JavaScript for interactivity in Intro to Computing - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to show an alert when the button is clicked.

Intro to Computing

document.getElementById('myButton').addEventListener('click', function() { alert([1]); });

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

A'Hello!'

BHello!

Calert('Hello!')

Dconsole.log('Hello!')

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the message string.

Using console.log instead of alert.

Passing a function call instead of a string.

The alert function requires a string argument inside quotes to display the message.

2fill in blank

medium

Complete the code to change the text of a paragraph with id 'text' when the button is clicked.

Intro to Computing

document.getElementById('myButton').addEventListener('click', function() { document.getElementById('text').[1] = 'Text changed!'; });

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

AinnerHTML

BtextContent

CinnerText

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'value' which works for input elements, not paragraphs.

Using innerHTML which can cause security issues if not careful.

Using innerText which behaves differently across browsers.

textContent changes the visible text inside an element safely without HTML parsing.

3fill in blank

hard

Fix the error in the code to toggle a CSS class 'active' on a div with id 'box' when clicked.

Intro to Computing

document.getElementById('box').addEventListener('click', function() { this.classList.[1]('active'); });

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

Aremove

Badd

Ctoggle

Dcontains

Attempts:

3 left

💡 Hint

Common Mistakes

Using add or remove which only add or remove but do not toggle.

Using contains which only checks if the class exists.

The toggle method adds the class if missing, or removes it if present, perfect for toggling.

4fill in blank

hard

Fill both blanks to create a function that changes the background color of the body to blue when called.

Intro to Computing

function changeBackground() { document.body.style.[1] = [2]; }

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

AbackgroundColor

Bbackground-color

C'blue'

Dblue

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'background-color' which is invalid in JS style property.

Not putting the color name in quotes.

Using the color name without quotes.

In JavaScript, CSS properties use camelCase like backgroundColor, and color values must be strings in quotes.

5fill in blank

hard

Fill all three blanks to create an event listener that changes the text of a button to 'Clicked!' when it is clicked.

Intro to Computing

document.getElementById([1]).addEventListener([2], function() { this.[3] = 'Clicked!'; });

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

A'myButton'

B'click'

CtextContent

Donclick

Attempts:

3 left

💡 Hint

Common Mistakes

Using onclick instead of 'click' event name.

Not quoting the id or event name strings.

Using innerHTML instead of textContent.

The element id is 'myButton' (string), the event is 'click' (string), and textContent changes the button's visible text.

Practice

1. What is the main purpose of JavaScript on a webpage?

easy

A. To style the webpage with colors and fonts

B. To make the webpage interactive by responding to user actions

C. To store data permanently on the server

D. To create static text content only

Solution

1. Step 1: Understand JavaScript's role

   JavaScript is used to add interactivity, meaning it reacts to what users do on the page.

2. Step 2: Compare with other web technologies

   Styling is done by CSS, and static content is HTML. Data storage is server-side, not JavaScript's main job.

3. Final Answer:

   To make the webpage interactive by responding to user actions -> Option B

4. Quick Check:

   JavaScript = Interactivity [OK]

Hint: JavaScript = webpage actions and reactions [OK]

Common Mistakes:

• Confusing JavaScript with CSS for styling
• Thinking JavaScript stores data permanently
• Believing JavaScript only creates static content

2. Which of the following is the correct way to add a click event listener to a button with id myBtn in JavaScript?

easy

A. document.querySelector('myBtn').addEventListener('click', alert('Clicked!'));

B. document.getElementById('myBtn').onClick = alert('Clicked!');

C. document.getElementById('myBtn').addEventListener('click', function() { alert('Clicked!'); });

D. document.getElementById('myBtn').addEventListener('onclick', function() { alert('Clicked!'); });

Solution

1. Step 1: Identify correct method and event name

   The method to add event listeners is addEventListener and the event name is 'click', not 'onclick'.

2. Step 2: Check function syntax

   The event handler should be a function, so function() { alert('Clicked!'); } is correct. Directly calling alert('Clicked!') passes the result, not the function.

