create-react-app/docusaurus/docs/user-guide.md

id, title, sidebar_label

id	title	sidebar_label
user-guide	User guide	User guide

This project was bootstrapped with Create React App.

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

Table of Contents

• Changing the Page <title>
• Installing a Dependency
• Importing a Component
• Code Splitting
• Adding a Stylesheet
• Adding a CSS Modules Stylesheet
• Adding a Sass Stylesheet
• Post-Processing CSS
• Adding Images, Fonts, and Files
• Adding SVGs
• Using the public Folder
  • Changing the HTML
  • Adding Assets Outside of the Module System
  • When to Use the public Folder
• Using Global Variables
• Adding Bootstrap
  • Using a Custom Theme
• Adding Flow
• Adding Relay
• Adding a Router
• 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?
• Fetching Data with AJAX Requests
• Integrating with an API Backend
  • Node
  • Ruby on Rails
• 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
• Making a Progressive Web App
  • Why Opt-in?
  • Offline-First Considerations
  • Progressive Web App Metadata
• Advanced Configuration
• Alternatives to Ejecting

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 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. Alternatively, you can pre-build each page as a static HTML file which then loads the JavaScript bundle, which is covered here.

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 react-router-dom

Alternatively you may use yarn:

yarn add react-router-dom

This works for any library, not just react-router-dom.

Importing a Component

This project setup supports ES6 modules thanks to Webpack.
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

Code Splitting

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 via dynamic import(). Its proposal is in stage 3. The import() function-like form takes the module name as an argument and returns a Promise which always resolves to the namespace object of the module.

Here is an example:

moduleA.js

const moduleA = 'Hello';

export { moduleA };

App.js

import React, { Component } from 'react';

class App extends Component {
  handleClick = () => {
    import('./moduleA')
      .then(({ moduleA }) => {
        // Use moduleA
      })
      .catch(err => {
        // Handle failure
      });
  };

  render() {
    return (
      <div>
        <button onClick={this.handleClick}>Load</button>
      </div>
    );
  }
}

export default App;

This will make moduleA.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 with async / await syntax if you prefer it.

With React Router

If you are using React Router check out this tutorial on how to use code splitting with it. You can find the companion GitHub repository here.

Also check out the Code Splitting section in React documentation.

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.

Adding a CSS Modules Stylesheet

Note: this feature is available with react-scripts@2.0.0 and higher.

This project supports CSS Modules alongside regular stylesheets using the [name].module.css file naming convention. CSS Modules allows the scoping of CSS by automatically creating a unique classname of the format [filename]\_[classname]\_\_[hash].

Tip: Should you want to preprocess a stylesheet with Sass then make sure to follow the installation instructions and then change the stylesheet file extension as follows: [name].module.scss or [name].module.sass.

CSS Modules let you use the same CSS class name in different files without worrying about naming clashes. Learn more about CSS Modules here.

Button.module.css

.error {
  background-color: red;
}

another-stylesheet.css

.error {
  color: red;
}

Button.js

import React, { Component } from 'react';
import styles from './Button.module.css'; // Import css modules stylesheet as styles
import './another-stylesheet.css'; // Import regular stylesheet

class Button extends Component {
  render() {
    // reference as a js object
    return <button className={styles.error}>Error Button</button>;
  }
}

Result

No clashes from other .error class names

<!-- This button has red background but not red text -->
<button class="Button_error_ax7yz"></div>

This is an optional feature. Regular <link> stylesheets and CSS files are fully supported. CSS Modules are turned on for files ending with the .module.css extension.

Adding a Sass Stylesheet

Note: this feature is available with react-scripts@2.0.0 and higher.

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.

To use Sass, first install node-sass:

$ npm install node-sass --save
$ # or
$ yarn add node-sass

Now you can rename src/App.css to src/App.scss and update src/App.js to import src/App.scss. This file and any other file will be automatically compiled if imported with the extension .scss or .sass.

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.

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

Tip: You can opt into using this feature with CSS modules too!

Note: You must prefix imports from node_modules with ~ as displayed above.

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.

Support for new CSS features like the all property, break properties, custom properties, and media query ranges are automatically polyfilled to add support for older browsers.

You can customize your target support browsers by adjusting the browserslist key in package.json accoring to the Browserslist specification.

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.

CSS Grid Layout prefixing is disabled by default, but it will not strip manual prefixing. If you'd like to opt-in to CSS Grid prefixing, first familiarize yourself about its limitations.
To enable CSS Grid prefixing, add /* autoprefixer grid: on */ to the top of your CSS file.

