JavaScript Objective Questions and Answers - Set 4

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. This is part 4 of the series.

Practice JavaScript MCQs with Detailed Explanations

Answer at least 12 questions to submit and verify answers.

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