Rendering elements in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to render a simple heading in React.

React

function Hello() {
  return <h1>[1]</h1>;
}

Drag options to blanks, or click blank then click option'

AHello, world!

B"Hello, world!"

C<h1>Hello, world!</h1>

D{Hello, world!}

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to put quotes around the text

Trying to put HTML tags inside JSX text

In React JSX, text inside tags must be a string or expression. Using quotes around the text renders it correctly.

2fill in blank

medium

Complete the code to render a list of items using React.

React

function ItemList() {
  const items = ['Apple', 'Banana', 'Cherry'];
  return (
    <ul>
      {items.map(item => <li key={item}>[1]</li>)}
    </ul>
  );
}

Drag options to blanks, or click blank then click option'

A<item>

B"item"

C{item}

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Putting quotes around item which renders the word 'item' literally

Using curly braces inside JSX text incorrectly

Inside JSX, to render a variable, you just write it directly inside the tag. Here, item is a string, so writing item works.

3fill in blank

hard

Fix the error in the React component that renders a button with a click handler.

React

function ClickMe() {
  function handleClick() {
    alert('Clicked!');
  }
  return <button onClick=[1]>Click me</button>;
}

Drag options to blanks, or click blank then click option'

AhandleClick

BhandleClick()

C"handleClick"

D{handleClick}

Attempts:

3 left

💡 Hint

Common Mistakes

Calling the function immediately instead of passing it

Passing the function name as a string

In React, event handlers should be passed as functions, not called immediately. So pass the function name without parentheses.

4fill in blank

hard

Fill both blanks to create a React component that renders a list of numbers doubled.

React

function DoubleList() {
  const numbers = [1, 2, 3];
  return (
    <ul>
      {numbers.map(n => <li key={n}>[1]</li>)}
    </ul>
  );
}

Drag options to blanks, or click blank then click option'

An - 2

Bn + 2

Cn * 2

Dn / 2

Attempts:

3 left

💡 Hint

Common Mistakes

Adding instead of multiplying

Using division or subtraction

To double each number, multiply it by 2 inside the map function.

5fill in blank

hard

Fill all three blanks to create a React component that renders a filtered list of even numbers doubled.

React

function FilteredDoubleList() {
  const numbers = [1, 2, 3, 4, 5];
  const doubledEvens = numbers.filter(n => n [1] 2 === 0).map(n => n [2] 2);
  return (
    <ul>
      {doubledEvens.map(n => <li key={n}>[3]</li>)}
    </ul>
  );
}

Drag options to blanks, or click blank then click option'

A%

B*

Cn

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of % for even check

Not doubling the number correctly

Rendering the wrong variable inside the list

To filter even numbers, use modulo % 2 === 0. Then multiply by 2 to double. Finally, render the number n inside the list.

Practice

1. What does rendering elements in React mean?

easy

A. Managing database connections

B. Writing CSS styles for components

C. Creating backend APIs

D. Showing content on the screen using React components

Solution

1. Step 1: Understand rendering in React

   Rendering means React creates and shows elements on the screen for users to see.

2. Step 2: Identify the correct description

   Only showing content on the screen matches rendering elements, not styling or backend tasks.

3. Final Answer:

   Showing content on the screen using React components -> Option D

4. Quick Check:

   Rendering = Showing content [OK]

Hint: Rendering means displaying content on screen [OK]

Common Mistakes:

• Confusing rendering with styling
• Thinking rendering means backend work
• Mixing rendering with database tasks

2. Which of the following is the correct way to render a simple React element?

easy

A. ReactDOM.render(<h1>Hello</h1>, document.getElementById('root'));

B. React.render(<h1>Hello</h1>, 'root');

C. ReactDOM.render('Hello', document.getElementById('root'));

D. ReactDOM.render(<h1>Hello</h1>);

Solution

1. Step 1: Recall ReactDOM.render syntax

   The correct syntax requires ReactDOM.render(element, container), where container is a DOM node.

2. Step 2: Check each option

   ReactDOM.render(<h1>Hello</h1>, document.getElementById('root')); uses ReactDOM.render with JSX element and document.getElementById('root'), which is correct.