Adding Images, Fonts, and Files

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

You can import 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 the src attribute of an image or the href 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 a data 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:

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

Adding SVGs

Note: this feature is available with react-scripts@2.0.0 and higher.

One way to add SVG files was described in the section above. You can also import SVGs directly as React components. You can use either of the two approaches. In your code it would look like this:

import { ReactComponent as Logo } from './logo.svg';
const App = () => (
  <div>
    {/* Logo is an actual React component */}
    <Logo />
  </div>
);

This is handy if you don't want to load SVG as a separate file. Don't forget the curly braces in the import! The ReactComponent import name is special and tells Create React App that you want a React component that renders an SVG, rather than its filename.

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. 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 and 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, 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 reactstrap together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:

Install reactstrap and Bootstrap from npm. reactstrap does not include Bootstrap CSS so this needs to be installed as well:

npm install --save reactstrap bootstrap@4

Alternatively you may use yarn:

yarn add bootstrap@4 reactstrap

Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your src/index.js file:

import 'bootstrap/dist/css/bootstrap.css';
// Put any other imports below so that CSS from your
// components takes precedence over default styles.

Import required reactstrap components within src/App.js file or your custom component files:

import { Button } from 'reactstrap';

Now you are ready to use the imported reactstrap components within your component hierarchy defined in the render method. Here is an example App.js redone using reactstrap.

Using a Custom Theme

Note: this feature is available with react-scripts@2.0.0 and higher.

Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).
As of react-scripts@2.0.0 you can import .scss files. This makes it possible to use a package's built-in Sass variables for global style preferences.

To customize Bootstrap, create a file called src/custom.scss (or similar) and import the Bootstrap source stylesheet. Add any overrides before the imported file(s). You can reference Bootstrap's documentation for the names of the available variables.

// Override default variables before the import
$body-bg: #000;

// Import Bootstrap and its default variables
@import '~bootstrap/scss/bootstrap.scss';

Note: You must prefix imports from node_modules with ~ as displayed above.

Finally, import the newly created .scss file instead of the default Bootstrap .css in the beginning of your src/index.js file, for example:

import './custom.scss';

Adding Flow

Flow is a static type checker that helps you write code with fewer bugs. Check out this introduction to using static types in JavaScript 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 flow-bin (or yarn add flow-bin).
2. Add "flow": "flow" to the scripts section of your package.json.
3. Run npm run flow init (or yarn flow init) to create a .flowconfig file 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 Relay

Relay is a framework for building data-driven React applications powered by GraphQL. The current release candidate of Relay works with Create React App projects out of the box using Babel Macros. Simply set up your project as laid out in the Relay documentation, then make sure you have a version of the babel plugin providing the macro.

To add it, run:

npm install --save --dev babel-plugin-relay@dev

Alternatively you may use yarn:

yarn upgrade babel-plugin-relay@dev

Then, wherever you use the graphql template tag, import the macro:

import graphql from 'babel-plugin-relay/macro';
// instead of:
//   import { graphql } from "babel-plugin-relay"

graphql`
  query UserQuery {
    viewer {
      id
    }
  }
`;

To learn more about Relay, check out its documentation.

Adding a Router

Create React App doesn't prescribe a specific routing solution, but React Router is the most popular one.

To add it, run:

npm install --save react-router-dom

Alternatively you may use yarn:

yarn add react-router-dom

To try it, delete all the code in src/App.js and replace it with any of the examples on its website. The Basic Example is a good place to get started.

Note that you may need to configure your production server to support client-side routing before deploying your app.

Adding Custom Environment Variables

Note: this feature is available with react-scripts@0.2.3 and higher.

Your project can consume variables declared in your environment as if they were declared locally in your JS files. By default you will have NODE_ENV defined for you, and any other environment variables starting with REACT_APP_.

The environment variables are embedded during the build time. Since Create React App produces a static HTML/CSS/JS bundle, it can’t possibly read them at runtime. To read them at runtime, you would need to load HTML into memory on the server and replace placeholders in runtime, just like described here. 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 (
    <div>
      <small>You are running this application in <b>{process.env.NODE_ENV}</b> mode.</small>
      <form>
        <input type="hidden" defaultValue={process.env.REACT_APP_SECRET_CODE} />
      </form>
    </div>
  );
}

During the build, process.env.REACT_APP_SECRET_CODE will be replaced with the current value of the REACT_APP_SECRET_CODE environment variable. Remember that the NODE_ENV variable will be set for you automatically.

