What is React - Visual Explanation

Concept Flow - What is React

Write React Component

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React Evaluates JSX

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React Creates Virtual DOM

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React Compares Virtual DOM with Previous

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React Updates Real DOM Efficiently

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User Sees Updated UI

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User Interacts -> Event Handler

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React Updates State -> Re-render

↩Back to React Evaluates JSX

React lets you write components that describe UI. It creates a virtual copy of the UI, compares changes, and updates the real page efficiently. User actions update state, causing React to re-render only what changed.

Execution Sample

React

function Hello() {
  return <h1>Hello, React!</h1>;
}

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

This code defines a simple React component that shows a greeting, then renders it inside the page element with id 'root'.

Execution Table

Step	Action	Evaluation	Result
1	Define Hello component	Function Hello returns JSX <h1>Hello, React!</h1>	Component ready to use
2	Call ReactDOM.render with <Hello />	React calls Hello(), gets JSX	Virtual DOM created with <h1>Hello, React!</h1>
3	React compares Virtual DOM with empty previous DOM	No previous DOM, so full update	Real DOM updated with <h1>Hello, React!</h1>
4	User sees 'Hello, React!' on page	UI rendered	Visible heading on screen
5	User clicks or interacts (not in this example)	Event triggers state update	React re-renders component (not shown here)
6	React updates Virtual DOM and Real DOM accordingly	Only changed parts updated	Efficient UI update
7	End of initial render	No more changes	UI stable

💡 Initial render completes after React updates the real DOM with the component's output

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
Hello	undefined	Function defined	Function defined	Function defined
Virtual DOM	empty	<h1>Hello, React!</h1>	<h1>Hello, React!</h1>	<h1>Hello, React!</h1>
Real DOM	empty	empty	<h1>Hello, React!</h1>	<h1>Hello, React!</h1>

Key Moments - 2 Insights

Why does React create a virtual DOM instead of updating the real DOM directly?

React creates a virtual DOM to quickly compare changes and update only what is necessary in the real DOM, making UI updates faster and smoother. See execution_table step 3 where React compares virtual DOM with previous DOM.

What happens when the user interacts with the UI in React?

User interaction triggers event handlers that update React state, causing React to re-render components and update the virtual and real DOM efficiently. This is shown in execution_table steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution table, what does React create after calling the Hello component?

A A virtual DOM with the component's JSX

B The real DOM directly

C An event handler

D A CSS style sheet

💡 Hint

Check execution_table step 2 where React calls Hello() and creates virtual DOM

At which step does React update the real DOM for the first time?

A Step 1

B Step 3

C Step 5

D Step 7

💡 Hint

Look at execution_table step 3 where React compares virtual DOM and updates real DOM

If the user interacts and changes state, what does React do next?

A Re-renders the component and updates the DOM efficiently

B Reloads the entire page

C Ignores the change

D Deletes the virtual DOM

💡 Hint

See execution_table steps 5 and 6 describing user interaction and React updates

Concept Snapshot

React is a library to build UI with components.
You write components returning JSX (HTML-like code).
React creates a virtual DOM to track changes.
It updates the real DOM efficiently.
User actions update state, causing re-render.
This makes UI fast and interactive.

Full Transcript

React is a tool to build user interfaces by creating components. Each component returns JSX, which looks like HTML but is JavaScript. React creates a virtual copy of the UI called the virtual DOM. When something changes, React compares the new virtual DOM with the old one and updates only the parts that changed in the real DOM. This makes updates fast and smooth. When users interact, React updates the component state, re-renders the virtual DOM, and updates the real DOM efficiently. This process helps build dynamic and responsive web pages.