Free React Course
For Beginners

L0 - What is React

React For Beginners by Itera

Key Points

  • The problem
  • What is the React
  • Three pillars of React
  • React - advantages and disadvantages
  • Why to learn React nowadays

About me

  • Vitalii Ruban
  • Head of Front-End department in Itera
  • Member of the program committee JsFest Ukraine
  • Love #webperf and #websec

The problem

How to sync the view and the data

Good old time

This days

When one field depends on another... 🤯

And the async calls comes...

And DOM modification is not fast

We have only 16.6 ms to render the screen

What is the React

Simply saying - React is a JavaScript library that:

  • Transforms your data into the markup
  • Redraw the markup when the data changed
  • Do this fast (enough)
React App
Fancy Web Site
            const user = { name: 'Vitalii' };
function App(props) {
    return <h2 className="green">Hello - {}</h2>
<App user={user}/>

Hello - Vitalii

This is not an HTML - JSX

<h2 className="green">Hello - {}</h2>

JSX - just a syntax sugar

function a() {
    return r.jsxs("h2", {
        children: [' className="green" Hello - ', l, " "],

More formal

React (also known as React.js or ReactJS) is a free and open-source front-end JavaScript library for building user interfaces based on UI components. React can be used as a base in the development of single-page, mobile, or server-rendered applications with frameworks like Next.js. (C) Wiki


React - open-source, JavaScript library that transforms your data into the markup and track the changes

Three pillars of React

View is derivative of your data

            function App(props) {
    return <h2 className="green">Hello - {props.userName}</h2>

To change the view - change the data

document.querySelect('.green').innerText = 'Anna'
setState({userName: 'Anna'})

Data is the single source of truth

Separating view and data is super important!

Declarative over Imperative

You write what you want to get, but not how you want to get


const form = document.createElement('form');
form.action = "/submit";
const header = document.createElement('h1');
header.innerText = 'Submit my email';
form.insertAdjacentElement('afterbegin', header);

Within imperative approach you defines a way to get the result


Submit my email

Within declarative approach you simply describe the result

Declarative approach is much more readable

JSX/TSX is an example of declarative approach

Based on components

All modern frameworks are based on components. But the most easy way to create a component - use React

Angular way

  selector: 'my-component',
  template: `<h1>Yes, I am a component</h1>`
export class MyComponent {}

React way

const MyComponent = () => 

Yes, I am a component

Creating a new component is very simple

You should not be afraid of creating as much components as needed

React - advantages and disadvantages


  • Declarative
  • React on changes automatically
  • Rich Ecosystem
  • Support client-side, server side rendering, mobile development
  • Not a framework
  • Fast


  • JSX
  • Own way to track changes
  • Not a framework*
  • Tends to put logic inside the View

Why to learn React nowadays

Popular - more chances to find a job

react is the most popular ui framework

Mature - good stability and support

  • 16.0.0 -September 26, 2017
  • 17.0.0 October 20, 2020
  • 18.0.0 March 29, 2022

Rich ecosystem - efficient development


  • React - JavaScript library for building user interfaces
  • React uses JSX - HTMLish syntax to describe UI
  • Data is the single source of truth

Useful links

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