React Library Example with multiple components

In the previous example you have seen how react basically works. In this example you will see how react works with multiple components.


<!DOCTYPE html>
<html lang="en">

  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>App Layout Demo</title>

  <div id="root"></div>
  <script src=""></script>
  <script src=""></script>

  <script src=""></script>

  <script src="Hello.js" type="text/jsx"></script>
  <script src="NumPicker.js" type="text/jsx"></script>
  <script src="index.js" type="text/jsx"></script>




class Hello extends React.Component {
  render() {
    return <h1>Hello There!!!</h1>


function getNum() {
  return Math.floor(Math.random() * 10) + 1;
class NumPicker extends React.Component {
  render() {
    const num = getNum();
    let msg;
    if (num === 7) {
      msg = <h2>CONGRATS YOU WIN!</h2>
    } else {
      msg = <p>Sorry, you lose!</p>
    return (
        <h1>Your number is: {num} </h1>


class App extends React.Component {
  render() {
    return (
        <Hello />
        <NumPicker />

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

The start of everything is the last line in the index.js

In this line the component <App /> will be rendered into the element with the id “root”.

The component <App /> has multiple components, which are <Hello /> as well as <NumPicker />. These components have to be defined before they are rendered. Thats why the order in which the JS files are placed in the index.html is important.

With that you get the output:

Or with the number 7:

Simple React Library Example

React can be used on your website by just adding some JS files from a CDN without having to setup a local Node.js instance.



<!DOCTYPE html>
<html lang="en">
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>First Component</title>
  <div id="root"></div>

  <script src=""></script>
  <script src=""></script>

  <script src=""></script>
  <script src="index.js" type="text/jsx"></script>

As you can see the index.html doesn’t contain very much:

  • The <head> contains only standard HTML5 meta tags and the title of the page, nothing React specific.
  • Directly after the <body> we only have a “root” element which is the base for our React app.
  • Before the </body> we can see some JS files which are partially external and internal:
    • react.development.js: Base React-Library
    • react-dom.development.js: React-DOM extension so React can interact with the DOM
    • babel-standalone: Babel is needed so JSX is possible
  • index.js: our React app


class Hello extends React.Component {
	render() {
		return (
				<h1>Hello there!</h1>
				<p>I am a React Component!</p>

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

So we define a component named “Hello“.
So we can render it via “<Hello />”.
Withing our component we have a render() funktion, which defines what happens when we render our Component.

At the end we just define, that our component “<Hello />” should be rendered into the element with the ID “root”.

So we get the following output after opening our index.html:

It is important, that you cant “just” open the index.html directly. You instead have to create a local web-server due to CORS restricting the embeding of local JS files.

Otherwise you can copy the JS React code directly in the index.html, but thats not good pratice if your project gets bigger and bigger. An easy way to create a “Live Server” is to use the Visual Studio Code Editor and its plugin called “Live Server”.

JS Frontend Libraries & Frameworks

Library vs Framework

First lets explain the difference between a “Library” and a “Framework”.

Libraries are mostly “small” code bundles which allow the developer to extend the functionality of an already present application.
Examples for libraries are Moment.js, jQuery or Data-Driven Documents.

Frameworks are most of the time “bigger” and usually provide a distinct structure how data should be handled or saved. Frameworks often provide easier ways to reuse already written blocks of code and already have built in security features like XSS Protection and CSRF tokens.

Why do JS Frontend Libraries/Frameworks exist?

There are many different reasons why JS Frontend Libraries/Frameworks exist:

  • Performance
  • Complete separation between frontend and backend logic
  • New technology / developer doesn’t want to work with “old” technology

Whats the principal behind JS Frontend Libraries/Frameworks?

Lets take a typical PHP as basis:

  1. Browser requests website
  2. Web server gets request from browser and looks for the defined document root
  3. If there is a PHP file present it will be interpreted
  4. Now all Database queries are being executed which can theoretically take a lot of time
  5. The resulting HTML and connected CSS and JS will be returned to the browser

Point 4. is the important part here: The database query is executed immediately when the request is being proccessed by the web server.

Now in comparison a website built with a JS Frontend Frameworks

  1. Browser requests website
  2. Web server gets request from browser and looks for the defined document root
  3. Web server delivers HTML, CSS and JS without executing DB Queries
  4. The browser renders the HTML and CSS and interprets the JS
  5. In JS the client now executes extra AJAX requests to the web server to dynamically load the needed data.

Therefore the initial HTML is very small because usually in JS Framworks the DOM only consists of 1 “root” element in which all the virtual DOM is rendered by JS.

Main difference to PHP is the fact, that the dynamic content will be added by the client, not the server!

This has also the advantage, that the backend code to manage the dynamic code doesn’t need to worry about frontend code.

Currently popular JS Libraries/Frameworks (June 2020)

  • React
  • VueJS
  • Angular

Basically there is no “ultimate” framework which is best for all scenarios. But bellow you can see a short description of each framework so you can make your own decission.

React is a “light” Library/Framework, which only has a few basic concepts (like e.g. “Components”) to build your project. Therfore you don’t have to learn that much and the start is not that difficult.

You can also use react just as a library (see HERE), but on the example of Next.js you can see a whole framework which is built on React.

VueJS basically has the same features as React. But one main difference to React is the fact, that VueJS doesn’t built its virtual DOM with JSX. See HERE for a description of what the virtual DOM and JSX is.

Angular is a “larger” framework and therefore has much more functionality built in. Thats why its better suited for bigger projects because you can already use “finished” functionality and don’t have to build everything yourself. But understand, using and maintaining these already build code blocks can be kind of tricky especially while updating Angular.

