App.css

.app {
    display: flex;
    flex-direction: column;
}

.header {
    display: flex;
    align-items: center;
    justify-content: center;
    min-width: 100vw;
    height: 96px;
    font-size: calc(10px + 2vmin);
    color: white;
}

.header p {
    margin-right: 24px;
}

.header.light {
    background-color: #ffba00;
}
.header.dark {
    background-color: #282c34;
}

.contentContainer {
    display: flex;
    flex-direction: column;
    position: absolute;
    width: 100vw;
    top: 96px;
    height: calc(100vh - 96px);
    align-items: center;
    justify-content: center;
}

.contentContainer.light {
    background-color: white;
}
.contentContainer.dark {
    background-color: #30353d;
}

p.light {
    color: black;
}
p.dark {
    color: white;
}

App.js

import React, { useState } from 'react';
import './App.css';
import Content from './components/content';
import { ThemeContext } from './shared/context';

function App() {
    const [theme, setTheme] = useState('dark');

    return (
        <ThemeContext.Provider value={theme}>
            <div className="app">
                <header className={`header ${theme}`}>
                    <p>Testing NEW 1...2...3...</p>
                    <button onClick={() => setTheme('light')}>Light</button>
                    <button onClick={() => setTheme('dark')}>Dark</button>
                </header>
                <div className={`contentContainer ${theme}`}>
                    <p className={theme}>Our gorgeous content</p>
                    <Content />
                </div>
            </div>
        </ThemeContext.Provider>
    );
}

export default App;

App.test.js

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);
  ReactDOM.unmountComponentAtNode(div);
});

AppOld.js

import React, { Component } from 'react';
import './App.css';
import ContentOld from './components/contentOld';

class AppOld extends Component {
    constructor(props) {
        super(props);
        this.state = {
            theme: 'dark'
        }
    }

    setTheme = (theme) => {
        this.setState({ theme });
    };

    render () {
        const { theme } = this.state;
        return (
            <div className="app">
                <header className={`header ${theme}`}>
                    <p>Testing OLD 1...2...3...</p>
                    <button onClick={() => this.setTheme('light')}>Light</button>
                    <button onClick={() => this.setTheme('dark')}>Dark</button>
                </header>
                <div className={`contentContainer ${theme}`}>
                    <p className={theme}>Our gorgeous content</p>
                    <ContentOld theme={theme} />
                </div>
            </div>
        );
    }
}

export default AppOld;

components__content.css

.content {
    display: flex;
    flex-direction: column;
}

.contentInput {
    display: flex;
    height: 32px;
    width: 240px;
    border-color: red;
    border-radius: 4px;
    margin-bottom: 24px;
}

p {
    text-align: center;
}

p.light {
    color: black;
}
p.dark {
    color: white;
}

components__content.js

import React, { useState, useContext, useEffect } from 'react';
import './content.css';

import { ThemeContext } from '../shared/context';
// Custom hook demo
// import { useWindowWidth, useFormInput } from '../shared/customHooks';

function Content() {
    const theme = useContext(ThemeContext);
    const [ text, setText ] = useState(theme);

    // Side effects demo
    const [ width, setWidth ] = useState(window.innerWidth);

    useEffect(() => {
        document.title = text;
    });

    useEffect(() => {
        window.addEventListener('resize', handleResize);
        return () => {
            window.removeEventListener('resize', handleResize);
        };
    });

    function handleResize() {
        setWidth(window.innerWidth);
    }

    return (
        <div className='content'>
            <input
                className='contentInput'
                defaultValue={theme}
                onChange={(e) => setText(e.target.value)}
            />
            <p className={theme}>{width}px</p>
        </div>
    );
}


export default Content;

components__contentOld.js

import React, { Component } from 'react';
import './content.css';


class ContentOld extends Component {
    constructor (props) {
        super(props);
        this.state = {
            val: 'dark',
            width: window.innerWidth
        };
        this.setText = this.setText.bind(this);
        this.handleResize = this.handleResize.bind(this);
    }

    componentDidMount() {
        document.title = this.state.val;
        window.addEventListener('resize', this.handleResize)
    }

    componentDidUpdate() {
        document.title = this.state.val;
    }

    componentWillUnmount() {
        window.removeEventListener('resize', this.handleResize)
    }

    setText(event) {
        this.setState({ val: event.target.value });
    }

    handleResize() {
        this.setState({
            width: window.innerWidth
        });
    }

    render () {
        return (
            <div className='content'>
                <input
                    className='contentInput'
                    defaultValue={this.props.theme}
                    onChange={this.setText}
                />
                <p className={this.props.theme}>{this.state.width}px</p>
            </div>
        );
    }
}

export default ContentOld;

index.css

body {
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", "Roboto", "Oxygen",
    "Ubuntu", "Cantarell", "Fira Sans", "Droid Sans", "Helvetica Neue",
    sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
}

code {
  font-family: source-code-pro, Menlo, Monaco, Consolas, "Courier New",
    monospace;
}

index.js

import React from 'react';
import ReactDOM from 'react-dom';
import './index.css';
import App from './App';
import AppOld from './AppOld';
import * as serviceWorker from './serviceWorker';

ReactDOM.render(<App />, document.getElementById('root'));
// ReactDOM.render(<AppOld />, document.getElementById('root'));

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

serviceWorker.js

// This optional code is used to register a service worker.
// register() is not called by default.

