JavaScript Code Challenges: DOM, Events, and Promises

This quiz covers foundational topics in JavaScript, including:

Variable Hoisting and Scoping: Understanding the behavior of var, let, and const in different contexts.
Promises and Async/Await: Handling asynchronous operations effectively in JavaScript.
Array and Object Manipulation: Using methods like map, filter, and the spread operator for data transformation.

This will help you evaluate your understanding of JavaScript’s core concepts and strengthen your problem-solving skills!

Question 1.

What does this code output?

var x = 10; function test() { console.log(x); var x = 20; } test();

A.
undefined
B.
10
C.
20
D.
Throws an error

Question 2.

What happens when you run this code?

document.querySelector("#btn").addEventListener("click", () => { alert("Button clicked!"); });

A.
Adds an alert when the button with ID btn is clicked
B.
Throws an error
C.
Doesn't execute anything
D.
Adds a console log instead of an alert

Question 3.

What will this code log to the console?

const arr = [1, 2, 3]; const result = arr.map(num => num * 2); console.log(result);

A.
Throws an error
B.
[1, 2, 3]
C.
undefined
D.
[2, 4, 6]

Question 4.

What will this code do?

const promise = new Promise((resolve, reject) => { setTimeout(() => resolve("Success!"), 2000); }); promise.then(data => console.log(data));

A.
Logs undefined
B.
Logs "Success!" after 2 seconds
C.
Logs "Success!" immediately
D.
Throws an error

Question 5.

What will this code output?

let obj = { a: 1, b: 2 }; let { a, b } = obj; console.log(a, b);

A.
{a: 1, b: 2}
B.
1 2
C.
undefined undefined
D.
Throws an error

Question 6.

What will the following log?

const a = [1, 2]; const b = [...a, 3, 4]; console.log(b);

A.
[1, 2]
B.
undefined
C.
[1, 2, 3, 4]
D.
Throws an error

Question 7.

What does this code log?

let x = 10; let y = "10"; console.log(x == y, x === y);

A.
true true
B.
false true
C.
true false
D.
false false

Question 8.

What will this function return?

function greet(name = "Guest") { return `Hello, ${name}!`; } console.log(greet());

A.
Hello, !
B.
undefined
C.
Throws an error
D.
Hello, Guest!

Question 9.

What will this code output?

const obj = { a: 1 }; Object.freeze(obj); obj.a = 2; console.log(obj.a);

A.
2
B.
1
C.
Throws an error
D.
undefined

Question 10.

What does this async function log?

async function fetchData() { return "Data fetched!"; } fetchData().then(console.log);

A.
undefined
B.
Throws an error
C.
Data fetched!
D.
Logs nothing