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" />;
}
}
@import 'styles/_colors.scss'; // assuming a styles directory under src/
@import 'nprogress/nprogress'; // importing a css file from the nprogress node module
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;
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'} />;
}
请记住这种方法的缺点：
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
通常我们建议导入 样式表、来自 JavaScript 的 图像和字体。
public 文件夹可用作许多不太常见情况的解决方法：
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.
请注意，如果您添加声明全局变量的
Using Global Variables
当您在 HTML 文件中包含定义全局变量的脚本并尝试在代码中使用这些变量之一时，linter 会抱怨，因为它看不到变量的定义。
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.
/ - 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
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.
{
// ...
"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
// ...
}
}
// ...
}
Using HTTPS in Development
注意：此功能适用于 react-scripts@0.4.0 及更高版本。
您可能需要开发服务器通过 HTTPS 提供页面。 这可能有用的一个特殊情况是使用“代理”功能将请求代理到 API 服务器本身时服务 HTTPS。
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, otherwise 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 Browsers
By default, the generated project uses the latest version of React.
While we recommend using 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.
Also note that using some newer syntax features like for...of or [...nonArrayValue] causes Babel to emit code that depends on ES6 runtime features and might not work without a polyfill. When in doubt, use Babel REPL to see what any specific syntax compiles down to.
Syntax Highlighting in the Editor
To configure the syntax highlighting in your favorite text editor, head to the relevant Babel documentation page and follow the instructions. Some of the most popular editors are covered.
Displaying Lint Output in the Editor
Note: this feature is available with react-scripts@0.2.0 and higher.
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 will only 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 using Prettier instead of ESLint style rules.
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.
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.
In the WebStorm menu Run select Edit Configurations.... Then click + and select JavaScript Debug. Paste http://localhost:3000 into the URL field and save the configuration.
Start your app by running npm start, then press ^D on macOS or F9 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.
Formatting Code Automatically
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 the Prettier's GitHub page for more information, and look at this page 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 use yarn:
yarn add husky lint-staged prettier
husky makes it easy to use githooks as if they are npm 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,json,css}" to format your entire project for the first time.
Next you might want to integrate Prettier in your favorite editor. Read the section on Editor Integration on the Prettier GitHub page.
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
Alternatively you may use yarn:
yarn add react-router
This works for any library, not just react-router.
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
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;
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.
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.
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.
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 --save node-sass-chokidar
Alternatively you may use yarn:
yarn add node-sass-chokidar
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.
To enable importing files without using relative paths, you can add the --include-path option to the command in package.json.
@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 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 npm-run-all
Alternatively you may use yarn:
yarn add npm-run-all
Then we can change start and build scripts to include the CSS preprocessor commands:
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.
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.
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.
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 React Bootstrap together with React but it is a popular library for integrating Bootstrap with React apps. If you need it, you can integrate it with Create React App by following these steps:
Install React Bootstrap and Bootstrap from npm. React Bootstrap does not include Bootstrap CSS so this needs to be installed as well:
npm install --save react-bootstrap bootstrap@3
Alternatively you may use yarn:
yarn add react-bootstrap bootstrap@3
Import Bootstrap CSS and optionally Bootstrap theme CSS in the beginning of your src/index.js file:
import 'bootstrap/dist/css/bootstrap.css';
import 'bootstrap/dist/css/bootstrap-theme.css';
// Put any other imports below so that CSS from your
// components takes precedence over default styles.
Import required React Bootstrap components within src/App.js file or your custom component files:
import { Navbar, Jumbotron, Button } from 'react-bootstrap';
Now you are ready to use the imported React Bootstrap components within your component hierarchy defined in the render method. Here is an example App.js redone using React Bootstrap.
Using a Custom Theme
Sometimes you might need to tweak the visual styles of Bootstrap (or equivalent package).
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.
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:
Run npm install --save flow-bin (or yarn add flow-bin).
Add "flow": "flow" to the scripts section of your package.json.
Run npm run flow init (or yarn flow init) to create a .flowconfig file in the root directory.
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.
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.)
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.
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:
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. Conveniently, Create React App includes a polyfill for fetch() so you can use it without worrying about the browser support.
The global fetch function allows to easily makes 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 fetchhere.
This project also includes a Promise polyfill which provides a full implementation of Promises/A+. 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.
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.
Proxying API Requests in Development
Note: this feature is available with react-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 is not required. However, if you do have a setup like this, it is convenient to write requests like fetch('/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 a proxy field to your package.json, for example:
"proxy": "http://localhost:4000",
This way, when you fetch('/api/todos') in development, the development server will recognize that it’s not a static asset, and will proxy your request to http://localhost:4000/api/todos as a fallback. The development server will only attempt to send requests without text/html in its Accept header to the proxy.
Conveniently, this avoids CORS 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 that proxy only has effect in development (with npm 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 a text/html accept header will be redirected to the specified proxy.
The proxy option supports HTTP, HTTPS and WebSocket connections.
If the proxy option is not flexible enough for you, alternatively you can:
"Invalid Host Header" Errors After Configuring Proxy
When you enable the proxy 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 in this article and this issue.
This shouldn’t affect you when developing on localhost, but if you develop remotely like described here, you will see this error in the browser after enabling the proxy 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.
Configuring the Proxy Manually
Note: this feature is available with react-scripts@1.0.0 and higher.
If the proxy option is not flexible enough for you, you can specify an object in the following form (in package.json).
You may also specify any configuration value http-proxy-middleware or http-proxy supports.
All requests matching this path will be proxies, no exceptions. This includes requests for text/html, which the standard proxy 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.
When setting up a WebSocket proxy, there are a some extra considerations to be aware of.
If you’re using a WebSocket engine like Socket.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 with the websocket.org echo test.
Standard WebSockets will work with a standard WebSocket server as well as the websocket.org echo test. You can use libraries like ws for the server, with native WebSockets in the browser.
Either way, you can proxy WebSocket requests manually in package.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
// ...
}
}
// ...
}
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
Windows (Powershell)
($env:HTTPS = $true) -and (npm start)
(Note: the lack of whitespace is intentional.)
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:
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.
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.
