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This project was bootstrapped withCreate React App.

Below you will find some information on how to perform common tasks.
You can find the most recent version of this guidehere.

• Updating to New Releases
• Sending Feedback
• Folder Structure
• Available Scripts
  • npm start
  • npm test
  • npm run build
  • npm run eject
• 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<title>
• Installing a Dependency
• Importing a Component
• Code Splitting
• Adding a Stylesheet
• Post-Processing CSS
• Adding a CSS Preprocessor (Sass, Less etc.)
• Adding Images, Fonts, and Files
• Using thepublic Folder
  • Changing the HTML
  • Adding Assets Outside of the Module System
  • When to Use thepublic Folder
• Using Global Variables
• Adding Bootstrap
  • Using a Custom Theme
• Adding Flow
• Adding Custom Environment Variables
  • Referencing Environment Variables in the HTML
  • Adding Temporary Environment Variables In Your Shell
  • Adding Development Environment Variables In.env
• Can I Use Decorators?
• Integrating with an API Backend
  • Node
  • Ruby on Rails
• Proxying API Requests in Development
  • "Invalid Host Header" Errors After Configuring Proxy
  • Configuring the Proxy Manually
  • Configuring a WebSocket Proxy
• Using HTTPS in Development
• Generating Dynamic<meta> Tags on the Server
• Pre-Rendering into Static HTML Files
• Injecting Data from the Server into the Page
• Running Tests
  • Filename Conventions
  • Command Line Interface
  • Version Control Integration
  • Writing Tests
  • Testing Components
  • Using Third Party Assertion Libraries
  • Initializing Test Environment
  • Focusing and Excluding Tests
  • Coverage Reporting
  • Continuous Integration
  • Disabling jsdom
  • Snapshot Testing
  • Editor Integration
• Developing Components in Isolation
  • Getting Started with Storybook
  • Getting Started with Styleguidist
• Making a Progressive Web App
  • Opting Out of Caching
  • Offline-First Considerations
  • Progressive Web App Metadata
• Analyzing the Bundle Size
• Deployment
  • Static Server
  • Other Solutions
  • Serving Apps with Client-Side Routing
  • Building for Relative Paths
  • Azure
  • Firebase
  • GitHub Pages
  • Heroku
  • Netlify
  • Now
  • S3 and CloudFront
  • Surge
• Advanced Configuration
• Troubleshooting
  • npm start doesn’t detect changes
  • npm test hangs on macOS Sierra
  • npm run build exits too early
  • npm run build fails on Heroku
  • npm run build fails to minify
  • Moment.js locales are missing
• Something Missing?

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 updatecreate-react-app itself: it delegates all the setup toreact-scripts.

When you runcreate-react-app, it always creates the project with the latest version ofreact-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 ofreact-scripts,open the changelog, find the version you’re currently on (checkpackage.json in this folder if you’re not sure), and apply the migration instructions for the newer versions.

In most cases bumping thereact-scripts version inpackage.json and runningnpm install in this folder should be enough, but it’s good to consult thechangelog for potential breaking changes.

We commit to keeping the breaking changes minimal so you can upgradereact-scripts painlessly.

We are always open toyour feedback.

After creation, your project should look like this:

my-app/  README.md  node_modules/  package.json  public/    index.html    favicon.ico  src/    App.css    App.js    App.test.js    index.css    index.js    logo.svg

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 insidesrc. For faster rebuilds, only files insidesrc are processed by Webpack.
You need toput any JS and CSS files insidesrc, otherwise Webpack won’t see them.

Only files insidepublic can be used frompublic/index.html.
Read instructions below for using assets from JavaScript and HTML.

You can, however, create more top-level directories.
They will not be included in the production build so you can use them for things like documentation.

In the project directory, you can run:

Runs the app in the development mode.
Openhttp://localhost:3000 to view it in the browser.

The page will reload if you make edits.
You will also see any lint errors in the console.

Launches the test runner in the interactive watch mode.
See the section aboutrunning tests for more information.

Builds the app for production to thebuild folder.
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.
Your app is ready to be deployed!

See the section aboutdeployment for more information.

Note: this is a one-way operation. Once youeject, you can’t go back!

If you aren’t satisfied with the build tool and configuration choices, you caneject 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 excepteject 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 useeject. 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.

This project supports a superset of the latest JavaScript standard.
In addition toES6 syntax features, it also supports:

• Exponentiation Operator (ES2016).
• Async/await (ES2017).
• Object Rest/Spread Properties (stage 3 proposal).
• Dynamic import() (stage 3 proposal)
• Class Fields and Static Properties (part of stage 3 proposal).
• JSX andFlow syntax.

Learn more aboutdifferent proposal stages.

While we recommend using experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to providecodemods if any of these proposals change in the future.

Note thatthe project only includes a few ES6polyfills:

• Object.assign() viaobject-assign.
• Promise viapromise.
• fetch() viawhatwg-fetch.

If you use any other ES6+ features that needruntime support (such asArray.from() orSymbol), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.

To configure the syntax highlighting in your favorite text editor, head to therelevant Babel documentation page and follow the instructions. Some of the most popular editors are covered.

Note: this feature is available withreact-scripts@0.2.0 and higher.
It also only works with npm 3 or 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. Then, add a file called.eslintrc to the project root:

{"extends":"react-app"}

Now your editor should report the linting warnings.

Note that even if you edit your.eslintrc file further, these changes willonly affect the editor integration. They won’t affect the terminal and in-browser lint output. This is because Create React App intentionally provides a minimal set of rules that find common mistakes.

