create-react-app is a global command-line utility that you use to create new projects.
react-scripts is a development dependency in the generated projects (including this one).
You almost never need to update create-react-app itself: it delegates all the setup to react-scripts.
When you run create-react-app, it always creates the project with the latest version of react-scripts so you’ll get all the new features and improvements in newly created apps automatically.
To update an existing project to a new version of react-scripts, open the changelog, find the version you’re currently on (check package.json in this folder if you’re not sure), and apply the migration instructions for the newer versions.
In most cases bumping the react-scripts version in package.json and running npm install in this folder should be enough, but it’s good to consult the changelog for potential breaking changes.
We commit to keeping the breaking changes minimal so you can upgrade react-scripts painlessly.
For the project to build, these files must exist with exact filenames:
public/index.html is the page template;
src/index.js is the JavaScript entry point.
You can delete or rename the other files.
You may create subdirectories inside src. For faster rebuilds, only files inside src are processed by Webpack.
You need to put any JS and CSS files inside src, or Webpack won’t see them.
Only files inside public can be used from public/index.html.
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.
Available Scripts
In the project directory, you can run:
npm start
Runs the app in the development mode.
Open http://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.
npm test
Launches the test runner in the interactive watch mode.
See the section about running tests for more information.
npm run build
Builds the app for production to the build 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 about deployment for more information.
npm run eject
Note: this is a one-way operation. Once you eject, you can’t go back!
If you aren’t satisfied with the build tool and configuration choices, you can eject at any time. This command will remove the single build dependency from your project.
Instead, it will copy all the configuration files and the transitive dependencies (Webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except eject will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.
You don’t have to ever use eject. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.
Supported Language Features and Polyfills
This project supports a superset of the latest JavaScript standard.
In addition to ES6 syntax features, it also supports:
While we recommend to use experimental proposals with some caution, Facebook heavily uses these features in the product code, so we intend to provide codemods if any of these proposals change in the future.
Note that the project only includes a few ES6 polyfills:
If you use any other ES6+ features that need runtime support (such as Array.from() or Symbol), make sure you are including the appropriate polyfills manually, or that the browsers you are targeting already support them.
Syntax Highlighting in the Editor
To configure the syntax highlighting in your favorite text editor, head to the relevant Babel documentation page and follow the instructions. Some of the most popular editors are covered.
Some editors, including Sublime Text, Atom, and Visual Studio Code, provide plugins for ESLint.
They are not required for linting. You should see the linter output right in your terminal as well as the browser console. However, if you prefer the lint results to appear right in your editor, there are some extra steps you can do.
You would need to install an ESLint plugin for your editor first.
A note for Atom linter-eslint users
If you are using the Atom linter-eslint plugin, make sure that Use global ESLint installation option is checked:
For Visual Studio Code users
VS Code ESLint plugin automatically detects Create React App's configuration file. So you do not need to create eslintrc.json at the root directory, except when you want to add your own rules. In that case, you should include CRA's config by adding this line:
{
// ...
"extends": "react-app"
}
Then add this block to the `package.json` file of your project:
```js
{
// ...
"eslintConfig": {
"extends": "react-app"
}
}
Finally, you will need to install some packages globally:
We recognize that this is suboptimal, but it is currently required due to the way we hide the ESLint dependency. The ESLint team is already working on a solution to this so this may become unnecessary in a couple of months.
Visual Studio Code supports live-editing and debugging out of the box with Create React App. This enables you as a developer to write and debug your React code without leaving the editor, and most importantly it enables you to have a continuous development workflow, where context switching is minimal, as you don’t have to switch between tools.
Start your app by running npm start, and start debugging in VS Code by pressing F5 or by clicking the green debug icon. You can now write code, set breakpoints, make changes to the code, and debug your newly modified code—all from your editor.
Changing the Page <title>
You can find the source HTML file in the public folder of the generated project. You may edit the <title> tag in it to change the title from “React App” to anything else.
Note that normally you wouldn’t edit files in the public folder very often. For example, adding a stylesheet 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 <library-name>
Importing a Component
This project setup supports ES6 modules thanks to Babel.
While you can still use require() and module.exports, we encourage you to use import and export instead.
For example:
Button.js
importReact,{Component}from'react';classButtonextendsComponent{render(){// ...}}exportdefaultButton;// Don’t forget to use export default!
DangerButton.js
importReact,{Component}from'react';importButtonfrom'./Button';// Import a component from another fileclassDangerButtonextendsComponent{render(){return<Buttoncolor="red"/>;}}exportdefaultDangerButton;
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.
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
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 approach here. However you should be aware that this makes your code less portable to other build tools and environments than Webpack.
In development, expressing dependencies this way allows your styles to be reloaded on the fly as you edit them. In production, all CSS files will be concatenated into a single minified .css file in the build output.
If you are concerned about using Webpack-specific semantics, you can put all your CSS right into src/index.css. It would still be imported from src/index.js, but you could always remove that import if you later migrate to a different build tool.
Post-Processing CSS
This project setup minifies your CSS and adds vendor prefixes to it automatically through Autoprefixer so you don’t need to worry about it.
Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a .Button CSS class in <AcceptButton> and <RejectButton> components, we recommend creating a <Button> component with its own .Button styles, that both <AcceptButton> and <RejectButton> can render (but not inherit).
Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.
First, let’s install the command-line interface for Sass:
npm install node-sass --save-dev
Then in package.json, add the following lines to scripts:
Note: To use a different preprocessor, replace build-css and watch-css commands according to your preprocessor’s documentation.
Now you can rename src/App.css to src/App.scss and run npm run watch-css. The watcher will find every Sass file in src subdirectories, and create a corresponding CSS file next to it, in our case overwriting src/App.css. Since src/App.js still imports src/App.css, the styles become a part of your application. You can now edit src/App.scss, and src/App.css will be regenerated.
To share variables between Sass files, you can use Sass imports. For example, src/App.scss and other component style files could include @import "./shared.scss"; with variable definitions.
At this point you might want to remove all CSS files from the source control, and add src/**/*.css to your .gitignore file. It is generally a good practice to keep the build products outside of the source control.
As a final step, you may find it convenient to run watch-css automatically with npm start, and run build-css as a part of npm run build. You can use the && operator to execute two scripts sequentially. However, there is no cross-platform way to run two scripts in parallel, so we will install a package for this:
npm install --save-dev npm-run-all
Then we can change start and build scripts to include the CSS preprocessor commands:
Now running npm start and npm run build also builds Sass files. Note that node-sass seems to have an issue recognizing newly created files on some systems so you might need to restart the watcher when you create a file until it’s resolved.
Adding Images and Fonts
With Webpack, using static assets like images and fonts works similarly to CSS.
You can import an image right in a JavaScript module. This tells Webpack to include that image in the bundle. Unlike CSS imports, importing an image or a font gives you a string value. This value is the final image path you can reference in your code.
Here is an example:
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.
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.
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<imgsrc={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.
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