JSX vs HTML differences in React - Practice Questions

Challenge - 5 Problems

❓ component_behavior

intermediate

JSX attribute naming difference

What will be the rendered output of this React component regarding the attribute on the <input> element?

React

function MyInput() {
  return <input className="my-class" />;
}

A The input will have no class because 'class' is invalid in JSX.

B The input will have an attribute 'class' but React will warn about it.

C The input will have a CSS class named 'my-class'.

D The input will throw a syntax error because 'class' is not allowed.

💡 Hint

Remember that 'class' is a reserved keyword in JavaScript.

In JSX, the HTML attribute class cannot be used because 'class' is a reserved JavaScript keyword, causing a syntax error in the transpiled code. Use className instead.

📝 Syntax

intermediate

JSX self-closing tags requirement

Which option correctly writes the <img> element in JSX?

React

function Image() {
  return <img src="logo.png" />;
}

A Change to <img src="logo.png" />

B Change to <img src="logo.png"></img> (no change needed)

C Change to <img src="logo.png">

D Change to <img src="logo.png" />;</img>

💡 Hint

Think about how void elements like <img> should be handled in JSX.

In JSX, void elements like <img> should be self-closed with /> to follow XML-like syntax rules. Using a separate closing tag is discouraged and may trigger warnings.

🔧 Debug

advanced

Why does this JSX code cause an error?

Consider this React component code. Why does the event handler not work?

React

function Button() {
  return <button onClick={() => alert('Clicked!')}>Click me</button>;
}

A Because the button element must be self-closing in JSX.

B Because alert is not allowed inside JSX event handlers.

C Because arrow functions cannot be used inside JSX attributes.

D Because JSX uses camelCase for event handlers, it should be onClick, not onclick.

💡 Hint

Check the casing of event handler attributes in JSX.

JSX requires event handler attributes to be camelCase. Using onclick instead of onClick causes React to not recognize the event handler, which can cause errors or unexpected behavior.

❓ state_output

advanced

JSX expression evaluation inside attributes

What will be the value of the alt attribute in the rendered <img> element?

React

function Logo() {
  const name = 'React';
  return <img src="logo.png" alt={name + ' Logo'} />;
}

A The alt attribute will be 'React Logo'.

B The alt attribute will be '{name + ' Logo'}' as a string literal.

C The alt attribute will be empty because expressions are not allowed.

D The code will cause a syntax error due to string concatenation.

💡 Hint

Remember how JSX handles JavaScript expressions inside curly braces.

JSX allows embedding JavaScript expressions inside curly braces. The expression {name + ' Logo'} evaluates to the string 'React Logo', which is assigned to the alt attribute.

🧠 Conceptual

expert

Why does JSX use 'htmlFor' instead of 'for' in labels?

In JSX, the <label> element uses htmlFor instead of the HTML attribute for. Why is this change necessary?

A Because 'for' is deprecated in HTML5 and replaced by 'htmlFor'.

B Because 'for' is a reserved keyword in JavaScript, so JSX uses 'htmlFor' to avoid conflicts.

C Because React automatically converts 'for' to 'htmlFor' internally.

D Because 'htmlFor' is the standard attribute in HTML, not 'for'.

💡 Hint

Think about JavaScript language rules and how JSX is JavaScript syntax.

The word for is a reserved keyword in JavaScript used for loops. JSX is JavaScript syntax, so using for as an attribute name would cause syntax errors. React uses htmlFor instead to avoid this conflict.

Practice

1. Which attribute should you use in JSX to add CSS classes to an element?

easy

A. cssClass

B. class

C. className

D. styleClass

Solution

1. Step 1: Recall JSX attribute rules

   In JSX, the HTML attribute class is replaced by className because class is a reserved word in JavaScript.

2. Step 2: Identify correct attribute for CSS classes

   Therefore, to add CSS classes in JSX, you must use className.