Virtual DOM and JSX

Virtual DOM

One main reason behind JS Web-Frameworks is the “virtual DOM”. But what is the Virtual DOM and why is it better then the “original DOM”?

Each change in the “original DOM” triggers a “redraw” of the whole DOM in the browser which can be (dependent on your DOM) very resource intensive.

Changes in the “virtual DOM” happen only at the level of the elements which are changed (therefore more performant) and only the difference between the “old” DOM and the “new” DOM is then pushed to the browser.

Also not every little change is directly transfered to the “original” DOM but all changes are gathered together, performed on the virtual DOM and then this difference is pushed to the original DOM which also increases performance.



JSX stands for Javascript XML or Javascript Syntax Extension and is an extension of the typical JavaScript grammar for React.

With JSX your are now “allowed” to write HTML in JS like that:

function render() {
  return <p>Hi everyone!</p>;

Native JavaScript would produce the following error:

Uncaught SyntaxError: Unexpected token <

“React” uses Babel to directly transform “React” HTML-Code in JavaScript Code. See here Online Babel Compiler

Therefore in React you write HTML in your .js files, not your .html files.

See JSX in depth for more details.

npm vs. yarn – Package Managers for JavaScript

Package managers are generally used to use and manage already made and tested code parts in your own project. The 2 most common JavaScript package managers are npm and yarn.

What is a package?

Packages are JavaScript modules, which contain reusable code for your own project.

You can find all available JavScript Packages here:

Each package can be dependent on other packages which are called “dependencies”. These dependencies are located in the package.json in the area “dependencies” and “devDependencies”. So if you install one package which is dependent on 3 other packages you will get a total number of 4 packages in your application.

What is NPM?

npm (“Node package manager”, released in 2010) will be automatically installed if you install Node.js to manage JavaScript packages.

What is yarn?

yarn is “another” Package-Manager (similar to NPM) developed by Facebook but it is not automatically installed via Node.js.

Released in 2016 yarn has been developed because in the past NPM wasn’t very performant and did’nt have all the features developers needed like lock files.

Command overview

Here an overview of which commands can be executed in NPM or in Yarn:

Install dependenciesnpm installyarn
Install packagenpm install [package]yarn add [package]
Install dev packagenpm install --save-dev [package]yarn add --dev [package]
Uninstall packagenpm uninstall [package]yarn remove [package]
Uninstall dev packagenpm uninstall --save-dev [package]yarn remove [package]
Updatenpm updateyarn upgrade
Update packagenpm update [package]yarn upgrade [package]
Global install packagenpm install --global [package]yarn global add [package]
Global uninstall packagenpm uninstall --global [package]yarn global remove [package]

But not all commands are named differently:

npm inityarn init
npm runyarn run
npm testyarn test
npm login (and logout)yarn login (and logout)
npm linkyarn link
npm publishyarn publish
npm cache cleanyarn cache clean

package-lock.json vs. yarn.lock

The package.json contains the desired packages you want to use in your application including the desired version of this package.

Usually a “version string” contains a special character like *, ^ or ~ (see for a detailed explenation)

Therefore the package.json alone is not enough to determine which package version should be installed in your application.

Thats why NPM generates a package-lock.json each time you execute “npm install” since version 5.x (May 2017).
Yarn already included this feature in its first version, but its file is named yarn.lock.

Which is better?

Basically yarn provides the same functionality as NPM. NPM has learned from its mistakes in the past and applied many features from yarn to itself.

Therfore its more of a preference which package manager you or your team uses. If your in a team you should only determine which of these 2 all developers should use to not get into more trouble then you need.

Other Package Managers

As you can see above there is not only 1 solution for package management in JavScript. Here are some more package managers:

Thread based vs. Event based

There are 2 main approaches how to handle requests from Clients to a web server:

Thread based

On a thread based system the web server creates one thread per request and all the work for this request is handled by that thread.

This also means, that the maximum of available threads on a computer determines how many concurrent users can connect to the web server.

Event based

On an event based system every request will be put in a “queue” which then is processed by 1 web server thread.

With that you avoid the “overhead” of creating a new thread for each request.

But this also means, that e.g. a very complex and long request blocks all request since it needs to be done before all the other requests can be processed.

But due to the fact, that JavaScript can be written asynchronously no “blocking” can occur and therefore no (direct) performance problem can be felt by the end-user.

Node.js “Event Loop” visualized

Node.js Event Loop


What is Node.js?

Node.js is a framework built upon the JavaScript Engine “V8“, which is currently the most used JavaScript Engine and built into Google Chrome.

Node.js was developed by the need of a resource efficient, performant, “nonblocking I/O” Framework to deliver fast and quick websites.

Here is an example of a very basic web-server written in Node.js

const http = require('http');

const hostname = '';
const port = 3000;

const server = http.createServer((req, res) => {
  res.statusCode = 200;
  res.setHeader('Content-Type', 'text/plain');
  res.end('Hello World\n');

server.listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);

Main requirement to run this code is the fact, that you need a Node.js version installed on your system (see
The current LTS version is fine.

Now we can start the webserver with

node index.js

and can see the following output in the terminal

Server running at

If we open up a browser with the given URL we see

Advantages of Node.js

  • Based on JavaScript there is very little to no new syntax to lean if you know the basics of JavaScript.
  • Node.js is event based (see HERE for more detials) and because of that very performant
  • Modularity with the Node-Package-Manager (NPM)

Disadvantages of Node.js

  • Backwards compatibility is somewhat a problem with upcoming Node.js version updates because the current development of the framework is so rapid.
  • The fact that Node.js works on an event loop implies only 1 thread has to carry all the work which can be overwhelmed by lots of requests and/or server logic.
  • Asynchron code notation can be tricky to understand and learn