Creating first React app - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import React and use the useState hook.

React

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

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

AuseRef

BuseState

CuseContext

DuseEffect

Attempts:

3 left

💡 Hint

Common Mistakes

Importing useEffect instead of useState

Forgetting to import any hooks

The useState hook is used to add state to functional components in React.

2fill in blank

medium

Complete the code to create a state variable called count with initial value 0.

React

const [count, [1]] = useState(0);

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

AcountSetter

BgetCount

CsetCount

DupdateCount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a wrong setter function name

Trying to update state directly without setter

The second element returned by useState is the setter function to update the state.

3fill in blank

hard

Fix the error in the JSX to display the count variable inside a paragraph.

React

<p>The count is: [1]</p>

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

Acount

B"count"

C(count)

D{count}

Attempts:

3 left

💡 Hint

Common Mistakes

Writing the variable name without braces

Using quotes which show the word literally

In JSX, JavaScript expressions must be inside curly braces to be evaluated.

4fill in blank

hard

Fill both blanks to create a button that increments count when clicked.

React

<button onClick=[1]>[2]</button>

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

A() => setCount(count + 1)

BIncrement

CDecrement

DsetCount

Attempts:

3 left

💡 Hint

Common Mistakes

Passing setCount directly without a function

Using wrong button text

The onClick needs a function to update state. The button text should say 'Increment'.

5fill in blank

hard

Fill all three blanks to create a simple React component that shows a count and a button to increase it.

React

function Counter() {
  const [[1], [2]] = useState(0);
  return (
    <div>
      <p>Count: [3]</p>
      <button onClick={() => [2]([1] + 1)}>Increase</button>
    </div>
  );
}

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

Acount

BsetCount

Dincrement

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for state and display

Forgetting to use the setter function in onClick

The state variable is count, the setter is setCount, and the displayed value is count.

Practice

1. What is the main purpose of the ReactDOM.createRoot method in a React app?

easy

A. It creates a new React component.

B. It tells React where to put the app in the HTML page.

C. It styles the React app automatically.

D. It fetches data from the server.

Solution

1. Step 1: Understand ReactDOM.createRoot usage

   This method is used to select the HTML element where the React app will appear, usually a div with id 'root'.

2. Step 2: Identify its role in rendering

   It creates a root for React to manage the app's UI inside that HTML element.

3. Final Answer:

   It tells React where to put the app in the HTML page. -> Option B

4. Quick Check:

   ReactDOM.createRoot sets the app's HTML container [OK]

Hint: ReactDOM.createRoot links React to HTML element [OK]

Common Mistakes:

• Thinking it creates components
• Confusing it with styling or data fetching
• Assuming it runs app logic

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

easy

A. class App extends React.Component { render() { return 'Hello'; } }

B. const App = Hello;

C. function App() { return Hello; }

D. function App { return Hello; }

Solution

1. Step 1: Check function syntax for React component

   A React functional component is a function that returns JSX, so it needs parentheses after the name.

2. Step 2: Validate JSX return and syntax

   function App() { return
   Hello; } correctly defines a function named App that returns JSX inside a div.

3. Final Answer:

   function App() { return
   Hello
   ; } -> Option C

4. Quick Check:

   Functional component = function with () and JSX return [OK]

Hint: React components are functions returning JSX with () [OK]

Common Mistakes:

• Missing parentheses after function name
• Assigning JSX directly to a variable without function
• Using class syntax incorrectly for functional components

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

import React from 'react';
import ReactDOM from 'react-dom/client';

function App() {
  return 
Welcome to React!
;
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(<App />);

medium

A. undefined

B. App

C. Nothing, there is an error

D. Welcome to React!

Solution

1. Step 1: Understand what the App component returns

   The App function returns an <h1> element with the text 'Welcome to React!'.

2. Step 2: Check rendering process

   The root renders the App component inside the HTML element with id 'root', so the heading will appear on the page.

3. Final Answer:

   Welcome to React! -> Option D

4. Quick Check:

   App returns <h1>Welcome to React!</h1> [OK]

Hint: Component returns JSX shown on page [OK]

Common Mistakes:

• Thinking component name shows instead of its return
• Assuming error without checking code carefully
• Confusing JSX tags with plain text

4. Identify the error in this React app code:

import React from 'react';
import ReactDOM from 'react-dom/client';

function App() {
  return <h1>Hello World</h1>;
}

const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

medium

A. App should be called as in render, not just App.

B. Missing import of ReactDOM.

C. Function App must return a string, not JSX.

D. document.getElementById('root') is incorrect.

Solution

1. Step 1: Check how render is called

   React expects a JSX element or component instance in render, so is correct, not just App.

2. Step 2: Understand difference between component and element

   App is a function, is JSX that creates an element from that function.

3. Final Answer:

   App should be called as <App /> in render, not just App. -> Option A

4. Quick Check:

   render needs JSX element like <App /> [OK]

Hint: Use to render component, not App alone [OK]

Common Mistakes:

• Passing function name instead of JSX element to render
• Confusing component function with element instance
• Ignoring JSX syntax rules

5. You want to create a React app that shows a greeting message inside a div with a class container. Which code correctly creates and renders this app?

hard

A. function App() { return Hello!; }
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render();

B. function App() { return Hello!; }
ReactDOM.render(, document.getElementById('root'));

C. const App = () => Hello!;
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

D. function App() { return Hello!; }
const root = ReactDOM.createRoot(document.getElementById('root'));
root.render(App);

Solution

1. Step 1: Use correct JSX attribute for class

   In React, use className instead of class for CSS classes.

2. Step 2: Render the component as JSX element

   Use root.render(<App />) to render the component, not just the function name.

3. Final Answer:

   function App() { return <div className="container">Hello!</div>; } const root = ReactDOM.createRoot(document.getElementById('root')); root.render(<App />); -> Option A

4. Quick Check:

   Use className and render JSX element [OK]

Hint: Use className and render with [OK]

Common Mistakes:

• Using class instead of className in JSX
• Passing function name instead of JSX element to render
• Using old ReactDOM.render instead of createRoot