What is a component in React - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What does a React component represent?

In React, what does a component mainly represent?

A A reusable piece of UI that can have its own logic and appearance

B A function that directly manipulates the browser's DOM elements

C A database where React stores user data

D A CSS style sheet that controls the look of the app

💡 Hint

Think about how React builds parts of the screen.

A React component is like a building block of the user interface. It can be reused and can have its own logic and look.

❓ component_behavior

intermediate

What will this React component render?

Given this React component, what will it display on the screen?

React

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

A A paragraph with the text 'Hello, friend!'

B A button labeled 'Hello, friend!'

C A heading with the text 'Hello, friend!'

D An empty screen with no text

💡 Hint

Look at the HTML tag used inside the return statement.

The component returns an

element with the text 'Hello, friend!', so it will render a heading with that text.

❓ state_output

advanced

What is the output after clicking the button twice?

Consider this React component. What will be the displayed count after clicking the button two times?

React

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increase</button>
    </div>
  );
}

A Count: undefined

B Count: 1

C Count: 0

D Count: 2

💡 Hint

Each click increases the count by 1.

The initial count is 0. Each button click adds 1. After two clicks, count is 2.

📝 Syntax

advanced

Which option correctly defines a React functional component?

Which of the following code snippets correctly defines a React functional component named 'Welcome' that returns a greeting?

A const Welcome = () => <h2>Welcome!</h2>;

B function Welcome() { <h2>Welcome!</h2>; }

C function Welcome() { <h2>Welcome!</h2> }

D const Welcome = () => { <h2>Welcome!</h2> }

💡 Hint

Remember that a component must return JSX to render.

Option A correctly defines a functional component using an arrow function with an implicit return of JSX.

❓ lifecycle

expert

What happens when the dependency array is empty in useEffect?

In React, what is the behavior of a useEffect hook with an empty dependency array like this? useEffect(() => { console.log('Effect ran'); }, []);

A The effect runs only when the component unmounts.

B The effect runs only once after the component mounts.

C The effect never runs.

D The effect runs after every render of the component.

💡 Hint

Think about when React runs effects with no dependencies.

An empty dependency array means the effect runs once after the component appears on screen (mount).

Practice

1. What is a React component?

easy

A. A reusable piece of UI built with a function or class

B. A special type of variable that stores data

C. A tool to style HTML elements

D. A command to run React apps

Solution

1. Step 1: Understand React components

   A React component is a building block of UI that can be reused multiple times.

2. Step 2: Identify the correct definition

   Only A reusable piece of UI built with a function or class describes a reusable UI piece built with a function or class.

3. Final Answer:

   A reusable piece of UI built with a function or class -> Option A

4. Quick Check:

   Component = reusable UI block [OK]

Hint: Components are reusable UI parts made with functions [OK]

Common Mistakes:

• Confusing components with variables
• Thinking components are styling tools
• Believing components are commands

2. Which of the following is the correct way to define a React functional component?

easy

A. const MyComponent = 'Hello';

B. function MyComponent() { return Hello; }

C. var MyComponent = Hello;

D. class MyComponent extends React.Component { }

Solution

1. Step 1: Review functional component syntax

   A functional component is a function returning JSX, like function MyComponent() { return
   Hello; }.

2. Step 2: Check each option

   function MyComponent() { return
   Hello
   ; } matches the correct syntax. var MyComponent =
   Hello
   ; assigns JSX directly to a variable, which is not a component. const MyComponent = 'Hello'; assigns a string, not JSX. class MyComponent extends React.Component { } is a class component, not functional.

3. Final Answer:

   function MyComponent() { return <div>Hello</div>; } -> Option B

4. Quick Check:

   Functional component = function returning JSX [OK]

Hint: Functional components are functions returning JSX [OK]

Common Mistakes:

• Assigning JSX directly to variables
• Using strings instead of JSX
• Confusing functional and class components

3. What will this React component render?

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

medium

A. Nothing, because the function is not called

B. An error because of missing parentheses

C. <h1>Hello, world!</h1> displayed on the page

D. A blank page with no content

Solution

1. Step 1: Understand component return value

   The component returns an <h1> element with text "Hello, world!" which React renders as HTML.

2. Step 2: Confirm rendering behavior

   When used in JSX, this component will display the heading on the page. No syntax errors exist.

3. Final Answer:

   <h1>Hello, world!</h1> displayed on the page -> Option C

4. Quick Check:

   Return JSX renders UI [OK]

Hint: Returned JSX shows on page when component is used [OK]

Common Mistakes:

• Thinking function must be called manually
• Assuming missing parentheses cause error here
• Believing component renders nothing by default

4. Identify the error in this React component:

function Welcome() {
  return <div>Welcome to React</div>
}

medium

A. Missing semicolon after return statement

B. JSX must be wrapped in parentheses if on multiple lines

C. Function name should start with lowercase letter

D. No error, component is correct

Solution

1. Step 1: Check JSX syntax

   The JSX is a single line and correctly returned without parentheses, which is valid.

2. Step 2: Verify naming and semicolon rules

   Function names for components should start uppercase, which is true. Semicolons are optional in JavaScript here.

3. Final Answer:

   No error, component is correct -> Option D

4. Quick Check:

   Valid JSX and naming means no error [OK]

Hint: JSX on one line doesn't need parentheses [OK]

Common Mistakes:

• Thinking semicolons are mandatory after return
• Believing lowercase function names are required
• Assuming JSX always needs parentheses

5. You want to create a reusable button component that shows different text based on a property. Which is the best way to do this?

hard

A. function Button(props) { return {props.text}; }

B. function Button() { return Click me; }

C. const Button = Click me;

D. function Button(text) { return {text}; }

Solution

1. Step 1: Understand passing data to components

   Props are used to pass data to components. Using props.text allows dynamic button text.

2. Step 2: Evaluate each option

   function Button(props) { return {props.text}; } correctly uses props to show different text. function Button() { return Click me; } always shows fixed text. const Button = Click me; is not a function component. function Button(text) { return {text}; } incorrectly uses a parameter instead of props object.

3. Final Answer:

   function Button(props) { return <button>{props.text}</button>; } -> Option A

4. Quick Check:

   Use props for dynamic content [OK]

Hint: Use props to pass dynamic text to components [OK]

Common Mistakes:

• Hardcoding text inside component
• Not using props object for parameters
• Assigning JSX directly to variables