Rendering elements in React - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What will this React component render?

Consider this React functional component. What will it display on the screen?

React

import React from 'react';

function Greeting() {
  const name = 'Alice';
  return <h1>Hello, {name}!</h1>;
}

export default Greeting;

A Hello, Alice!

B Syntax error

C Hello, !

D Hello, {name}!

💡 Hint

Look at how JSX expressions inside curly braces are evaluated.

In JSX, expressions inside curly braces are evaluated. The variable 'name' holds 'Alice', so the component renders 'Hello, Alice!'.

📝 Syntax

intermediate

Which option correctly renders a list of items in React?

You want to render a list of fruits using React. Which code snippet will render the list correctly without errors?

React

const fruits = ['Apple', 'Banana', 'Cherry'];

function FruitList() {
  return (
    <ul>
      {/* Fill in here */}
    </ul>
  );
}

A fruits.map(fruit => <li>{fruit}</li>)

B fruits.forEach(fruit => <li>{fruit}</li>)

C fruits.map(fruit => <li key={fruits.indexOf(fruit)}>{fruit}</li>)

D fruits.map(fruit => <li key={fruit}>{fruit}</li>)

💡 Hint

Remember React requires a unique 'key' prop for list items.

Option D uses map to create list items and provides a unique 'key' prop using the fruit name, which is best practice. Option D misses the key prop, B uses forEach which returns undefined, and D uses indexOf which can cause issues if fruits repeat.

❓ state_output

advanced

What will be displayed after clicking the button twice?

Given this React component, what will the displayed count be after clicking the button two times?

React

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  function handleClick() {
    setCount(count + 1);
    setCount(count + 1);
  }

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={handleClick}>Increase</button>
    </div>
  );
}

export default Counter;

A Count: 2

B Count: 1

C Count: 0

D Count: 3

💡 Hint

React batches state updates and uses the current state value in closures.

Both setCount calls use the same 'count' value from the closure because state updates are asynchronous and batched. Each button click increases the count by 1 only. After two clicks, the count will be 2.

🔧 Debug

advanced

Why does this React component throw an error?

This component throws an error when rendered. What is the cause?

React

import React from 'react';

function List() {
  const items = ['a', 'b', 'c'];
  return (
    <ul>
      {items.map(item => {
        <li>{item}</li>
      })}
    </ul>
  );
}

export default List;

A The <li> elements need a unique 'key' prop to render.

B The 'items' array is empty, so nothing renders.

C The arrow function in map lacks a return statement, so it returns undefined.

D JSX elements must be wrapped in a fragment <>...</> inside map.

💡 Hint

Check how arrow functions with curly braces behave regarding return values.

When using curly braces in arrow functions, you must explicitly return a value. Here, the function returns undefined, so React tries to render undefined, causing an error.

🧠 Conceptual

expert

What is the main reason React uses a virtual DOM?

Why does React use a virtual DOM instead of updating the real DOM directly every time?

A To batch and minimize real DOM updates for better performance.

B To allow React to run without a browser environment.

C To enable server-side rendering without client-side code.

D To store component state separately from the UI.

💡 Hint

Think about what makes updating the real DOM slow.

React uses a virtual DOM to compare changes and update only what is necessary in the real DOM, improving performance by reducing costly DOM operations.

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