What is JSX in React - Visual Explanation

Concept Flow - What is JSX

Write JSX code

↓

React compiles JSX

↓

JSX becomes React.createElement calls

↓

React renders elements to DOM

↓

User sees UI in browser

JSX is a syntax that looks like HTML but is transformed by React into JavaScript calls that create UI elements.

Execution Sample

React

const element = <h1>Hello, world!</h1>;
ReactDOM.render(element, document.getElementById('root'));

This code creates a JSX element and renders it inside the page element with id 'root'.

Execution Table

Step	Action	JSX Code	Transformed Code	Result
1	Write JSX	<h1>Hello, world!</h1>	N/A	JSX element created in code
2	Compile JSX	<h1>Hello, world!</h1>	React.createElement('h1', null, 'Hello, world!')	JSX transformed to React call
3	Render element	React.createElement('h1', null, 'Hello, world!')	N/A	React renders <h1>Hello, world!</h1> in DOM
4	Display UI	N/A	N/A	User sees 'Hello, world!' as heading on page

💡 Rendering completes and user sees the heading on the webpage.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
element	undefined	<h1>Hello, world!</h1>	React.createElement('h1', null, 'Hello, world!')	Rendered in DOM	Displayed on page

Key Moments - 2 Insights

Why does JSX look like HTML but is not a string?

JSX looks like HTML but is actually JavaScript syntax that React compiles into function calls, as shown in step 2 of the execution_table.

What happens to JSX before it reaches the browser?

JSX is transformed by React's compiler into React.createElement calls before rendering, as seen in step 2 and 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what does the JSX <h1>Hello, world!</h1> become after compilation?

A '<h1>Hello, world!</h1>' string

B document.createElement('h1')

C React.createElement('h1', null, 'Hello, world!')

D A plain JavaScript object

💡 Hint

Check step 2 in the execution_table where JSX is compiled.

At which step does React actually put the element into the webpage DOM?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Render element' action in the execution_table.

If we change the JSX to <p>Hi!</p>, what changes in the execution_table?

A The transformed code changes to React.createElement('p', null, 'Hi!')

B The rendering step is skipped

C The variable 'element' becomes undefined

D The user sees 'Hello, world!' still

💡 Hint

Focus on how JSX transforms in step 2 of the execution_table.

Concept Snapshot

JSX is a syntax that looks like HTML inside JavaScript.
React compiles JSX into React.createElement calls.
These calls create React elements.
React renders these elements to the browser DOM.
JSX makes writing UI easier and clearer.

Full Transcript

JSX is a special syntax used in React that looks like HTML but is actually JavaScript. When you write JSX, React compiles it into calls to React.createElement. These calls create React elements that React uses to build the user interface. Finally, React renders these elements into the browser's DOM so the user can see the UI. This process lets developers write UI code that looks like HTML but works as JavaScript behind the scenes.

Practice

1. What is JSX in React?

easy

A. A syntax that lets you write HTML-like code inside JavaScript

B. A CSS framework for styling React components

C. A database query language used with React

D. A tool to compile JavaScript into machine code

Solution

1. Step 1: Understand JSX purpose

   JSX allows writing HTML-like tags directly in JavaScript code, making UI code easier to write and read.

2. Step 2: Identify incorrect options

   Options B, C, and D describe unrelated tools or concepts not connected to JSX.

3. Final Answer:

   A syntax that lets you write HTML-like code inside JavaScript -> Option A

4. Quick Check:

   JSX = HTML-like code in JavaScript [OK]

Hint: JSX looks like HTML but is inside JavaScript code [OK]

Common Mistakes:

• Confusing JSX with CSS or styling tools
• Thinking JSX is a separate language
• Believing JSX is a database query language

2. Which of the following is the correct way to write a JSX element?

easy

A. <div>Hello World

B. div>Hello World

C. Hello World

D. {div}Hello World{/div}

Solution

1. Step 1: Check JSX tag syntax

   JSX elements must have opening and closing tags properly enclosed in angle brackets, like <div>Hello World</div>.

2. Step 2: Identify syntax errors in other options

   div>Hello World</div> misses the opening angle bracket, C misses the closing tag, and D uses curly braces incorrectly.

3. Final Answer:

   <div>Hello World</div> -> Option C

4. Quick Check:

   Proper JSX tags use <tag>content</tag> [OK]

Hint: JSX tags look like HTML tags with < and > [OK]

Common Mistakes:

• Omitting angle brackets around tags
• Forgetting closing tags
• Using curly braces instead of angle brackets for tags

3. What will this JSX code render?

const element = <h1>Hello, {"React"} & JSX!</h1>;
ReactDOM.render(element, document.getElementById('root'));

medium

A. Hello, {"React"} & JSX!

B. SyntaxError due to curly braces

C. Hello, React & JSX!

D. Hello, React & JSX!

Solution

1. Step 1: Understand JSX expression inside curly braces

   The string "React" inside curly braces is evaluated and inserted as text inside the h1 element.

2. Step 2: Recognize HTML entities and output

   The & symbol renders as & in the browser, so the final rendered text is Hello, React & JSX! inside an h1 tag.

3. Final Answer:

   <h1>Hello, React & JSX!</h1> -> Option D

4. Quick Check:

   JSX expressions render evaluated content [OK]

Hint: Curly braces insert JavaScript values inside JSX [OK]

Common Mistakes:

• Thinking curly braces print raw code
• Confusing HTML entities with literal text
• Expecting syntax error from curly braces

4. Identify the error in this JSX code:

const element = <div>Hello {name}</div>
ReactDOM.render(element, document.getElementById('root'));

Assuming name is not defined anywhere.

medium

A. ReferenceError because 'name' is not defined

B. SyntaxError due to missing quotes around name

C. Renders 'Hello name' as plain text

D. No error, renders empty space after Hello

Solution

1. Step 1: Check variable usage inside JSX

   JSX expressions inside curly braces expect valid JavaScript variables or expressions. Here, 'name' is not defined.

2. Step 2: Understand runtime behavior

   Using an undefined variable causes a ReferenceError at runtime, stopping rendering.

3. Final Answer:

   ReferenceError because 'name' is not defined -> Option A

4. Quick Check:

   Undefined variables in JSX cause ReferenceError [OK]

Hint: Variables in JSX must be defined before use [OK]

Common Mistakes:

• Assuming missing quotes cause syntax error
• Thinking JSX prints variable names as text
• Believing undefined variables render empty strings

5. How can you include a JavaScript expression inside JSX to display the sum of 2 + 3?

hard

A. The sum is {"2 + 3"}

B. The sum is {2 + 3}

C. The sum is (2 + 3)

D. The sum is "2 + 3"

Solution

1. Step 1: Use curly braces for JavaScript expressions in JSX

   JSX lets you insert JavaScript expressions inside curly braces to evaluate and display their result.

2. Step 2: Evaluate each option's expression

   The sum is {2 + 3}

   correctly uses curly braces with 2 + 3, which evaluates to 5. Options A and B treat it as a string, and C uses parentheses without curly braces, so it won't evaluate.

3. Final Answer:

   <p>The sum is {2 + 3}</p> -> Option B

4. Quick Check:

   Curly braces evaluate JS expressions inside JSX [OK]

Hint: Use curly braces to evaluate expressions inside JSX [OK]

Common Mistakes:

• Putting expressions inside quotes disables evaluation
• Using parentheses without curly braces
• Confusing strings with expressions