3. Final Answer:

   ReactDOM.render(<h1>Hello</h1>, document.getElementById('root')); -> Option A

4. Quick Check:

   ReactDOM.render(element, container) = ReactDOM.render(<h1>Hello</h1>, document.getElementById('root')); [OK]

Hint: ReactDOM.render needs element and container node [OK]

Common Mistakes:

• Using React.render instead of ReactDOM.render
• Passing string instead of JSX element
• Omitting container DOM node

3. What will be displayed on the screen after running this React code?

const element = <h2>Welcome to React!</h2>;
ReactDOM.render(element, document.getElementById('root'));

medium

A. A plain text 'element' string

B. An <h2> heading with text 'Welcome to React!'

C. Nothing, because of a syntax error

D. An empty page with no content

Solution

1. Step 1: Understand JSX element creation

   The variable 'element' holds an <h2> JSX element with text 'Welcome to React!'.

2. Step 2: ReactDOM.render displays the element

   ReactDOM.render inserts this <h2> element inside the DOM node with id 'root', showing the heading on screen.

3. Final Answer:

   An <h2> heading with text 'Welcome to React!' -> Option B

4. Quick Check:

   JSX element renders as HTML heading [OK]

Hint: JSX inside render shows as HTML element [OK]

Common Mistakes:

• Thinking JSX is a string
• Expecting no output due to misunderstanding JSX
• Confusing variable name with output

4. Identify the error in this React rendering code:

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

medium

A. No error, code runs correctly

B. document.getElementById('root') is undefined

C. Missing semicolon after JSX element declaration

D. JSX element must be wrapped in parentheses

Solution

1. Step 1: Check JSX element syntax

   The JSX element is correctly assigned to 'element' variable; semicolon is optional in JavaScript.

2. Step 2: Verify ReactDOM.render usage

   ReactDOM.render is called with valid arguments; assuming 'root' exists in HTML, no error occurs.

3. Final Answer:

   No error, code runs correctly -> Option A

4. Quick Check:

   Valid JSX and render call = No error, code runs correctly [OK]

Hint: JSX semicolon optional; check DOM node exists [OK]

Common Mistakes:

• Thinking semicolon is mandatory after JSX
• Assuming JSX needs parentheses always
• Not verifying if 'root' element exists in HTML

5. You want to render a list of names as <li> items inside a <ul>. Which React code correctly renders this list?

const names = ['Anna', 'Bob', 'Cara'];

// Choose the correct rendering code

hard

A. ReactDOM.render(<ul><li>names</li></ul>, document.getElementById('root'));

B. const listItems = names.forEach(name => <li>{name}</li>);
ReactDOM.render(<ul>{listItems}</ul>, document.getElementById('root'));

C. const listItems = names.map(name => <li key={name}>{name}</li>);
ReactDOM.render(<ul>{listItems}</ul>, document.getElementById('root'));

D. const listItems = names.map(name => <li>{name}</li>);
ReactDOM.render(<ul>listItems</ul>, document.getElementById('root'));

Solution

1. Step 1: Understand list rendering with keys

   React requires keys on list items for efficient updates; map returns a new array of elements.

2. Step 2: Analyze each option

   const listItems = names.map(name => <li key={name}>{name}</li>); ReactDOM.render(<ul>{listItems}</ul>, document.getElementById('root')); uses map with key attribute and renders list correctly. const listItems = names.forEach(name => <li>{name}</li>); ReactDOM.render(<ul>{listItems}</ul>, document.getElementById('root')); uses forEach which returns undefined. ReactDOM.render(<ul><li>names</li></ul>, document.getElementById('root')); renders literal 'names' text. const listItems = names.map(name => <li>{name}</li>); ReactDOM.render(<ul>listItems</ul>, document.getElementById('root')); misses {} around listItems, rendering literal 'listItems' text.

3. Final Answer:

   const listItems = names.map(name => <li key={name}>{name}</li>); ReactDOM.render(<ul>{listItems}</ul>, document.getElementById('root')); -> Option C

4. Quick Check:

   Use map with keys for lists [OK]

Hint: Use map with keys to render lists in React [OK]

Common Mistakes:

• Using forEach instead of map for rendering
• Omitting key attribute on list items
• Rendering array as plain text