If you want to enforce a coding style for your project, consider usingPrettier instead of ESLint style rules.

This feature is currently only supported byVisual Studio Code andWebStorm.

Visual Studio Code and WebStorm support 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 ofVS Code and VS CodeChrome Debugger Extension installed.

Then add the block below to yourlaunch.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":"${workspaceRoot}/src","sourceMapPathOverrides": {"webpack:///src/*":"${webRoot}/*"    }  }]}

Note: the URL may be different if you've made adjustments via theHOST or PORT environment variables.

Start your app by runningnpm start, and start debugging in VS Code by pressingF5 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.

Having problems with VS Code Debugging? Please see theirtroubleshooting guide.

You would need to haveWebStorm andJetBrains IDE Support Chrome extension installed.

In the WebStorm menuRun selectEdit Configurations.... Then click+ and selectJavaScript Debug. Pastehttp://localhost:3000 into the URL field and save the configuration.

Note: the URL may be different if you've made adjustments via theHOST or PORT environment variables.

Start your app by runningnpm start, then press^D on macOS orF9 on Windows and Linux or click the green debug icon to start debugging in WebStorm.

The same way you can debug your application in IntelliJ IDEA Ultimate, PhpStorm, PyCharm Pro, and RubyMine.

Prettier is an opinionated code formatter with support for JavaScript, CSS and JSON. With Prettier you can format the code you write automatically to ensure a code style within your project. See thePrettier's GitHub page for more information, and look at thispage to see it in action.

To format our code whenever we make a commit in git, we need to install the following dependencies:

npm install --save husky lint-staged prettier

Alternatively you may useyarn:

yarn add husky lint-staged prettier

• husky makes it easy to use githooks as if they are npm scripts.
• lint-staged allows us to run scripts on staged files in git. See thisblog post about lint-staged to learn more about it.
• prettier is the JavaScript formatter we will run before commits.

Now we can make sure every file is formatted correctly by adding a few lines to thepackage.json in the project root.

Add the following line toscripts section:

  "scripts": {+   "precommit": "lint-staged",    "start": "react-scripts start",    "build": "react-scripts build",

Next we add a 'lint-staged' field to thepackage.json, for example:

  "dependencies": {    // ...  },+ "lint-staged": {+   "src/**/*.{js,jsx,json,css}": [+     "prettier --single-quote --write",+     "git add"+   ]+ },  "scripts": {

Now, whenever you make a commit, Prettier will format the changed files automatically. You can also run./node_modules/.bin/prettier --single-quote --write "src/**/*.{js,jsx}" to format your entire project for the first time.

Next you might want to integrate Prettier in your favorite editor. Read the section onEditor Integration on the Prettier GitHub page.

You can find the source HTML file in thepublic 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 thepublic folder very often. For example,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 browserdocument.title API. For more complex scenarios when you want to change the title from React components, you can useReact 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 inthis section. Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is coveredhere.

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) withnpm:

npm install --save react-router

Alternatively you may useyarn:

yarn add react-router

This works for any library, not justreact-router.

This project setup supports ES6 modules thanks to Babel.
While you can still userequire() andmodule.exports, we encourage you to useimport andexport instead.

For example:

importReact,{Component}from'react';classButtonextendsComponent{render(){// ...}}exportdefaultButton;// Don’t forget to use export default!

importReact,{Component}from'react';importButtonfrom'./Button';// Import a component from another fileclassDangerButtonextendsComponent{render(){return<Buttoncolor="red"/>;}}exportdefaultDangerButton;

Be aware of thedifference 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 useexport default Button andimport 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

Instead of downloading the entire app before users can use it, code splitting allows you to split your code into small chunks which you can then load on demand.

This project setup supports code splitting viadynamicimport(). Itsproposal is in stage 3. Theimport() function-like form takes the module name as an argument and returns aPromise which always resolves to the namespace object of the module.

Here is an example:

constmoduleA='Hello';export{moduleA};

importReact,{Component}from'react';classAppextendsComponent{handleClick=()=>{import('./moduleA').then(({ moduleA})=>{// Use moduleA}).catch(err=>{// Handle failure});};render(){return(<div><buttononClick={this.handleClick}>Load</button></div>);}}exportdefaultApp;

This will makemoduleA.js and all its unique dependencies as a separate chunk that only loads after the user clicks the 'Load' button.

You can also use it withasync /await syntax if you prefer it.

If you are using React Router check outthis tutorial on how to use code splitting with it. You can find the companion GitHub repositoryhere.

This project setup usesWebpack for handling all assets. Webpack offers a custom way of “extending” the concept ofimport beyond JavaScript. To express that a JavaScript file depends on a CSS file, you need toimport the CSS from the JavaScript file:

.Button {padding:20px;}

importReact,{Component}from'react';import'./Button.css';// Tell Webpack that Button.js uses these stylesclassButtonextendsComponent{render(){// You can use them as regular CSS stylesreturn<divclassName="Button"/>;}}

This is not required for React but many people find this feature convenient. You can read about the benefits of this approachhere. 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 intosrc/index.css. It would still be imported fromsrc/index.js, but you could always remove that import if you later migrate to a different build tool.

This project setup minifies your CSS and adds vendor prefixes to it automatically throughAutoprefixer 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.

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 (butnot 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 --save node-sass-chokidar

Alternatively you may useyarn:

yarn add node-sass-chokidar

Then inpackage.json, add the following lines toscripts:

   "scripts": {+    "build-css": "node-sass-chokidar src/ -o src/",+    "watch-css": "npm run build-css && node-sass-chokidar 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, replacebuild-css andwatch-css commands according to your preprocessor’s documentation.

