tramyardg / reactjs-notes

ReactJS notes and tutorials.

This project was bootstrapped with Create React App.

Folder Structure

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

For the project to build, these files must exist with exact filenames:

• public/index.html is the page template;
• src/index.js is the JavaScript entry point.

You can delete or rename the other files.

You may create subdirectories 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.

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;

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.

Post-Processing CSS

This project setup minifies your CSS and adds vendor prefixes to it automatically through Autoprefixer so you don’t need to worry about it.

For example, this:

.App {
  display: flex;
  flex-direction: row;
  align-items: center;
}

becomes this:

.App {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  -webkit-box-orient: horizontal;
  -webkit-box-direction: normal;
      -ms-flex-direction: row;
          flex-direction: row;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
}

If you need to disable autoprefixing for some reason, follow this section.

Adding a CSS Preprocessor (Sass, Less etc.)

Generally, we recommend that you don’t reuse the same CSS classes across different components. For example, instead of using a .Button CSS class in <AcceptButton> and <RejectButton> components, we recommend creating a <Button> component with its own .Button styles, that both <AcceptButton> and <RejectButton> can render (but not inherit).

Following this rule often makes CSS preprocessors less useful, as features like mixins and nesting are replaced by component composition. You can, however, integrate a CSS preprocessor if you find it valuable. In this walkthrough, we will be using Sass, but you can also use Less, or another alternative.

First, let’s install the command-line interface for Sass:

npm install --save node-sass-chokidar

Alternatively you may use yarn:

yarn add node-sass-chokidar

Then in package.json, add the following lines to scripts:

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

Note: To use a different preprocessor, 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.

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

This will allow you to do imports like

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

At this point you might want to remove all CSS files from the source control, and 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:

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

Now running npm start and npm run build also builds Sass files.

Why node-sass-chokidar?

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

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

• Infinite styles compiling #1939

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

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

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.

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 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.

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 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?

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

• It is an experimental proposal and is subject to change.
• The current specification version is not officially supported by Babel.
• If the specification changes, we won’t be able to write a codemod because we don’t use them internally at Facebook.

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

• #214
• #411

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

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 fetch here.

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.

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.

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:

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

"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.

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

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.

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

Configuring a WebSocket Proxy

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.

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

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:

<!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.

Running Tests

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

Create React App uses Jest as its test runner. To prepare for this integration, we did a major revamp of Jest so if you heard bad things about it years ago, give it another try.

Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.

While Jest provides browser globals such as window thanks to jsdom, they are only approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.

We recommend that you use a separate tool for browser end-to-end tests if you need them. They are beyond the scope of Create React App.

Filename Conventions

Jest will look for test files with any of the following popular naming conventions:

• Files with .js suffix in __tests__ folders.
• Files with .test.js suffix.
• Files with .spec.js suffix.

The .test.js / .spec.js files (or the __tests__ folders) can be located at any depth under the src top level folder.

We recommend to put the test files (or __tests__ folders) next to the code they are testing so that relative imports appear shorter. For example, if App.test.js and App.js are in the same folder, the test just needs to import App from './App' instead of a long relative path. Colocation also helps find tests more quickly in larger projects.

Writing Tests

To create tests, add it() (or test()) blocks with the name of the test and its code. You may optionally wrap them in describe() blocks for logical grouping but this is neither required nor recommended.

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

import sum from './sum';

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

All expect() matchers supported by Jest are extensively documented here.
You can also use jest.fn() and expect(fn).toBeCalled() to create “spies” or mock functions.

Testing Components

There is a broad spectrum of component testing techniques. They range from a “smoke test” verifying that a component renders without throwing, to shallow rendering and testing some of the output, to full rendering and testing component lifecycle and state changes.

Different projects choose different testing tradeoffs based on how often components change, and how much logic they contain. If you haven’t decided on a testing strategy yet, we recommend that you start with creating simple smoke tests for your components:

import React from 'react';
import ReactDOM from 'react-dom';
import App from './App';

it('renders without crashing', () => {
  const div = document.createElement('div');
  ReactDOM.render(<App />, div);
});

This test mounts a component and makes sure that it didn’t throw during rendering. Tests like this provide a lot of value with very little effort so they are great as a starting point, and this is the test you will find in src/App.test.js.

