Top 100 JavaScript Interview Questions

JavaScript Interview Questions For Freshers

1. What are the data types in JavaScript ?
2. What is an object in JavaScript ?
3. In how many ways we can define an object in JavaScript ?
4. How to define a class in JavaScript ?
5. What is defineProperty() method of javascript Object ?
6. What is scope in JavaScript ?
7. Explain variable hoisting ?
8. Explain closures in javascript ?
9. Difference between null and undefined ?
10. Difference between == and === ?
11. What are the new features in ES6 , which were not there in previous JavaScript version ?
12. Explain time complexity ?
13. What is space complexity ?

14. What will be the output of the below code: let obj ={ a: 10, vir : function(){ x(); console.log(this.a); function x(){ console.log(this.a) } } } obj.vir();

15. What will be the output of the below code for (var i= 0; i < 5; i++){ setTimeout(() => console.log(i)); }
16. What will be the output of the below code :- var a = 90; doit(); function doit(){ console.log(a); var a = 10; }
17. Explain call(), bind() and apply() in JavaScript ?
18. Advantage of using arrow functions ?
19. What is webpack and babel ?
20. How can we add an element at the start of a JavaScript array ?
21. How to capitalise all the first letters of a given string ?
22. What will be the output of below code snippet :-function x() {return 2}alert(x());function x() {return 3}
23. What will be output of following code doit(); var doit = function (){ alert('x'); }
24. Difference between pass by value and pass by reference ?
25. What will be the output of following code:- for (var i= 0; i < 5; i++){ (function(i) { setTimeout( () => console.log(i)) })(i); }
26. What is InstanceOf operator ?
27. What is the use of constructor() in a class ?
28. What will be the output of the following :
    console.log(4.toString());
    console.log(4.2.toString());
    console.log(4*undefined);
29. How can we use javascript Set to remove duplicate elements of an array ?
30. What is the data storage limit of a cookie ? and what is the maximum number of characters that can be stored in a cookie string ?
31. How to make deep copy and shallow copy of objects ?
32. What's the difference between for loop and for each loop ?
33. What is a unary function ?
34. What is JSON in javascript and what are its advantages ?
35. What is JSON.stringify() ?
36. What is JSON.parse() ?





Source:- JavaScript Developer Interview Questions for Freshers 

JavaScript Interview Questions For Experienced 

1. What is Callback hell ?
2. Explain promises in JavaScript ?
3. Difference between observable and a promise
4. How to create custom events in JavaScript ?
5. Difference between prototype and _proto_ ?
6. Explain prototypal inheritance in JavaScript ?

7. Difference between for in and for of loop ?

8. Difference between get and post methods ?
9. Difference between post and put methods ?
10. When we pass an object to a function are they passed by reference or by value ?
11. What do u understand by the terms mutable and immutable in JavaScript ?
12. What is Async and Await in JavaScript ?
13. Is array a data type ?
14. How can we overwrite any JavaScript inbuilt property (like array.push())
15. Explain exception handling in JavaScript ?
16. Difference between a transpiler and a compiler ?
17. How to check if a number is an integer without using math function and without converting number into string or array ?
18. Use of JavaScript clearInterval() ?
19. How to define a private method or variable inside a class in JavaScript or ES6 ?
20. Can arrow functions be used as constructor functions ?
21. What is Map in JavaScript ?
22. How to implement authentication in a website ?
23. How would you implement pagination using JavaScript ?
24. Difference between a browser tab and a window in context of localstorage and sessionstorage ?
25. What is oauth in context of jwt ?
26. What is nginx used for ?
27. How to change the redirection URLs after the build has been made ?
28. What is a JWT , what are it's parts and what is its use ?
29. What are ‘set’ in javascript ?
30. Do we have reverse() method on javascript string ?
31. Name some array methods in JavaScript ?
32. Which operation is faster among == and === ?
33. Explain Array's sort method for sorting numbers. What is the time complexity of this method ?
34. What is Micro frontend architecture ?
35. What is the difference between first class function and higher order function ?    
36. What is temporal dead zone ?
37. Difference between put and patch method in javascript ?
38. How to convert an object to an Array in JavaScript ?
39. What is memoization in JavaScript ?
40. How to write a function ‘Sum’ such that on calling sum(a)(b)(c)....() where a,b,c etc are numbers , the output will be a+b+c+....(function calls with parameters) ?
41. What are the design patterns in JavaScript ?
42. What is event loop in JavaScript ?
43. How to deep clone in JavaScript ?
44. How to flatten an array in JavaScript ?
45. What is debounce in JavaScript ?
46. Write a code to flatten the contents of an object ?
47. How to create virtual dom in JavaScript ?
48. What is critical rendering path ?
49. What is a freeze method in JavaScript ?
50. What are solid principles in JavaScript ?
51. What is the seal method in JavaScript ?
52. Difference between Angular and React ?
53. How to watch variable changes in JavaScript ?
54. How to create a change detection mechanism of JavaScript framework ?
55. What is functional programming in JavaScript ?
56. What is the use of function currying in JavaScript ?
57. What is quick sort and what is it’s time complexity ?
58. Explain bubble sort and insertion sort with example ?
59. What is a REST API ?
60. How to apply conditional breakpoints in chrome browser debugging tool ?
61. Why storing JWT in cookie is safer than storing JWT in session storage or local storage? Is storing in cookie 100 percent safe ? what if I copy JWT from cookie in the browser debug panel and use it in a CSRF attack ?
62. What is an execution context and lexical environment in JavaScript ?
63. What is generator function in JavaScript ?
64. What are JavaScript iterators ?

Source: JavaScript Interview Questions For 5 Years Of Experience

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