Now you can renamesrc/App.css tosrc/App.scss and runnpm run watch-css. The watcher will find every Sass file insrc subdirectories, and create a corresponding CSS file next to it, in our case overwritingsrc/App.css. Sincesrc/App.js still importssrc/App.css, the styles become a part of your application. You can now editsrc/App.scss, andsrc/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.

To enable importing files without using relative paths, you can add the--include-path option to the command inpackage.json.

"build-css": "node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/","watch-css": "npm run build-css && node-sass-chokidar --include-path ./src --include-path ./node_modules src/ -o src/ --watch --recursive",

This will allow you to do imports like

@import'styles/_colors.scss';// assuming a styles directory under src/@import'nprogress/nprogress';// importing a css file from the nprogress node module

At this point you might want to remove all CSS files from the source control, and addsrc/**/*.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 runwatch-css automatically withnpm start, and runbuild-css as a part ofnpm 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 npm-run-all

Alternatively you may useyarn:

yarn add npm-run-all

Then we can changestart andbuild scripts to include the CSS preprocessor commands:

   "scripts": {     "build-css": "node-sass-chokidar src/ -o src/",     "watch-css": "npm run build-css && node-sass-chokidar 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-js": "react-scripts build",+    "build": "npm-run-all build-css build-js",     "test": "react-scripts test --env=jsdom",     "eject": "react-scripts eject"   }

Now runningnpm start andnpm run build also builds Sass files.

Whynode-sass-chokidar?

node-sass has been reported as having the following issues:

• node-sass --watch has been reported to haveperformance issues in certain conditions when used in a virtual machine or with docker.

• Infinite styles compiling#1939

• node-sass has been reported as having issues with detecting new files in a directory#1891

node-sass-chokidar is used here as it addresses these issues.

With Webpack, using static assets like images and fonts works similarly to CSS.

You canimport a file right in a JavaScript module. This tells Webpack to include that file in the bundle. Unlike CSS imports, importing a file gives you a string value. This value is the final path you can reference in your code, e.g. as thesrc attribute of an image or thehref of a link to a PDF.

To reduce the number of requests to the server, importing images that are less than 10,000 bytes returns adata URI instead of a path. This applies to the following file extensions: bmp, gif, jpg, jpeg, and png. SVG files are excluded due to#1153.

Here is an example:

importReactfrom'react';importlogofrom'./logo.png';// Tell Webpack this JS file uses this imageconsole.log(logo);// /logo.84287d09.pngfunctionHeader(){// Import result is the URL of your imagereturn<imgsrc={logo}alt="Logo"/>;}exportdefaultHeader;

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).
An alternative way of handling static assets is described in the next section.

Note: this feature is available withreact-scripts@0.5.0 and higher.

Thepublic folder contains the HTML file so you can tweak it, for example, toset the page title.The<script> tag with the compiled code will be added to it automatically during the build process.

You can also add other assets to thepublic folder.

Note that we normally encourage you toimport assets in JavaScript files instead.For example, see the sections onadding a stylesheet andadding 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 anescape hatch that you can use to add an asset outside of the module system.

If you put a file into thepublic folder, it willnot be processed by Webpack. Instead it will be copied into the build folder untouched. To reference assets in thepublic folder, you need to use a special variable calledPUBLIC_URL.

Insideindex.html, you can use it like this:

<linkrel="shortcut icon"href="%PUBLIC_URL%/favicon.ico">

Only files inside thepublic folder will be accessible by%PUBLIC_URL% prefix. If you need to use a file fromsrc ornode_modules, you’ll have to copy it there to explicitly specify your intention to make this file a part of the build.

When you runnpm 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 useprocess.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<imgsrc={process.env.PUBLIC_URL+'/img/logo.png'}/>;}

Keep in mind the downsides of this approach:

• None of the files inpublic 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.

Normally we recommend importingstylesheets,images, and fonts from JavaScript.Thepublic 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 asmanifest.webmanifest.
• You have thousands of images and need to dynamically reference their paths.
• You want to include a small script likepace.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.

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 thewindow 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.

You don’t have to useReact 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 --save react-bootstrap bootstrap@3

Alternatively you may useyarn:

yarn add react-bootstrap bootstrap@3

Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of yoursrc/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 withinsrc/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 exampleApp.js redone using React Bootstrap.

Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).
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 acustomized Bootstrap that follows these steps.

Flow is a static type checker that helps you write code with fewer bugs. Check out thisintroduction to using static types in JavaScript if you are new to this concept.

Recent versions ofFlow work with Create React App projects out of the box.

To add Flow to a Create React App project, follow these steps:

1. Runnpm install --save flow-bin (oryarn add flow-bin).
2. Add"flow": "flow" to thescripts section of yourpackage.json.
3. Runnpm run flow init (oryarn flow init) to create a.flowconfig file in the root directory.
4. Add// @flow to any files you want to type check (for example, tosrc/App.js).

Now you can runnpm run flow (oryarn flow) to check the files for type errors.You can optionally use an IDE likeNuclide 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 outits documentation.

Note: this feature is available withreact-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. Bydefault you will haveNODE_ENV defined for you, and any other environment variables starting withREACT_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 likedescribed here. Alternatively you can rebuild the app on the server anytime you change them.

Note: You must create custom environment variables beginning withREACT_APP_. Any other variables exceptNODE_ENV will be ignored to avoid accidentallyexposing 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 onprocess.env. For example, having an environmentvariable namedREACT_APP_SECRET_CODE will be exposed in your JS asprocess.env.REACT_APP_SECRET_CODE.

