and you have no other option but to include it as a `<script>` tag.
Note that if you add a `<script>` that declares global variables, you also need to read the next section on using them.
## Using Global Variables
When you include a script in the HTML file that defines global variables and try to use one of these variables in the code, the linter will complain because it cannot see the definition of the variable.
You can avoid this by reading the global variable explicitly from the `window` object, for example:
const $ = window.$;
This makes it obvious you are using a global variable intentionally rather than because of a typo.
Alternatively, you can force the linter to ignore any line by adding `// eslint-disable-line` after it.
## Adding Bootstrap
You don’t have to use [React Bootstrap]( together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:
Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:
npm install react-bootstrap --save
npm install bootstrap@3 --save
Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your ```src/index.js``` file:
import 'bootstrap/dist/css/bootstrap.css';
import 'bootstrap/dist/css/bootstrap-theme.css';
// Put any other imports below so that CSS from your
// components takes precedence over default styles.
Import required React Bootstrap components within ```src/App.js``` file or your custom component files:
import { Navbar, Jumbotron, Button } from 'react-bootstrap';
Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example [`App.js`]( redone using React Bootstrap.
### Using a Custom Theme
Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).<br>
We suggest the following approach:
* Create a new package that depends on the package you wish to customize, e.g. Bootstrap.
* Add the necessary build steps to tweak the theme, and publish your package on npm.
* Install your own theme npm package as a dependency of your app.
Here is an example of adding a [customized Bootstrap]( that follows these steps.
## Adding Flow
Flow is a static type checker that helps you write code with fewer bugs. Check out this [introduction to using static types in JavaScript]( if you are new to this concept.
Recent versions of [Flow]( 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-dev flow-bin` (or `yarn add --dev 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]( 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]( 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](
## 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
**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]( 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>`:
render() {
return (
<small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
<input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
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`:
<small>You are running this application in <b>development</b> mode.</small>
<input type="hidden" value="abcdef" />
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:
if (process.env.NODE_ENV !== 'production') {
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:
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)
set REACT_APP_SECRET_CODE=abcdef&&npm start
(Note: the lack of whitespace is intentional.)
#### Linux, macOS (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: