What is JavaScript - Practice Questions & Exercises

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is JavaScript used for?

JavaScript is a popular programming language. What is its main use?

A To style web pages with colors and fonts

B To create databases for storing data

C To write server operating systems

D To add interactivity and dynamic behavior to web pages

💡 Hint

Think about what happens when you click buttons or see animations on websites.

JavaScript runs in the browser to make web pages interactive, like responding to clicks or updating content without reloading.

❓ Predict Output

intermediate

What is the output of this JavaScript code?

Look at this code and choose the correct output.

Javascript

console.log(typeof []);

A "array"

B "list"

C "object"

D "undefined"

💡 Hint

In JavaScript, arrays are a type of object.

The typeof operator returns "object" for arrays because arrays are special objects in JavaScript.

❓ Predict Output

advanced

What is the output of this JavaScript code snippet?

What will this code print to the console?

Javascript

console.log(0.1 + 0.2 === 0.3);

A true

B false

C TypeError

D undefined

💡 Hint

Think about how computers handle decimal numbers.

Due to floating-point precision, 0.1 + 0.2 is not exactly 0.3, so the comparison is false.

🧠 Conceptual

advanced

Which statement about JavaScript is true?

Choose the correct statement about JavaScript.

A JavaScript supports asynchronous programming with promises and async/await.

B JavaScript is a statically typed language.

C JavaScript can only run in web browsers.

D JavaScript code must be compiled before running.

💡 Hint

Think about how JavaScript handles tasks like fetching data from the internet.

JavaScript supports asynchronous programming using promises and async/await to handle tasks without blocking the program.

❓ Predict Output

expert

What is the output of this JavaScript code with closures?

What will this code print to the console?

Javascript

function makeCounter() {
  let count = 0;
  return function() {
    count += 1;
    return count;
  };
}
const counter1 = makeCounter();
const counter2 = makeCounter();
console.log(counter1());
console.log(counter1());
console.log(counter2());

A

1
2
1

B

1
1
1

C

0
1
0

D

2
3
2

💡 Hint

Each call to makeCounter creates a new independent counter.

counter1 and counter2 have separate count variables. counter1 increments twice, then counter2 starts at 0 and increments once.

Practice

1. What is JavaScript primarily used for on websites?

easy

A. To write the webpage content

B. To style the webpage layout

C. To add actions and interactivity

D. To store data on the server

Solution

1. Step 1: Understand JavaScript's role

   JavaScript is a programming language that adds actions and interactivity to websites.

2. Step 2: Compare with other web technologies

   HTML writes content, CSS styles layout, and server stores data, so JavaScript adds interactivity.

3. Final Answer:

   To add actions and interactivity -> Option C

4. Quick Check:

   JavaScript = Interactivity [OK]

Hint: JavaScript makes websites interactive, not just static [OK]

Common Mistakes:

• Confusing JavaScript with CSS for styling
• Thinking JavaScript writes webpage text
• Assuming JavaScript stores server data

2. Which of the following is the correct way to include JavaScript in an HTML file?

easy

A. <link href='app.js'>

B. <script src='app.js'></script>

C. <js>app.js</js>

D. <style>app.js</style>

Solution

1. Step 1: Identify correct HTML tag for JavaScript

   The <script> tag is used to include JavaScript files or code in HTML.

2. Step 2: Check the options

   <script src='app.js'></script> uses <script src='app.js'></script>, which is correct syntax to include an external JS file.

3. Final Answer:

   <script src='app.js'></script> -> Option B

4. Quick Check:

   Use <script> for JavaScript [OK]

Hint: JavaScript files go inside <script> tags [OK]

Common Mistakes:

• Using <style> tag for JavaScript
• Using non-existent <js> tag
• Using <link> tag which is for CSS

3. What will the following JavaScript code output in the browser console?

console.log('Hello, ' + 'world!');

medium

A. Hello, world!

B. Hello, + world!

C. Hello world!

D. Error

Solution

1. Step 1: Understand string concatenation

   The + operator joins two strings into one continuous string.

2. Step 2: Combine the strings

   'Hello, ' + 'world!' becomes 'Hello, world!'. console.log prints this to the console.

3. Final Answer:

   Hello, world! -> Option A

4. Quick Check:

   String + String = Combined string [OK]

Hint: Plus (+) joins strings in JavaScript [OK]

Common Mistakes:

• Expecting plus sign to print
• Missing comma or space in output
• Thinking it causes an error

4. Find the error in this JavaScript code snippet:

function greet() {
  console.log('Hi there!')
}

medium

A. Missing semicolon after console.log statement

B. Missing parentheses in function declaration

C. Missing curly braces around function body

D. No error, code is correct

Solution

1. Step 1: Check function syntax

   The function greet() has parentheses and curly braces properly placed.

2. Step 2: Check statement termination

   JavaScript allows optional semicolons; missing semicolon here does not cause error.

3. Final Answer:

   No error, code is correct -> Option D

4. Quick Check:

   Semicolons optional in JS [OK]

Hint: Semicolons are optional in many JS cases [OK]

Common Mistakes:

• Thinking semicolon is always required
• Confusing function syntax rules
• Assuming missing semicolon causes error

5. You want to make a webpage respond when a user clicks a button. Which JavaScript concept should you use?

hard

A. Event handling

B. Variable declaration

C. CSS styling

D. HTML structuring

Solution

1. Step 1: Understand user interaction

   Responding to clicks means reacting to user events on the page.

2. Step 2: Identify JavaScript feature for user actions

   Event handling lets JavaScript listen and respond to user actions like clicks.

3. Final Answer:

   Event handling -> Option A

4. Quick Check:

   User clicks = Event handling [OK]

Hint: Use event handlers to react to user actions [OK]

Common Mistakes:

• Confusing CSS styling with interactivity
• Thinking variables handle clicks
• Assuming HTML alone manages user actions