There is also a special built-in environment variable calledNODE_ENV. You can read it fromprocess.env.NODE_ENV. When you runnpm start, it is always equal to'development', when you runnpm test it is always equal to'test', and when you runnpm run build to make a production bundle, it is always equal to'production'.You cannot overrideNODE_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 isdeployed 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 definedin the environment inside a<form>:

render(){return(<div><small>You are running this application in<b>{process.env.NODE_ENV}</b> mode.</small><form><inputtype="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 theREACT_APP_SECRET_CODE environment variable. Remember that theNODE_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 toabcdef, and the bold text will show the environment provided when usingnpm start:

<div><small>You are running this application in<b>development</b> mode.</small><form><inputtype="hidden"value="abcdef"/></form></div>

The above form is looking for a variable calledREACT_APP_SECRET_CODE from the environment. In order to consume thisvalue, we need to have it defined in the environment. This can be done using two ways: either in your shell or ina.env file. Both of these ways are described in the next few sections.

Having access to theNODE_ENV is also useful for performing actions conditionally:

if(process.env.NODE_ENV!=='production'){analytics.disable();}

When you compile the app withnpm run build, the minification step will strip out this condition, and the resulting bundle will be smaller.

Note: this feature is available withreact-scripts@0.9.0 and higher.

You can also access the environment variables starting withREACT_APP_ in thepublic/index.html. For example:

<title>%REACT_APP_WEBSITE_NAME%</title>

Note that the caveats from the above section apply:

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

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

setREACT_APP_SECRET_CODE=abcdef&&npmstart

(Note: the lack of whitespace is intentional.)

REACT_APP_SECRET_CODE=abcdef npm start

Note: this feature is available withreact-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

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

Note: this feature isavailable withreact-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 thedotenv documentation for more details.

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

Many popular libraries usedecorators 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.

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

Check outthis tutorial.You can find the companion GitHub repositoryhere.

Check outthis tutorial.You can find the companion GitHub repositoryhere.

Note: this feature is available withreact-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 isnot required. However, if youdo have a setup like this, it is convenient to write requests likefetch('/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 aproxy field to yourpackage.json, for example:

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

This way, when youfetch('/api/todos') in development, the development server will recognize that it’s not a static asset, and will proxy your request tohttp://localhost:4000/api/todos as a fallback. The development server willonly attempt to send requests withouttext/html in itsAccept header to the proxy.

Conveniently, this avoidsCORS 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 thatproxy only has effect in development (withnpm 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 atext/html accept header will be redirected to the specifiedproxy.

Theproxy option supports HTTP, HTTPS and WebSocket connections.
If theproxy option isnot flexible enough for you, alternatively you can:

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

When you enable theproxy 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 inthis article andthis issue.

This shouldn’t affect you when developing onlocalhost, but if you develop remotely likedescribed here, you will see this error in the browser after enabling theproxy 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.

Note: this feature is available withreact-scripts@1.0.0 and higher.

If theproxy option isnot flexible enough for you, you can specify an object in the following form (inpackage.json).
You may also specify any configuration valuehttp-proxy-middleware orhttp-proxy supports.

{// ..."proxy":{"/api":{"target":"<url>","ws":true// ...}}// ...}

All requests matching this path will be proxies, no exceptions. This includes requests fortext/html, which the standardproxy 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.

{// ..."proxy":{// Matches any request starting with /api"/api":{"target":"<url_1>","ws":true// ...},// Matches any request starting with /foo"/foo":{"target":"<url_2>","ssl":true,"pathRewrite":{"^/foo":"/foo/beta"}// ...},// Matches /bar/abc.html but not /bar/sub/def.html"/bar/[^/]*[.]html":{"target":"<url_3>",// ...},// Matches /baz/abc.html and /baz/sub/def.html"/baz/.*/.*[.]html":{"target":"<url_4>"// ...}}// ...}

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

If you’re using a WebSocket engine likeSocket.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 withthe websocket.org echo test.

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

Standard WebSocketswill work with a standard WebSocket server as well as the websocket.org echo test. You can use libraries likews for the server, withnative WebSockets in the browser.

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

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

Note: this feature is available withreact-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 usingthe "proxy" feature to proxy requests to an API server when that API server is itself serving HTTPS.

To do this, set theHTTPS environment variable totrue, then start the dev server as usual withnpm start:

setHTTPS=true&&npmstart

(Note: the lack of whitespace is intentional.)

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.

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:

<!doctype html><htmllang="en"><head><metaproperty="og:title"content="__OG_TITLE__"><metaproperty="og:description"content="__OG_DESCRIPTION__">

Then, on the server, regardless of the backend you use, you can readindex.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.

If you’re hosting yourbuild with a static hosting provider you can usereact-snapshot orreact-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 pagewith 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 aboutzero-configuration pre-rendering (also called snapshotting) here.

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

<!doctypehtml><htmllang="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 readwindow.SERVER_DATA to use it.Make sure tosanitize the JSON before sending it to the client as it makes your app vulnerable to XSS attacks.

Note: this feature is available withreact-scripts@0.3.0 and higher.
Read the migration guide to learn how to enable it in older projects!

Create React App usesJest as its test runner. To prepare for this integration, we did amajor 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 aswindow thanks tojsdom, 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.

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 thesrc 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, ifApp.test.js andApp.js are in the same folder, the test just needs toimport App from './App' instead of a long relative path. Colocation also helps find tests more quickly in larger projects.

When you runnpm test, Jest will launch in the watch mode. Every time you save a file, it will re-run the tests, just likenpm 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:

By default, when you runnpm 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 pressa in the watch mode to force Jest to run all tests.