When you load the app in the browser and inspect the <input>, you will see its value set to abcdef, and the bold text will show the environment provided when using npm start:

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

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

Having access to the NODE_ENV is also useful for performing actions conditionally:

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

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

Referencing Environment Variables in the HTML

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

You can also access the environment variables starting with REACT_APP_ in the public/index.html. For example:

<title>%REACT_APP_WEBSITE_NAME%</title>

Note that the caveats from the above section apply:

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

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

Windows (Powershell)

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

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:

REACT_APP_SECRET_CODE=abcdef

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

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

What other .env files can be used?

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

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

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

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

These variables will act as the defaults if the machine does not explicitly set them.
Please refer to the dotenv documentation for more details.

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

Expanding Environment Variables In .env

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

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

For example, to get the environment variable npm_package_version:

REACT_APP_VERSION=$npm_package_version
# also works:
# REACT_APP_VERSION=${npm_package_version}

Or expand variables local to the current .env file:

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

Can I Use Decorators?

Some popular libraries use decorators in their documentation.
Create React App intentionally doesn’t support decorator syntax at the moment because:

• It is an experimental proposal and is subject to change (in fact, it has already changed once, and will change again).
• Most libraries currently support only the old version of the proposal — which will never be a standard.

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

• #214
• #411

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

Fetching Data with AJAX Requests

React doesn't prescribe a specific approach to data fetching, but people commonly use either a library like axios or the fetch() API provided by the browser.

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

A Promise represents the eventual result of an asynchronous operation, you can find more information about Promises here and here. Both axios and fetch() use Promises under the hood. You can also use the async / await syntax to reduce the callback nesting.

Make sure the fetch() API and Promises are available in your target audience's browsers. For example, support in Internet Explorer requires a polyfill.

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

Integrating with an API Backend

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

Node

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

Ruby on Rails

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

API Platform (PHP and Symfony)

API Platform is a framework designed to build API-driven projects. It allows to create hypermedia and GraphQL APIs in minutes. It is shipped with an official Progressive Web App generator as well as a dynamic administration interface, both built for Create React App. Check out this tutorial.

Using HTTPS in Development

Note: this feature is available with react-scripts@0.4.0 and higher.

You may require the dev server to serve pages over HTTPS. One particular case where this could be useful is when using the "proxy" feature to proxy requests to an API server when that API server is itself serving HTTPS.

To do this, set the HTTPS environment variable to true, then start the dev server as usual with npm start:

Windows (cmd.exe)

set HTTPS=true&&npm start

(Note: the lack of whitespace is intentional.)

Windows (Powershell)

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

Linux, macOS (Bash)

HTTPS=true npm start

Note that the server will use a self-signed certificate, so your web browser will almost definitely display a warning upon accessing the page.

Generating Dynamic <meta> Tags on the Server

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

<!doctype html>
<html lang="en">
  <head>
    <meta property="og:title" content="__OG_TITLE__">
    <meta property="og:description" content="__OG_DESCRIPTION__">

Then, on the server, regardless of the backend you use, you can read index.html into memory and replace __OG_TITLE__, __OG_DESCRIPTION__, and any other placeholders with values depending on the current URL. Just make sure to sanitize and escape the interpolated values so that they are safe to embed into HTML!

If you use a Node server, you can even share the route matching logic between the client and the server. However duplicating it also works fine in simple cases.

Pre-Rendering into Static HTML Files

If you’re hosting your build with a static hosting provider you can use react-snapshot or react-snap to generate HTML pages for each route, or relative link, in your application. These pages will then seamlessly become active, or “hydrated”, when the JavaScript bundle has loaded.

There are also opportunities to use this outside of static hosting, to take the pressure off the server when generating and caching routes.

The primary benefit of pre-rendering is that you get the core content of each page with the HTML payload—regardless of whether or not your JavaScript bundle successfully downloads. It also increases the likelihood that each route of your application will be picked up by search engines.

You can read more about zero-configuration pre-rendering (also called snapshotting) here.

Injecting Data from the Server into the Page

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

<!doctype html>
<html lang="en">
  <head>
    <script>
      window.SERVER_DATA = __SERVER_DATA__;
    </script>

Then, on the server, you can replace __SERVER_DATA__ with a JSON of real data right before sending the response. The client code can then read window.SERVER_DATA to use it. Make sure to sanitize the JSON before sending it to the client as it makes your app vulnerable to XSS attacks.

Making a Progressive Web App

The production build has all the tools necessary to generate a first-class Progressive Web App, but the offline/cache-first behavior is opt-in only. By default, the build process will generate a service worker file, but it will not be registered, so it will not take control of your production web app.

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

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

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

Why Opt-in?

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

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

However, they can make debugging deployments more challenging so, starting with Create React App 2, service workers are opt-in.