When you encounter bugs caused by changing components, you will gain a deeper insight into which parts of them are worth testing in your application. This might be a good time to introduce more specific tests asserting specific expected output or behavior.

If you’d like to test components in isolation from the child components they render, we recommend using shallow() rendering API from Enzyme. To install it, run:

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

Alternatively you may use yarn:

yarn add enzyme enzyme-adapter-react-16 react-test-renderer

As of Enzyme 3, you will need to install Enzyme along with an Adapter corresponding to the version of React you are using. (The examples above use the adapter for React 16.)

The adapter will also need to be configured in your global setup file:

src/setupTests.js

import { configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() });

Note: Keep in mind that if you decide to "eject" before creating src/setupTests.js, the resulting package.json file won't contain any reference to it. Read here to learn how to add this after ejecting.

Now you can write a smoke test with it:

import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders without crashing', () => {
  shallow(<App />);
});

Unlike the previous smoke test using ReactDOM.render(), this test only renders <App> and doesn’t go deeper. For example, even if <App> itself renders a <Button> that throws, this test will pass. Shallow rendering is great for isolated unit tests, but you may still want to create some full rendering tests to ensure the components integrate correctly. Enzyme supports full rendering with mount(), and you can also use it for testing state changes and component lifecycle.

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

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

import React from 'react';
import { shallow } from 'enzyme';
import App from './App';

it('renders welcome message', () => {
  const wrapper = shallow(<App />);
  const welcome = <h2>Welcome to React</h2>;
  // expect(wrapper.contains(welcome)).to.equal(true);
  expect(wrapper.contains(welcome)).toEqual(true);
});

All Jest matchers are extensively documented here.
Nevertheless you can use a third-party assertion library like Chai if you want to, as described below.

Additionally, you might find jest-enzyme helpful to simplify your tests with readable matchers. The above contains code can be written more simply with jest-enzyme.

expect(wrapper).toContainReact(welcome)

To enable this, install jest-enzyme:

npm install --save jest-enzyme

Alternatively you may use yarn:

yarn add jest-enzyme

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

import 'jest-enzyme';

Using Third Party Assertion Libraries

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

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

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

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

Initializing Test Environment

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

If your app uses a browser API that you need to mock in your tests or if you just need a global setup before running your tests, add a src/setupTests.js to your project. It will be automatically executed before running your tests.

For example:

src/setupTests.js

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

Note: Keep in mind that if you decide to "eject" before creating src/setupTests.js, the resulting package.json file won't contain any reference to it, so you should manually create the property setupTestFrameworkScriptFile in the configuration for Jest, something like the following:

"jest": {
  // ...
  "setupTestFrameworkScriptFile": "<rootDir>/src/setupTests.js"
 }

Focusing and Excluding Tests

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

Coverage Reporting

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

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

Configuration

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

Supported overrides:

• collectCoverageFrom
• coverageReporters
• coverageThreshold
• snapshotSerializers

Example package.json:

{
  "name": "your-package",
  "jest": {
    "collectCoverageFrom" : [
      "src/**/*.{js,jsx}",
      "!<rootDir>/node_modules/",
      "!<rootDir>/path/to/dir/"
    ],
    "coverageThreshold": {
      "global": {
        "branches": 90,
        "functions": 90,
        "lines": 90,
        "statements": 90
      }
    },
    "coverageReporters": ["text"],
    "snapshotSerializers": ["my-serializer-module"]
  }
}

Continuous Integration

By default npm test runs the watcher with interactive CLI. However, you can force it to run tests once and finish the process by setting an environment variable called CI.

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

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

On CI servers

Travis CI

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

language: node_js
node_js:
  - 6
cache:
  directories:
    - node_modules
script:
  - npm run build
  - npm test

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

CircleCI

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

On your own environment

Windows (cmd.exe)

set CI=true&&npm test

set CI=true&&npm run build

(Note: the lack of whitespace is intentional.)