Jest will always run all tests on acontinuous integration server or if the project is not inside a Git or Mercurial repository.

To create tests, addit() (ortest()) blocks with the name of the test and its code. You may optionally wrap them indescribe() blocks for logical grouping but this is neither required nor recommended.

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

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

Allexpect() matchers supported by Jest areextensively documented here.
You can also usejest.fn() andexpect(fn).toBeCalled() to create “spies” or mock functions.

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:

importReactfrom'react';importReactDOMfrom'react-dom';importAppfrom'./App';it('renders without crashing',()=>{constdiv=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 value with very little effort so they are great as a starting point, and this is the test you will find insrc/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 usingshallow() rendering API fromEnzyme. To install it, run:

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

Alternatively you may useyarn:

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 yourglobal setup file:

import{configure}from'enzyme';importAdapterfrom'enzyme-adapter-react-16';configure({adapter:newAdapter()});

Now you can write a smoke test with it:

importReactfrom'react';import{shallow}from'enzyme';importAppfrom'./App';it('renders without crashing',()=>{shallow(<App/>);});

Unlike the previous smoke test usingReactDOM.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 supportsfull rendering withmount(), and you can also use it for testing state changes and component lifecycle.

You can read theEnzyme documentation for more testing techniques. Enzyme documentation uses Chai and Sinon for assertions but you don’t have to use them because Jest provides built-inexpect() andjest.fn() for spies.

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

importReactfrom'react';import{shallow}from'enzyme';importAppfrom'./App';it('renders welcome message',()=>{constwrapper=shallow(<App/>);constwelcome=<h2>Welcome to React</h2>;// expect(wrapper.contains(welcome)).to.equal(true);expect(wrapper.contains(welcome)).toEqual(true);});

All Jest matchers areextensively documented here.
Nevertheless you can use a third-party assertion library likeChai if you want to, as described below.

Additionally, you might findjest-enzyme helpful to simplify your tests with readable matchers. The abovecontains code can be written simpler with jest-enzyme.

expect(wrapper).toContainReact(welcome)

To enable this, installjest-enzyme:

npm install --save jest-enzyme

Alternatively you may useyarn:

yarn add jest-enzyme

Import it insrc/setupTests.js to make its matchers available in every test:

import'jest-enzyme';

We recommend that you useexpect() for assertions andjest.fn() for spies. If you are having issues with them pleasefile those against Jest, and we’ll fix them. We intend to keep making them better for React, supporting, for example,pretty-printing React elements as JSX.

However, if you are used to other libraries, such asChai andSinon, or if you have existing code using them that you’d like to port over, you can import them normally like this:

importsinonfrom'sinon';import{expect}from'chai';

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

Note: this feature is available withreact-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 asrc/setupTests.js to your project. It will be automatically executed before running your tests.

For example:

constlocalStorageMock={getItem:jest.fn(),setItem:jest.fn(),clear:jest.fn()};global.localStorage=localStorageMock

You can replaceit() withxit() to temporarily exclude a test from being executed.
Similarly,fit() lets you focus on a specific test without running any other tests.

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

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

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

Supported overrides:

• collectCoverageFrom
• coverageReporters
• coverageThreshold
• snapshotSerializers

Example package.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"]  }}

By defaultnpm 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 calledCI.

When creating a build of your application withnpm run build linter warnings are not checked by default. Likenpm test, you can force the build to perform a linter warning check by setting the environment variableCI. If any warnings are encountered then the build fails.

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

1. Following theTravis Getting started guide for syncing your GitHub repository with Travis. You may need to initialize some settings manually in yourprofile page.
2. Add a.travis.yml file to your git repository.

language: node_jsnode_js:  - 6cache:  directories:    - node_modulesscript:  - npm run build  - npm test

1. Trigger your first build with a git push.
2. Customize your Travis CI Build if needed.

Followthis article to set up CircleCI with a Create React App project.

setCI=true&&npm test

setCI=true&&npm run build

(Note: the lack of whitespace is intentional.)

CI=true npmtest

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, pleasefile an issue 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.

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

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

If you know that none of your tests depend onjsdom, you can safely remove--env=jsdom, and your tests will run faster:

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

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

• Any browser globals likewindow anddocument
• ReactDOM.render()
• TestUtils.renderIntoDocument() (a shortcut for the above)
• mount() inEnzyme

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

• TestUtils.createRenderer() (shallow rendering)
• shallow() inEnzyme

Finally, jsdom is also not needed forsnapshot 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.

If you useVisual Studio Code, there is aJest extension 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.

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 addStorybook for React (source) orReact Styleguidist (source) to your project.These are third-party tools that let you develop components and see all their states in isolation from your app.

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.

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:

npm install -g @storybook/cli

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

getstorybook

After that, follow the instructions on the screen.

Learn more about React Storybook:

• Screencast:Getting Started with React Storybook
• GitHub Repo
• Documentation
• Snapshot Testing UI with Storybook + addon/storyshot

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:

npm install --save react-styleguidist

Alternatively you may useyarn:

yarn add react-styleguidist

Then, add these scripts to yourpackage.json:

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

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

npm run styleguide

After that, follow the instructions on the screen.

Learn more about React Styleguidist:

• GitHub Repo
• Documentation

By default, the production build is a fully functional, offline-firstProgressive Web App.

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

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

Thesw-precache-webpack-pluginis integrated into production configuration,and it will take care of generating a service worker file that will automaticallyprecache all of your local assets and keep them up to date as you deploy updates.The service worker will use acache-first strategyfor handling all requests for local assets, including the initial HTML, ensuringthat your web app is reliably fast, even on a slow or unreliable network.

If you would prefer not to enable service workers prior to your initialproduction deployment, then remove the call toregisterServiceWorker()fromsrc/index.js.

If you had previously enabled service workers in your production deployment andhave decided that you would like to disable them for all your existing users,you can swap out the call toregisterServiceWorker() insrc/index.js first by modifying the service worker import:

import{unregister}from'./registerServiceWorker';

and then callunregister() instead.After the user visits a page that hasunregister(),the service worker will be uninstalled. Note that depending on how/service-worker.js is served,it may take up to 24 hours for the cache to be invalidated.

1. Service workersrequire HTTPS,although to facilitate local testing, that policydoes not apply tolocalhost.If your production web server does not support HTTPS, then the service workerregistration will fail, but the rest of your web app will remain functional.

2. Service workers arenot currently supportedin all web browsers. Service worker registrationwon't be attemptedon browsers that lack support.

3. The service worker is only enabled in theproduction environment,e.g. the output ofnpm run build. It's recommended that you do not enable anoffline-first service worker in a development environment, as it can lead tofrustration when previously cached assets are used and do not include the latestchanges you've made locally.

4. If youneed to test your offline-first service worker locally, buildthe application (usingnpm run build) and run a simple http server from yourbuild directory. After running the build script,create-react-app will giveinstructions for one way to test your production build locally and thedeployment instructions haveinstructions for using other methods.Be sure to always use anincognito window to avoid complications with your browser cache.

5. If possible, configure your production environment to serve the generatedservice-worker.jswith HTTP caching disabled.If that's not possible—GitHub Pages, for instance, does notallow you to change the default 10 minute HTTP cache lifetime—then be awarethat if you visit your production site, and then revisit again beforeservice-worker.js has expired from your HTTP cache, you'll continue to getthe previously cached assets from the service worker. If you have an immediateneed to view your updated production deployment, performing a shift-refreshwill temporarily disable the service worker and retrieve all assets from thenetwork.

6. Users aren't always familiar with offline-first web apps. It can be useful tolet the user knowwhen the service worker has finished populating your caches (showing a "This webapp works offline!" message) and also let them know when the service worker hasfetched the latest updates that will be available the next time they load thepage (showing a "New content is available; please refresh." message). Showingthis messages is currently left as an exercise to the developer, but as astarting point, you can make use of the logic included insrc/registerServiceWorker.js, whichdemonstrates which service worker lifecycle events to listen for to detect eachscenario, and which as a default, just logs appropriate messages to theJavaScript console.

7. By default, the generated service worker file will not intercept or cache anycross-origin traffic, like HTTPAPI requests,images, or embeds loaded from a different domain. If you would like to use aruntime caching strategy for those requests, you canejectand then configure theruntimeCachingoption in theSWPrecacheWebpackPlugin section ofwebpack.config.prod.js.

The default configuration includes a web app manifest located atpublic/manifest.json, that you can customize withdetails specific to your web application.

When a user adds a web app to their homescreen using Chrome or Firefox onAndroid, the metadata inmanifest.json determines whaticons, names, and branding colors to use when the web app is displayed.The Web App Manifest guideprovides more context about what each field means, and how your customizationswill affect your users' experience.

Source map explorer analyzesJavaScript bundles using the source maps. This helps you understand where codebloat is coming from.

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

npm install --save source-map-explorer

Alternatively you may useyarn:

yarn add source-map-explorer

Then inpackage.json, add the following line toscripts:

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

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

npm run buildnpm run analyze

npm run build creates abuild directory with a production build of your app. Set up your favourite HTTP server so that a visitor to your site is servedindex.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.

For environments usingNode, the easiest way to handle this would be to installserve and let it handle the rest:

npm install -g serveserve -s build

The last command shown above will serve your static site on the port5000. Like many ofserve’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:

serve -h

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 usingNode andExpress:

constexpress=require('express');constpath=require('path');constapp=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.

Thebuild 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.

If you use routers that use the HTML5pushState history API under the hood (for example,React Router withbrowserHistory), many static file servers will fail. For example, if you used React Router with a route for/todos/42, the development server will respond tolocalhost: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 filebuild/todos/42 and does not find it. The server needs to be configured to respond to a request to/todos/42 by servingindex.html. For example, we can amend our Express example above to serveindex.html for any unknown paths:

 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 usingApache HTTP Server, you need to create a.htaccess file in thepublic folder that looks like this:

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

It will get copied to thebuild folder when you runnpm run build.

If you’re usingApache Tomcat, you need to followthis Stack Overflow answer.

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

On a production build, and in a browser that supportsservice workers,the service worker will automatically handle all navigation requests, like for/todos/42, by serving the cached copy of yourindex.html. Thisservice worker navigation routing can be configured or disabled byejecting and then modifying thenavigateFallbackandnavigateFallbackWhitelistoptions of theSWPreachePluginconfiguration.

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 atpublic/manifest.json and changestart_url to match the required URL scheme, for example:

"start_url":".",

By default, Create React App produces a build assuming your app is hosted at the server root.
To override this, specify thehomepage in yourpackage.json, for example:

"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 usingreact-router@^4, you can root<Link>s using thebasename prop on any<Router>.
More informationhere.

For example:

<BrowserRouterbasename="/calendar"/><Linkto="/today"/>// renders <a href="/calendar/today">

Note: this feature is available withreact-scripts@0.9.0 and higher.

If you are not using the HTML5pushState 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 yourpackage.json:

"homepage":".",

This will make sure that all the asset paths are relative toindex.html. You will then be able to move your app fromhttp://mywebsite.com tohttp://mywebsite.com/relativepath or evenhttp://mywebsite.com/relative/path without having to rebuild it.

