Why React is used - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a React component that displays a greeting.

React

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

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

A"Hello, world!"

BHello, world!

C<Hello, world!>

D{Hello, world!}

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around text inside JSX.

Using curly braces without quotes for plain text.

The text inside JSX must be a string wrapped in quotes.

2fill in blank

medium

Complete the code to use React's useState hook to create a counter.

React

import React, { [1] } from 'react';

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

AuseRef

BuseEffect

CuseContext

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Importing useEffect instead of useState.

Trying to use state without importing useState.

useState is the hook to add state in functional components.

3fill in blank

hard

Fix the error in the React component that updates state on button click.

React

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

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

A() => setCount(count + 1)

Bcount + 1

C() => count + 1

DsetCount(count + 1)

Attempts:

3 left

💡 Hint

Common Mistakes

Calling setCount directly instead of passing a function.

Using expressions instead of functions for onClick.

The onClick handler must be a function. Using an arrow function calls setCount correctly on click.

4fill in blank

hard

Fill both blanks to create a controlled input component in React.

React

function NameInput() {
  const [name, setName] = React.useState('');
  return (
    <input
      value=[1]
      onChange={e => [2](e.target.value)}
      aria-label="Name input"
    />
  );
}

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

Aname

BsetName

Cname()

DsetName()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name()' instead of 'name' for value.

Calling 'setName()' without argument.

The input's value is the state variable 'name'. The onChange calls 'setName' to update it.

5fill in blank

hard

Fill all three blanks to create a React component that fetches data on mount using useEffect.

React

import React, { [1], [2] } from 'react';

function DataFetcher() {
  const [data, setData] = [3](null);

  [1](() => {
    fetch('https://api.example.com/data')
      .then(response => response.json())
      .then(json => setData(json));
  }, []);

  if (!data) return <p>Loading...</p>;
  return <pre>{JSON.stringify(data, null, 2)}</pre>;
}

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

AuseEffect

BuseState

DuseContext

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to import useEffect or useState.

Using useContext instead of useState for state.

useEffect runs code on mount. useState creates state. The component uses both to fetch and store data.

Practice

1. Why do developers use React for building websites?

easy

A. To design database schemas

B. To write backend server code easily

C. To manage network security settings

D. To create reusable components that make development faster

Solution

1. Step 1: Understand React's main feature

   React breaks websites into small, reusable parts called components, which helps developers build faster and maintainable code.

2. Step 2: Compare options with React's purpose

   Only To create reusable components that make development faster describes creating reusable components, which is React's core use. Other options relate to backend, database, or security, which React does not handle.

3. Final Answer:

   To create reusable components that make development faster -> Option D

4. Quick Check:

   Reusable components = C [OK]

Hint: React is mainly for building UI components fast [OK]

Common Mistakes:

• Confusing React with backend frameworks
• Thinking React manages databases
• Assuming React handles security

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

easy

A. component MyComponent() { return Hello; }

B. function MyComponent() { return Hello; }

C. def MyComponent() { return Hello; }

D. func MyComponent() { return Hello; }

Solution

1. Step 1: Recall React functional component syntax

   React functional components are JavaScript functions that return JSX, defined using the keyword 'function'.

2. Step 2: Check each option's syntax

   function MyComponent() { return
   Hello; } uses 'function' correctly. Options B, C, and D use invalid keywords not recognized in JavaScript or React.

3. Final Answer:

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

4. Quick Check:

   React components use 'function' keyword [OK]

Hint: React components start with 'function' keyword [OK]

Common Mistakes:

• Using Python or other language keywords
• Confusing component declaration syntax
• Omitting return statement or JSX

3. What will be the result of this React component rendering?

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

medium

A. Syntax Error

B. Hello, {name}!

C. <h1>Hello, Alice!</h1>

D. <h1>Hello, name!</h1>

Solution

1. Step 1: Understand JSX expression embedding

   In JSX, curly braces { } embed JavaScript expressions. Here, {name} inserts the value of the variable 'name', which is 'Alice'.

2. Step 2: Predict rendered output

   The component returns an h1 element with text 'Hello, Alice!'. So the rendered HTML is <h1>Hello, Alice!</h1>.

3. Final Answer:

   <h1>Hello, Alice!</h1> -> Option C

4. Quick Check:

   JSX {name} inserts variable value [OK]

Hint: JSX uses { } to insert variables [OK]

Common Mistakes:

• Thinking {name} prints literally
• Confusing JSX with plain HTML
• Expecting syntax error for valid JSX

4. Identify the error in this React component code:

function Welcome() {
  return (
    <div>
      <h1>Welcome</h1>
      <p>Have a nice day!</p>
    </div>
  )
  console.log('Rendered');
}

medium

A. console.log is unreachable after return statement

B. Missing semicolon after return

C. JSX tags are not closed properly

D. Function name should be lowercase

Solution

1. Step 1: Analyze function flow

   The return statement ends the function execution, so any code after it is unreachable.

2. Step 2: Check the console.log placement

   console.log('Rendered') is after return, so it will never run, which is a logical error.

3. Final Answer:

   console.log is unreachable after return statement -> Option A

4. Quick Check:

   Code after return is unreachable [OK]

Hint: Code after return won't run [OK]

Common Mistakes:

• Ignoring unreachable code warnings
• Thinking semicolon is mandatory after return
• Assuming function names must be lowercase

5. How does React improve website speed when updating the user interface?

hard

A. By updating only the parts of the page that change using a virtual DOM

B. By reloading the entire page on every change

C. By using server-side rendering exclusively

D. By disabling user interactions during updates

Solution

1. Step 1: Understand React's update mechanism

   React uses a virtual DOM to track changes and updates only the parts of the real DOM that need to change, avoiding full page reloads.

2. Step 2: Compare options with React's behavior

   By updating only the parts of the page that change using a virtual DOM correctly describes this selective update process. Options B, C, and D describe incorrect or unrelated behaviors.

3. Final Answer:

   By updating only the parts of the page that change using a virtual DOM -> Option A

4. Quick Check:

   Virtual DOM updates only changed parts [OK]

Hint: React updates only changed parts for speed [OK]

Common Mistakes:

• Thinking React reloads whole page
• Confusing client and server rendering
• Assuming React disables interactions