Windows (Powershell)

($env:CI = $true) -and (npm test)

($env:CI = $true) -and (npm run build)

Linux, macOS (Bash)

CI=true npm test

CI=true npm run build

The test command will force Jest to run tests once instead of launching the watcher.

If you find yourself doing this often in development, please file an issue to tell us about your use case because we want to make watcher the best experience and are open to changing how it works to accommodate more workflows.

The build command will check for linter warnings and fail if any are found.

Disabling jsdom

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

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

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

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

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

• Any browser globals like window and document
• ReactDOM.render()
• TestUtils.renderIntoDocument() (a shortcut for the above)
• mount() in Enzyme

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

• TestUtils.createRenderer() (shallow rendering)
• shallow() in Enzyme

Finally, jsdom is also not needed for snapshot testing.

Debugging Tests in Chrome

Add the following to the scripts section in your project's package.json

"scripts": {
    "test:debug": "react-scripts --inspect-brk test --runInBand --env=jsdom"
  }

Place debugger; statements in any test and run:

$ npm run test:debug

This will start running your Jest tests, but pause before executing to allow a debugger to attach to the process.

Open the following in Chrome

about:inspect

After opening that link, the Chrome Developer Tools will be displayed. Select inspect on your process and a breakpoint will be set at the first line of the react script (this is done simply to give you time to open the developer tools and to prevent Jest from executing before you have time to do so). Click the button that looks like a "play" button in the upper right hand side of the screen to continue execution. When Jest executes the test that contains the debugger statement, execution will pause and you can examine the current scope and call stack.

Note: the --runInBand cli option makes sure Jest runs test in the same process rather than spawning processes for individual tests. Normally Jest parallelizes test runs across processes but it is hard to debug many processes at the same time.

Opting Out of Caching

If you would prefer not to enable service workers prior to your initial production deployment, then remove the call to registerServiceWorker() from src/index.js.

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

import { unregister } from './registerServiceWorker';

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

Deployment

npm run build creates a build directory with a production build of your app. Set up your favorite HTTP server so that a visitor to your site is served index.html, and requests to static paths like /static/js/main.<hash>.js are served with the contents of the /static/js/main.<hash>.js file.

Static Server

For environments using Node, the easiest way to handle this would be to install serve and let it handle the rest:

npm install -g serve
serve -s build

The last command shown above will serve your static site on the port 5000. Like many of serve’s internal settings, the port can be adjusted using the -p or --port flags.

Run this command to get a full list of the options available:

serve -h

Serving Apps with Client-Side Routing

If you use routers that use the HTML5 pushState history API under the hood (for example, React Router with browserHistory), many static file servers will fail. For example, if you used React Router with a route for /todos/42, the development server will respond to localhost:3000/todos/42 properly, but an Express serving a production build as above will not.

This is because when there is a fresh page load for a /todos/42, the server looks for the file build/todos/42 and does not find it. The server needs to be configured to respond to a request to /todos/42 by serving index.html. For example, we can amend our Express example above to serve index.html for any unknown paths:

 app.use(express.static(path.join(__dirname, 'build')));

-app.get('/', function (req, res) {
+app.get('/*', function (req, res) {
   res.sendFile(path.join(__dirname, 'build', 'index.html'));
 });

If you’re using Apache HTTP Server, you need to create a .htaccess file in the public folder that looks like this:

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

It will get copied to the build folder when you run npm run build.

If you’re using Apache Tomcat, you need to follow this Stack Overflow answer.

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

On a production build, and in a browser that supports service workers, the service worker will automatically handle all navigation requests, like for /todos/42, by serving the cached copy of your index.html. This service worker navigation routing can be configured or disabled by ejecting and then modifying the navigateFallback and navigateFallbackWhitelist options of the SWPreachePlugin configuration.

When users install your app to the homescreen of their device the default configuration will make a shortcut to /index.html. This may not work for client-side routers which expect the app to be served from /. Edit the web app manifest at public/manifest.json and change start_url to match the required URL scheme, for example:

  "start_url": ".",

Building for Relative Paths

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

  "homepage": "http://mywebsite.com/relativepath",

This will let Create React App correctly infer the root path to use in the generated HTML file.