Seethis blog post on how to deploy your React app to Microsoft Azure.

Install the Firebase CLI if you haven’t already by runningnpm install -g firebase-tools. Sign up for aFirebase account and create a new project. Runfirebase login and login with your previous created Firebase account.

Then run thefirebase init command from your project’s root. You need to choose theHosting: Configure and deploy Firebase Hosting sites and choose the Firebase project you created in the previous step. You will need to agree withdatabase.rules.json being created, choosebuild as the public directory, and also agree toConfigure as a single-page app by replying withy.

    === Project Setup    First, let's associate this project directory with a Firebase project.    You can create multiple project aliases by running firebase use --add,    but for now we'll justset up a default project.? What Firebase projectdo you want to associate as default? Example app (example-app-fd690)    === Database Setup    Firebase Realtime Database Rules allow you to define how your data should be    structured and when your data can beread from and written to.? What file should be usedfor Database Rules? database.rules.json    ✔  Database Rulesfor example-app-fd690 have been downloaded to database.rules.json.    Future modifications to database.rules.json will update Database Rules when you run    firebase deploy.    === Hosting Setup    Your public directory is the folder (relative to your project directory) that    will contain Hosting assets to uploaded with firebase deploy. If you    have a build processfor your assets, use your build's output directory.    ? What do you want to use as your public directory? build    ? Configure as a single-page app (rewrite all urls to /index.html)? Yes    ✔  Wrote build/index.html    i  Writing configuration info to firebase.json...    i  Writing project information to .firebaserc...    ✔  Firebase initialization complete!

Now, after you create a production build withnpm run build, you can deploy it by runningfirebase deploy.

    === Deploying to'example-app-fd690'...    i  deploying database, hosting    ✔  database: rules ready to deploy.    i  hosting: preparing build directoryfor upload...    Uploading: [==============================          ] 75%✔  hosting: build folder uploaded successfully    ✔  hosting: 8 files uploaded successfully    i  starting release process (may take several minutes)...    ✔  Deploy complete!    Project Console: https://console.firebase.google.com/project/example-app-fd690/overview    Hosting URL: https://example-app-fd690.firebaseapp.com

For more information seeAdd Firebase to your JavaScript Project.

Note: this feature is available withreact-scripts@0.2.0 and higher.

The step below is important!
If you skip it, your app will not deploy correctly.

Open yourpackage.json and add ahomepage field:

"homepage":"https://myusername.github.io/my-app",

Create React App uses thehomepage field to determine the root URL in the built HTML file.

Now, whenever you runnpm run build, you will see a cheat sheet with instructions on how to deploy to GitHub Pages.

To publish it athttps://myusername.github.io/my-app, run:

npm install --save gh-pages

Alternatively you may useyarn:

yarn add gh-pages

Add the following scripts in yourpackage.json:

  "scripts": {+   "predeploy": "npm run build",+   "deploy": "gh-pages -d build",    "start": "react-scripts start",    "build": "react-scripts build",

Thepredeploy script will run automatically beforedeploy is run.

Then run:

npm run deploy

Finally, make sureGitHub Pages option in your GitHub project settings is set to use thegh-pages branch:

You can configure a custom domain with GitHub Pages by adding aCNAME file to thepublic/ folder.

GitHub Pages doesn’t support routers that use the HTML5pushState history API under the hood (for example, React Router usingbrowserHistory). This is because when there is a fresh page load for a url likehttp://user.github.io/todomvc/todos/42, where/todos/42 is a frontend route, the GitHub Pages server returns 404 because it knows nothing of/todos/42. If you want to add a router to a project hosted on GitHub Pages, here are a couple of solutions:

• You could switch from using HTML5 history API to routing with hashes. If you use React Router, you can switch tohashHistory for this effect, but the URL will be longer and more verbose (for example,http://user.github.io/todomvc/#/todos/42?_k=yknaj).Read more about different history implementations in React Router.
• Alternatively, you can use a trick to teach GitHub Pages to handle 404 by redirecting to yourindex.html page with a special redirect parameter. You would need to add a404.html file with the redirection code to thebuild folder before deploying your project, and you’ll need to add code handling the redirect parameter toindex.html. You can find a detailed explanation of this techniquein this guide.

Use theHeroku Buildpack for Create React App.
You can find instructions inDeploying React with Zero Configuration.

Sometimesnpm run build works locally but fails during deploy via Heroku. Following are the most common cases.

If you get something like this:

remote: Failed to create a production build. Reason:remote: Module not found: Error: Cannot resolve 'file' or 'directory'MyDirectory in /tmp/build_1234/src

It means you need to ensure that the lettercase of the file or directory youimport matches the one you see on your filesystem or on GitHub.

This is important because Linux (the operating system used by Heroku) is case sensitive. SoMyDirectory andmydirectory are two distinct directories and thus, even though the project builds locally, the difference in case breaks theimport statements on Heroku remotes.

If you exclude or ignore necessary files from the package you will see a error similar this one:

remote: Could not find a required file.remote:   Name: `index.html`remote:   Searched in: /tmp/build_a2875fc163b209225122d68916f1d4df/publicremote:remote: npm ERR! Linux 3.13.0-105-genericremote: npm ERR! argv "/tmp/build_a2875fc163b209225122d68916f1d4df/.heroku/node/bin/node" "/tmp/build_a2875fc163b209225122d68916f1d4df/.heroku/node/bin/npm" "run" "build"

In this case, ensure that the file is there with the proper lettercase and that’s not ignored on your local.gitignore or~/.gitignore_global.

