JavaScript Objective Questions and Answers

This JavaScript quiz contains carefully curated objective questions with correct answers and clear explanations. It is designed for frontend and backend developers, interview preparation, and deep understanding of modern JavaScript concepts including ES6+, asynchronous behavior, scope, closures, and performance pitfalls.

Practice JavaScript MCQs with Detailed Explanations

Answer at least 12 questions to submit.

46

What does setTimeout(fn, 0) guarantee?

High
47

Which is NOT a primitive type?

Medium
48

What will be logged? console.log(typeof []);

Medium
49

Which scenario causes callback hell?

Medium
50

What is the output? console.log('5' - 2);

Medium
51

Which causes implicit type coercion?

Medium
52

What will be logged? console.log(Promise.resolve() === Promise.resolve());

High
53

Which avoids blocking the UI thread?

High
54

What happens when an exception is thrown inside async?

High
55

Which prevents prototype pollution?

High
56

What will be logged? console.log(typeof class A {});

Medium
57

Which pattern reduces memory usage?

High
58

What is the output? console.log(+'');

Medium
59

Which structure allows garbage collection of keys?

High
60

Why should deep cloning be handled carefully in JavaScript?

High