Mathilde Lacombe

JavaScript remains one of the most essential programming languages in the world of web development. Whether you’re preparing for a front-end, full-stack, or software engineering interview, a strong understanding of JavaScript fundamentals and modern ES6+ features is crucial. In this comprehensive guide, we’ve compiled the top 100 JavaScript interview questions and answers for 2025 — perfect for both beginners brushing up their basics and professionals sharpening their technical skills.

JavaScript is the backbone of interactive web applications. Its flexibility allows developers to create dynamic interfaces, manage data efficiently, and build scalable front-end frameworks like React, Vue, and Angular. As interviewers increasingly focus on practical JavaScript knowledge, it’s vital to understand not just “what” works, but also why it works. This list covers everything from the fundamentals — such as variables, functions, and objects — to advanced topics like closures, asynchronous programming, promises, and the event loop.

Each section is organized by topic to make learning easier. You’ll find questions about scope, hoisting, prototype inheritance, ES6 features, DOM manipulation, async/await, and more. Understanding these areas will help you confidently tackle both coding challenges and conceptual discussions during your interview.

We’ve also included modern JavaScript concepts like arrow functions, destructuring, modules, localStorage, and performance optimization techniques like debouncing and throttling — commonly asked in 2025 tech interviews. Whether you’re applying for a junior developer or senior engineer role, this question bank will give you a competitive edge.

By the end of this guide, you’ll have a well-rounded understanding of the language that powers the modern web. Review these JavaScript interview questions and answers regularly to stay sharp, boost confidence, and make a strong impression in your next interview.

🟢 Basics & Fundamentals

1. What is JavaScript?

2. How is JavaScript different from Java?

3. What are the data types in JavaScript?

4. What are primitive and non-primitive data types?

5. What are variables in JavaScript?

6. What is the difference between var, let, and const?

7. What is hoisting in JavaScript?

8. What are closures in JavaScript?

9. What is the difference between == and ===?

10. What is NaN in JavaScript?

11. What is typeof operator?

12. What is the use of this keyword?

13. What is the difference between null and undefined?

14. What are template literals?

15. How do you write comments in JavaScript?

16. What are truthy and falsy values?

17. What is coercion in JavaScript?

18. What is a callback function?

19. What is the use of parseInt() and parseFloat()?

20. What is the purpose of isNaN() function?

🔵 Functions & Scope

21. What are function declarations and expressions?

22. What are arrow functions?

23. What is an Immediately Invoked Function Expression (IIFE)?

24. What is the difference between call, apply, and bind?

25. What is lexical scope?

26. What are higher-order functions?

27. What is function currying?

28. What are default parameters in functions?

29. Can functions return other functions?

30. What is recursion in JavaScript?

🟣 Objects & Arrays

31. What are objects in JavaScript?

32. How do you create an object?

33. How do you clone an object in JavaScript?

34. What is object destructuring?

35. What is array destructuring?

36. What are array methods like map(), filter(), and reduce()?

37. How do you merge two arrays?

38. How do you remove duplicates from an array?

39. What is the difference between forEach and map?

40. How do you sort an array of objects by a property?

🟠 Events & DOM

41. What is the DOM?

42. How do you select elements in the DOM?

43. How do you add or remove HTML elements dynamically?

44. What are event listeners?

45. What is event bubbling and capturing?

46. What is event delegation?

47. What is the difference between window.onload and document.ready?

48. How can you prevent the default action in an event?

49. How do you stop event propagation?

50. How do you handle forms using JavaScript?

🟡 Asynchronous JavaScript

51. What is asynchronous programming?

52. What are callbacks?

53. What are Promises?

54. What is the difference between synchronous and asynchronous code?

55. What are async and await?

56. What is the event loop in JavaScript?

57. What are microtasks and macrotasks?

58. What is the difference between setTimeout and setInterval?

59. What is Promise chaining?

60. What are Fetch API and AJAX?

🟤 Error Handling & Debugging

61. How do you handle errors in JavaScript?

62. What are try, catch, and finally?

63. What are custom errors in JavaScript?

64. What is throw used for?

65. How can you debug JavaScript code in the browser?

⚫ Advanced Concepts

66. What is prototypal inheritance?

67. What is the prototype chain?

68. What is the difference between classical and prototypal inheritance?

69. What is a constructor function?

70. What are classes in JavaScript?

71. What is encapsulation in JavaScript?

72. What is the difference between shallow copy and deep copy?

73. What are symbols in JavaScript?

74. What is the difference between Object.seal() and Object.freeze()?

75. What are generators and iterators?

⚪ Browser & Storage

76. What are cookies, localStorage, and sessionStorage?

77. What’s the difference between localStorage and sessionStorage?

78. How do you store data in localStorage?

79. How do you delete an item from localStorage?

80. How do you handle cross-origin requests?

🟩 Modules & Imports

81. What are ES6 modules?

82. What is the difference between default export and named export?

83. How do you import and export functions or variables?

84. What is CommonJS vs ES6 module system?

85. What is tree-shaking in JavaScript?

🟥 Performance & Optimization

86. How do you improve JavaScript performance?

87. What is debouncing?

88. What is throttling?

89. What are memory leaks and how do you prevent them?

90. What is lazy loading in JavaScript?

🟧 Testing & Tools

91. What are unit tests in JavaScript?

92. What are some popular JavaScript testing frameworks?

93. What is Jest used for?

94. What is Mocha vs Jasmine?

95. What is the difference between console.log() and debugging tools?

🟪 ES6+ Features

96. What are template literals?

97. What are destructuring assignments?

98. What are spread and rest operators?

99. What are Promises and how do they improve callbacks?

100. What new features were introduced in ES6 and later (ES7–ES13)?