To do a manual deploy to Netlify’s CDN:

npm install netlify-clinetlify deploy

Choosebuild as the path to deploy.

To setup continuous delivery:

With this setup Netlify will build and deploy when you push to git or open a pull request:

1. Start a new netlify project
2. Pick your Git hosting service and select your repository
3. ClickBuild your site

Support for client-side routing:

To supportpushState, make sure to create apublic/_redirects file with the following rewrite rules:

/*  /index.html  200

When you build the project, Create React App will place thepublic folder contents into the build output.

Now offers a zero-configuration single-command deployment. You can usenow to deploy your app for free.

1. Install thenow command-line tool either via the recommendeddesktop tool or via node withnpm install -g now.

2. Build your app by runningnpm run build.

3. Move into the build directory by runningcd build.

4. Runnow --name your-project-name from within the build directory. You will see anow.sh URL in your output like this:

   > Ready! https://your-project-name-tpspyhtdtk.now.sh (copied to clipboard)

   Paste that URL into your browser when the build is complete, and you will see your deployed app.

Details are available inthis article.

See thisblog post on how to deploy your React app to Amazon Web Services S3 and CloudFront.

Install the Surge CLI if you haven’t already by runningnpm install -g surge. Run thesurge command and log in you or create a new account.

When asked about the project path, make sure to specify thebuild folder, for example:

       project path: /path/to/project/build

Note that in order to support routers that use HTML5pushState API, you may want to rename theindex.html in your build folder to200.html before deploying to Surge. Thisensures that every URL falls back to that file.

You can adjust various development and production settings by setting environment variables in your shell or with.env.

When you save a file whilenpm start is running, the browser should refresh with the updated code.
If this doesn’t happen, try one of the following workarounds:

• If your project is in a Dropbox folder, try moving it out.
• If the watcher doesn’t see a file calledindex.js and you’re referencing it by the folder name, youneed to restart the watcher due to a Webpack bug.
• Some editors like Vim and IntelliJ have a “safe write” feature that currently breaks the watcher. You will need to disable it. Follow the instructions in“Adjusting Your Text Editor”.
• If your project path contains parentheses, try moving the project to a path without them. This is caused by aWebpack watcher bug.
• On Linux and macOS, you might need totweak system settings to allow more watchers.
• If the project runs inside a virtual machine such as (a Vagrant provisioned) VirtualBox, create an.env file in your project directory if it doesn’t exist, and addCHOKIDAR_USEPOLLING=true to it. This ensures that the next time you runnpm start, the watcher uses the polling mode, as necessary inside a VM.

If none of these solutions help please leave a commentin this thread.

If you runnpm test and the console gets stuck after printingreact-scripts test --env=jsdom to the console there might be a problem with yourWatchman installation as described infacebookincubator/create-react-app#713.

We recommend deletingnode_modules in your project and runningnpm install (oryarn if you use it) first. If it doesn't help, you can try one of the numerous workarounds mentioned in these issues:

• facebook/jest#1767
• facebook/watchman#358
• ember-cli/ember-cli#6259

It is reported that installing Watchman 4.7.0 or newer fixes the issue. If you useHomebrew, you can run these commands to update it:

watchman shutdown-serverbrew updatebrew reinstall watchman

You can findother installation methods on the Watchman documentation page.

If this still doesn’t help, try runninglaunchctl unload -F ~/Library/LaunchAgents/com.github.facebook.watchman.plist.

There are also reports thatuninstalling Watchman fixes the issue. So if nothing else helps, remove it from your system and try again.

It is reported thatnpm run build can fail on machines with limited memory and no swap space, which is common in cloud environments. Even with small projects this command can increase RAM usage in your system by hundreds of megabytes, so if you have less than 1 GB of available memory your build is likely to fail with the following message:

The build failed because the process exited too early. This probably means the system ran out of memory or someone calledkill -9 on the process.

If you are completely sure that you didn't terminate the process, consideradding some swap space to the machine you’re building on, or build the project locally.

This may be a problem with case sensitive filenames.Please refer tothis section.

If you use aMoment.js, you might notice that only the English locale is available by default. This is because the locale files are large, and you probably only need a subset ofall the locales provided by Moment.js.

To add a specific Moment.js locale to your bundle, you need to import it explicitly.
For example:

importmomentfrom'moment';import'moment/locale/fr';

If import multiple locales this way, you can later switch between them by callingmoment.locale() with the locale name:

importmomentfrom'moment';import'moment/locale/fr';import'moment/locale/es';// ...moment.locale('fr');

This will only work for locales that have been explicitly imported before.

Some third-party packages don't compile their code to ES5 before publishing to npm. This often causes problems in the ecosystem because neither browsers (except for most modern versions) nor some tools currently support all ES6 features. We recommend to publish code on npm as ES5 at least for a few more years.

1. Open an issue on the dependency's issue tracker and ask that the package be published pre-compiled.

• Note: Create React App can consume both CommonJS and ES modules. For Node.js compatibility, it is recommended that the main entry point is CommonJS. However, they can optionally provide an ES module entry point with themodule field inpackage.json. Note thateven if a library provides an ES Modules version, it should still precompile other ES6 features to ES5 if it intends to support older browsers.

2. Fork the package and publish a corrected version yourself.

3. If the dependency is small enough, copy it to yoursrc/ folder and treat it as application code.

In the future, we might start automatically compiling incompatible third-party modules, but it is not currently supported. This approach would also slow down the production builds.

If you have ideas for more “How To” recipes that should be on this page,let us know orcontribute some!