React Introduction

JSX & Virtual DOM

Learning Outcome

5

React Fragments

4

JSX in React and How It Works

3

Diffing Algorithm and Reconciliation

2

Virtual DOM & Real DOM

1

How React Updates UI Without Page Reload

Ever Wonder How React Updates UI Without Reloading the Page?

  • UI updates feel instant

  • Pages don’t reload

In React:

Page reloads or heavy DOM updates happen frequently

In traditional web apps:

This is possible because of the Virtual DOM

Why Not Update the Real DOM Directly?

DOM manipulation is slow

Frequent updates reduce performance

Complex UI changes are hard to manage

Problems with direct DOM updates

So React introduces:

Virtual DOM → Efficient updates

Controlled rendering process

What is Virtual DOM?

How React Updates UI Efficiently

Steps:

React creates a new Virtual DOM

  • A lightweight JavaScript copy of the Real DOM

  • Stored in memory

  • Faster to update than Real DOM

React never updates the Real DOM directly first.

This comparison is called Diffing Algorithm

Compares it with the previous Virtual DOM

What is Virtual DOM?

React identifies what changed

How React Updates UI Efficiently.

Steps

React creates a new Virtual DOM.

Compares it with the previous Virtual DOM.

Compares it with the previous Virtual DOM.

Updates only those parts in the Real DOM​

This process is called Reconciliation

This comparison is called Diffing Algorithm.

JSX in React?

JSX stands for JavaScript XML.

In simple terms, JSX lets you write HTML-like code inside JavaScript when building React apps

Example without JSX

const element = React.createElement(
  "h1",
  null,
  "Hello, World!"
);

This works, but it’s hard to read.

Same example with JSX

const element = <h1>Hello, World!</h1>;

Much simpler and cleaner

JSX  →  Babel  →  JavaScript  →  Browser renders HTML

Babel converts JSX into JavaScript

React Fragment ?

A React Fragment lets you group multiple elements without adding an extra HTML element (like a <div>) to the page.

 Solution 1: Using a <div> (not always ideal)

return (
  <div>
    <h1>Hello</h1>
    <p>Welcome</p>
  </div>
);

Works
Adds an extra <div> to the DOM

In React, a component must return only one parent element.

Why do we need it?

This causes an error:

return (
  <h1>Hello</h1>
  <p>Welcome</p>
);

Solution 2: Using React Fragment

return (
  <React.Fragment>
    <h1>Hello</h1>
    <p>Welcome</p>
  </React.Fragment>
);

No extra HTML element added

Short Syntax (Most Common)

return (
  <>
    <h1>Hello</h1>
    <p>Welcome</p>
  </>
);

Why React Fragment is Important

Avoids unnecessary <div>s
Keeps HTML clean
Helps with CSS & layout issues
Required for tables (<tr>, <td>)

Summary

4

JSX simplifies and cleans UI code

5

Fragments group elements without extra DOM

3

Diffing updates only changed elements

2

Virtual DOM makes UI updates faster

1

React updates UI without page reload

Quiz

Who converts JSX into JavaScript?

A. Browser

B. React

C. Babel

D. Node

Quiz-Answer

Who converts JSX into JavaScript?

A. Browser

B. React

C. Babel

D. Node