// This lets the app load faster on subsequent visits in production, and gives
// it offline capabilities. However, it also means that developers (and users)
// will only see deployed updates on subsequent visits to a page, after all the
// existing tabs open on the page have been closed, since previously cached
// resources are updated in the background.

// To learn more about the benefits of this model and instructions on how to
// opt-in, read http://bit.ly/CRA-PWA

const isLocalhost = Boolean(
  window.location.hostname === 'localhost' ||
    // [::1] is the IPv6 localhost address.
    window.location.hostname === '[::1]' ||
    // 127.0.0.1/8 is considered localhost for IPv4.
    window.location.hostname.match(
      /^127(?:\.(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)){3}$/
    )
);

export function register(config) {
  if (process.env.NODE_ENV === 'production' && 'serviceWorker' in navigator) {
    // The URL constructor is available in all browsers that support SW.
    const publicUrl = new URL(process.env.PUBLIC_URL, window.location.href);
    if (publicUrl.origin !== window.location.origin) {
      // Our service worker won't work if PUBLIC_URL is on a different origin
      // from what our page is served on. This might happen if a CDN is used to
      // serve assets; see https://github.com/facebook/create-react-app/issues/2374
      return;
    }

    window.addEventListener('load', () => {
      const swUrl = `${process.env.PUBLIC_URL}/service-worker.js`;

      if (isLocalhost) {
        // This is running on localhost. Let's check if a service worker still exists or not.
        checkValidServiceWorker(swUrl, config);

        // Add some additional logging to localhost, pointing developers to the
        // service worker/PWA documentation.
        navigator.serviceWorker.ready.then(() => {
          console.log(
            'This web app is being served cache-first by a service ' +
              'worker. To learn more, visit http://bit.ly/CRA-PWA'
          );
        });
      } else {
        // Is not localhost. Just register service worker
        registerValidSW(swUrl, config);
      }
    });
  }
}

function registerValidSW(swUrl, config) {
  navigator.serviceWorker
    .register(swUrl)
    .then(registration => {
      registration.onupdatefound = () => {
        const installingWorker = registration.installing;
        if (installingWorker == null) {
          return;
        }
        installingWorker.onstatechange = () => {
          if (installingWorker.state === 'installed') {
            if (navigator.serviceWorker.controller) {
              // At this point, the updated precached content has been fetched,
              // but the previous service worker will still serve the older
              // content until all client tabs are closed.
              console.log(
                'New content is available and will be used when all ' +
                  'tabs for this page are closed. See http://bit.ly/CRA-PWA.'
              );

              // Execute callback
              if (config && config.onUpdate) {
                config.onUpdate(registration);
              }
            } else {
              // At this point, everything has been precached.
              // It's the perfect time to display a
              // "Content is cached for offline use." message.
              console.log('Content is cached for offline use.');

              // Execute callback
              if (config && config.onSuccess) {
                config.onSuccess(registration);
              }
            }
          }
        };
      };
    })
    .catch(error => {
      console.error('Error during service worker registration:', error);
    });
}

function checkValidServiceWorker(swUrl, config) {
  // Check if the service worker can be found. If it can't reload the page.
  fetch(swUrl)
    .then(response => {
      // Ensure service worker exists, and that we really are getting a JS file.
      const contentType = response.headers.get('content-type');
      if (
        response.status === 404 ||
        (contentType != null && contentType.indexOf('javascript') === -1)
      ) {
        // No service worker found. Probably a different app. Reload the page.
        navigator.serviceWorker.ready.then(registration => {
          registration.unregister().then(() => {
            window.location.reload();
          });
        });
      } else {
        // Service worker found. Proceed as normal.
        registerValidSW(swUrl, config);
      }
    })
    .catch(() => {
      console.log(
        'No internet connection found. App is running in offline mode.'
      );
    });
}

export function unregister() {
  if ('serviceWorker' in navigator) {
    navigator.serviceWorker.ready.then(registration => {
      registration.unregister();
    });
  }
}

shared__context.js

import React from 'react';

export const ThemeContext = React.createContext('dark');

shared__customHooks.js

import { useEffect, useState } from 'react';

export function useWindowWidth() {
    const [width, setWidth] = useState(window.innerWidth);

    useEffect(() => {
        window.addEventListener('resize', handleResize);
        return () => {
            window.removeEventListener('resize', handleResize);
        };
    });

    function handleResize() {
        setWidth(window.innerWidth);
    }

    return width;
}

export function useFormInput(initValue) {
    const [value, setValue] = useState(initValue);

    function handleChange(event) {
        setValue(event.target.value);
    }

    return {
        value,
        onChange: handleChange
    }
}