The workbox-webpack-plugin is integrated into production configuration, and it will take care of generating a service worker file that will automatically precache all of your local assets and keep them up to date as you deploy updates. The service worker will use a cache-first strategy for handling all requests for local assets, including navigation requests for your HTML, ensuring that your web app is consistently fast, even on a slow or unreliable network.

Offline-First Considerations

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

1. Service workers require HTTPS, although to facilitate local testing, that policy does not apply to localhost. If your production web server does not support HTTPS, then the service worker registration will fail, but the rest of your web app will remain functional.

2. Service workers are not supported in older web browsers. Service worker registration won't be attempted on browsers that lack support.

3. The service worker is only enabled in the production environment, e.g. the output of npm run build. It's recommended that you do not enable an offline-first service worker in a development environment, as it can lead to frustration when previously cached assets are used and do not include the latest changes you've made locally.

4. If you need to test your offline-first service worker locally, build the application (using npm run build) and run a simple http server from your build directory. After running the build script, create-react-app will give instructions for one way to test your production build locally and the deployment instructions have instructions for using other methods. Be sure to always use an incognito window to avoid complications with your browser cache.

5. Users aren't always familiar with offline-first web apps. It can be useful to let the user know when the service worker has finished populating your caches (showing a "This web app works offline!" message) and also let them know when the service worker has fetched the latest updates that will be available the next time they load the page (showing a "New content is available; please refresh." message). Showing this messages is currently left as an exercise to the developer, but as a starting point, you can make use of the logic included in src/registerServiceWorker.js, which demonstrates which service worker lifecycle events to listen for to detect each scenario, and which as a default, just logs appropriate messages to the JavaScript console.

6. By default, the generated service worker file will not intercept or cache any cross-origin traffic, like HTTP API requests, images, or embeds loaded from a different domain.

Progressive Web App Metadata

The default configuration includes a web app manifest located at public/manifest.json, that you can customize with details specific to your web application.

When a user adds a web app to their homescreen using Chrome or Firefox on Android, the metadata in manifest.json determines what icons, names, and branding colors to use when the web app is displayed. The Web App Manifest guide provides more context about what each field means, and how your customizations will affect your users' experience.

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

Advanced Configuration

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

Variable	Development	Production	Usage
BROWSER	✅	❌	By default, Create React App will open the default system browser, favoring Chrome on macOS. Specify a browser to override this behavior, or set it to none to disable it completely. If you need to customize the way the browser is launched, you can specify a node script instead. Any arguments passed to npm start will also be passed to this script, and the url where your app is served will be the last argument. Your script's file name must have the .js extension.
HOST	✅	❌	By default, the development web server binds to localhost. You may use this variable to specify a different host.
PORT	✅	❌	By default, the development web server will attempt to listen on port 3000 or prompt you to attempt the next available port. You may use this variable to specify a different port.
HTTPS	✅	❌	When set to true, Create React App will run the development server in https mode.
PUBLIC_URL	❌	✅	Create React App assumes your application is hosted at the serving web server's root or a subpath as specified in package.json (homepage). Normally, Create React App ignores the hostname. You may use this variable to force assets to be referenced verbatim to the url you provide (hostname included). This may be particularly useful when using a CDN to host your application.
CI	🔶	✅	When set to true, Create React App treats warnings as failures in the build. It also makes the test runner non-watching. Most CIs set this flag by default.
REACT_EDITOR	✅	❌	When an app crashes in development, you will see an error overlay with clickable stack trace. When you click on it, Create React App will try to determine the editor you are using based on currently running processes, and open the relevant source file. You can send a pull request to detect your editor of choice. Setting this environment variable overrides the automatic detection. If you do it, make sure your systems PATH environment variable points to your editor’s bin folder. You can also set it to none to disable it completely.
CHOKIDAR_USEPOLLING	✅	❌	When set to true, the watcher runs in polling mode, as necessary inside a VM. Use this option if npm start isn't detecting changes.
GENERATE_SOURCEMAP	❌	✅	When set to false, source maps are not generated for a production build. This solves OOM issues on some smaller machines.
NODE_PATH	✅	✅	Same as NODE_PATH in Node.js, but only relative folders are allowed. Can be handy for emulating a monorepo setup by setting NODE_PATH=src.

Alternatives to Ejecting

Ejecting lets you customize anything, but from that point on you have to maintain the configuration and scripts yourself. This can be daunting if you have many similar projects. In such cases instead of ejecting we recommend to fork react-scripts and any other packages you need. This article dives into how to do it in depth. You can find more discussion in this issue.