Note: If you are using react-router@^4, you can root <Link>s using the basename prop on any <Router>.
More information here.

For example:

<BrowserRouter basename="/calendar"/>
<Link to="/today"/> // renders <a href="/calendar/today">

Serving the Same Build from Different Paths

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

If you are not using the HTML5 pushState history API or not using client-side routing at all, it is unnecessary to specify the URL from which your app will be served. Instead, you can put this in your package.json:

  "homepage": ".",

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

• create ReactJS app
• two important imports
• self closing tag
• curly braces in JSX
• comments
• styles in JSX
• conditional statement in JSX
• createElement
• handle event
• class name
• render a component
• importing a component
• making an ajax call

---

create ReactJS app

npx create-react-app myapp
cd myapp
npm start

two important imports

import React from 'react'; {// for creating component}
import ReactDOM from 'react-dom'; {// for rendering}

self closing tag

In JSX, you have to include the slash.
Fine in JSX: <br />
NOT FINE AT ALL in JSX: <br>

curly braces in JSX

Even though I am located in between JSX tags, treat me like ordinary JavaScript and not like JSX.
You can do this by wrapping your code in curly braces.
<h1>{2 + 3}</h1>

comments

• {// This is a comment }
• {/* This is a comment */}
• /* Multi Line Comment */

styles in JSX

We can represent this as a JavaScript object in the CamelCase fashion.
var headingStyle = { backgroundColor: 'FloralWhite', fontSize: '19px'};
return (
  <th style={headingStyle}>heading</th>
);

conditional statement in JSX

// Moving if/else logic to a function
var showResult = function() {
  if(this.props.success === true)
    return <SuccessComponent />
  else
    return <ErrorComponent />
};
// Using ternary operator
<div className={ success ? 'green' : 'red' }/>
React.createElement("div", {className: success ? 'green' : 'red'});
---
if (coinToss() === 'heads') {
    img = <img src={pics.kitty} />
} else {
    img = <img src={pics.doggy} />
}
const img = <img src={pics[coinToss() == 'heads' ? 'kitty' : 'doggy']} />;

createElement

React.createElement(type, [props], [...children])

• type: (such as 'div' or 'span'
• [props]: React component type (a class or a function), or a React fragment type.
• [...children]

// Using HTML entity inside JSX tags.
<div> Mike &amp; Shawn </div>
// will produce
React.createElement("div", null, " Mike & Shawn ")

handle event

class Button extends React.Component {
  scream() {
    alert('AAAAAAAAHHH!!!!!');
  }
  render() {
    return <button onClick={this.scream}>AAAAAH!</button>;
  }
}

class name

Class name should be written in UpperCamelCase.

class MyComponentClass extends Component {}

render a component

ReactDOM.render(<MyComponentClass />, document.getElementById('app'));

importing a component

import React from 'react';
export class Greeting extends React.Component {
  render() {
    return <h1>Hi there, {this.props.name}!</h1>;
  }
}
import React from 'react';
import ReactDOM from 'react-dom';
import { Greeting } from './Greeting';
class App extends React.Component {
  render() {
    return (
      <div>
        <h1>Hullo and, "Welcome to The Newzz," "On Line!"</h1>
        {<Greeting name="Leo" />}
        <article>Latest newzz:  where is my phone?</article>
      </div>
    );
  }
}
ReactDOM.render(<App />, document.getElementById('app'));

making an ajax call

  constructor(props) {
    super(props);
    this.state = {
      error: null,
      isLoaded: false,
      items: []
    };
  }
  componentDidMount() {
    const apiURI = 'https://api.myjson.com/bins/17wm14';
    fetch(apiURI)
        .then(res => res.json())
        .then(
            (result) => {
              this.setState({
                isLoaded: true,
                items: result.items
              });
            },
            (error) => {
              this.setState({
                isLoaded: true,
                error: {message: 'There was an error!'}
              });
            }
        )
  }