3. Final Answer:

   className -> Option C

4. Quick Check:

   JSX uses className, not class [OK]

Hint: Remember: JSX uses className, not class for CSS classes [OK]

Common Mistakes:

• Using 'class' instead of 'className' causes errors
• Confusing 'className' with 'cssClass'
• Trying to use 'styleClass' which doesn't exist

2. Which of the following is the correct way to write a label with a 'for' attribute in JSX?

easy

A. Name

B. Name

C. Name

D. Name

Solution

1. Step 1: Understand JSX attribute naming

   In JSX, the HTML attribute for is replaced by htmlFor because for is a reserved word in JavaScript.

2. Step 2: Check correct casing and spelling

   The correct attribute is htmlFor with capital F, so <label htmlFor="inputId">Name</label> is correct.

3. Final Answer:

   <label htmlFor="inputId">Name</label> -> Option A

4. Quick Check:

   Use htmlFor, not for in JSX labels [OK]

Hint: Use htmlFor (camelCase) instead of for in JSX labels [OK]

Common Mistakes:

• Using 'for' attribute directly causes syntax errors
• Writing 'htmlfor' with lowercase f is incorrect
• Using non-existent attributes like 'forId'

3. What will be the rendered output of this JSX code?

const element = <input type="checkbox" checked="true" />;
ReactDOM.render(element, document.getElementById('root'));

medium

A. A checkbox input with checked attribute as string

B. A checked checkbox input

C. Syntax error due to checked attribute value

D. An unchecked checkbox input

Solution

1. Step 1: Analyze checked attribute in JSX

   In JSX, checked="true" without curly braces sets the prop to the string "true".

2. Step 2: Understand rendering of boolean attributes

   React renders <input type="checkbox" checked="true" />. Browsers treat the presence of the checked attribute (any non-empty value) as checked for uncontrolled checkboxes.

3. Final Answer:

   A checked checkbox input -> Option B

4. Quick Check:

   Attribute presence checks uncontrolled checkbox [OK]

Hint: checked="true" renders checked checkbox (attribute presence) [OK]

Common Mistakes:

• Thinking checked="true" leaves checkbox unchecked
• Expecting string value to cause syntax error
• Confusing uncontrolled render with controlled components

4. Identify the error in this JSX snippet:

const element = <div class="container">Hello</div>;

medium

A. Using class instead of className

B. Using double quotes instead of single quotes

C. Missing closing tag for div

D. Using semicolon inside JSX

Solution

1. Step 1: Check attribute names in JSX

   JSX does not use the HTML attribute class. Instead, it uses className because class is a reserved word in JavaScript.

2. Step 2: Verify other parts of the snippet

   The quotes and closing tag are correct. The semicolon is outside JSX and valid in JavaScript.

3. Final Answer:

   Using class instead of className -> Option A

4. Quick Check:

   JSX requires className, not class [OK]

Hint: Replace class with className in JSX attributes [OK]

Common Mistakes:

• Using class attribute causes React warnings or errors
• Thinking quotes style causes errors
• Confusing semicolon placement inside JSX

5. You want to apply inline styles to a JSX element to make text red and bold. Which is the correct way?

hard

A. Text

B. Text

C. Text

D. Text

Solution

1. Step 1: Understand JSX style attribute syntax

   In JSX, the style attribute expects a JavaScript object, so you use double curly braces: the outer for embedding JS, the inner for the object.

2. Step 2: Check property names and values

   CSS properties use camelCase in JSX, so fontWeight is correct. Values are strings like 'red' and 'bold'.

3. Final Answer:

   <div style={{color: 'red', fontWeight: 'bold'}}>Text</div> -> Option D

4. Quick Check:

   Use double braces and camelCase for inline styles [OK]

Hint: Use double braces and camelCase keys for JSX inline styles [OK]

Common Mistakes:

• Using string style attribute instead of object
• Missing curly braces around style object
• Using CSS property names with hyphens instead of camelCase