3. Final Answer:

   document.getElementById('myBtn').addEventListener('click', function() { alert('Clicked!'); }); -> Option C

4. Quick Check:

   Use addEventListener('click', function) [OK]

Hint: Use addEventListener with event name and function [OK]

Common Mistakes:

• Using 'onclick' instead of 'click' in addEventListener
• Assigning event handler directly instead of a function
• Using querySelector without '#' for id

3. What will be the output when the following code runs and the button is clicked?

const btn = document.getElementById('btn');
btn.addEventListener('click', () => {
  btn.textContent = 'Clicked!';
});

medium

A. The button text changes to 'Clicked!' when clicked

B. An error occurs because textContent is not valid

C. Nothing happens when the button is clicked

D. The button disappears from the page

Solution

1. Step 1: Understand event listener effect

   The code listens for a click on the button and changes its text content to 'Clicked!'.

2. Step 2: Confirm textContent usage

   textContent is the correct property to change the visible text inside an element.

3. Final Answer:

   The button text changes to 'Clicked!' when clicked -> Option A

4. Quick Check:

   Click event changes button text [OK]

Hint: textContent changes visible text on click [OK]

Common Mistakes:

• Thinking textContent is invalid property
• Expecting the button to disappear
• Assuming no change happens without page reload

4. Identify the error in this code snippet that tries to change the background color of a div with id box when clicked:

const box = document.getElementById('box');
box.addEventListener('click', changeColor);

function changeColor() {
  box.style.background = 'blue';
}

medium

A. The getElementById method is incorrect and should be querySelector

B. The event listener should use 'onclick' instead of 'click'

C. The function changeColor should be anonymous inside addEventListener

D. The property 'background' should be 'backgroundColor' to change color

Solution

1. Step 1: Check CSS property used in JavaScript

   To change background color, the correct style property is backgroundColor, not background.

2. Step 2: Verify event listener and function usage

   Using 'click' event and named function is valid. getElementById is correct for id selection.

3. Final Answer:

   The property 'background' should be 'backgroundColor' to change color -> Option D

4. Quick Check:

   Use style.backgroundColor to change background color [OK]

Hint: Use style.backgroundColor, not style.background [OK]

Common Mistakes:

• Using 'background' instead of 'backgroundColor'
• Confusing 'click' event with 'onclick'
• Thinking function must be anonymous

5. You want to create a button that toggles the visibility of a paragraph with id text each time it is clicked. Which code snippet correctly implements this behavior?

hard

A. const btn = document.getElementById('btn');
const text = document.getElementById('text');
btn.addEventListener('click', () => {
if (text.style.display === 'none') {
text.style.display = 'block';
} else {
text.style.display = 'none';
}
});

B. const btn = document.getElementById('btn');
const text = document.getElementById('text');
btn.addEventListener('click', () => {
text.style.visibility = 'hidden';
});

C. const btn = document.getElementById('btn');
const text = document.getElementById('text');
btn.onclick = function() {
text.style.display = 'visible';
};

D. const btn = document.getElementById('btn');
const text = document.getElementById('text');
btn.addEventListener('click', () => {
text.style.display = 'block';
});

Solution

1. Step 1: Understand toggle logic

   To toggle visibility, check if display is 'none' and switch it to 'block', else set to 'none'.

2. Step 2: Analyze each option

   const btn = document.getElementById('btn'); const text = document.getElementById('text'); btn.addEventListener('click', () => { if (text.style.display === 'none') { text.style.display = 'block'; } else { text.style.display = 'none'; } }); correctly toggles between 'none' and 'block'. const btn = document.getElementById('btn'); const text = document.getElementById('text'); btn.addEventListener('click', () => { text.style.visibility = 'hidden'; }); only hides once. const btn = document.getElementById('btn'); const text = document.getElementById('text'); btn.onclick = function() { text.style.display = 'visible'; }; uses invalid 'visible' for display. const btn = document.getElementById('btn'); const text = document.getElementById('text'); btn.addEventListener('click', () => { text.style.display = 'block'; }); only shows without toggling.

3. Final Answer:

   const btn = document.getElementById('btn'); const text = document.getElementById('text'); btn.addEventListener('click', () => { if (text.style.display === 'none') { text.style.display = 'block'; } else { text.style.display = 'none'; } }); -> Option A

4. Quick Check:

   Toggle display between 'none' and 'block' [OK]

Hint: Toggle display style between 'none' and 'block' [OK]

Common Mistakes:

• Using visibility instead of display for toggling
• Setting display to invalid values like 'visible'
• Not toggling, only hiding or showing once