JavaScript Interview Questions

JavaScript is everywhere! Whether you're building interactive websites, dynamic web apps, or even backend services, mastering JavaScript is an essential skill for any developer.

Why Learn JavaScript? 🤔

JavaScript is the backbone of the web. With it, you can:

• Make web pages interactive 🎨
• Build full-fledged applications 🏗️
• Work with modern frameworks like React, Vue, and Angular ⚛️
• Even create mobile apps and server-side applications 📱🖥️

Beginner-Level Questions

Basics

1. What is JavaScript?
2. What are the key features of JavaScript?
3. What are the differences between JavaScript and Java?
4. How do you include JavaScript in an HTML file?
5. What are the different data types in JavaScript?
6. What are the different data types present in JavaScript?
7. What is the difference between =and = =?
8. Difference between =and = = operators.
9. What is the difference between null and undefined?
10. What is the use of the typeof operator in JavaScript?
11. What are JavaScript variables? How do you declare them?
12. What is the difference between let, const, and var?
13. Differences between declaring variables using var, let, and const.
14. What is hoisting in JavaScript?
15. Explain Hoisting in JavaScript.
16. Explain the difference in hoisting between var, let, and const.
17. How does hoisting affect function declarations and expressions?
18. What are the potential issues caused by hoisting?
19. How can you avoid problems related to hoisting?
20. What is scope in JavaScript?
21. Explain Scope and Scope Chain in JavaScript.
22. What is the difference between global and local scope?
23. What is an arrow function? How is it different from a regular function?
24. What are arrow functions?
25. What is a closure in JavaScript?
26. Explain Closures in JavaScript.
27. What is an IIFE (Immediately Invoked Function Expression)?
28. What is an Immediately Invoked Function in JavaScript?
29. What is the this keyword in JavaScript?
30. Explain the this keyword.
31. What is JSON? How do you parse and stringify JSON in JavaScript?
32. What are JavaScript promises?
33. What is the use of promises in JavaScript?
34. What is the difference between call, apply, and bind?
35. Explain call(), apply(), and bind() methods.
36. What is the difference between exec() and test() methods in JavaScript?
37. What is currying in JavaScript?
38. What is the rest parameter and spread operator?
39. What is Implicit Type Coercion in JavaScript?
40. Is JavaScript a statically typed or a dynamically typed language?
41. What is the NaN property in JavaScript?
42. Explain passed by value and passed by reference.
43. Is JavaScript a pass-by-reference or pass-by-value language?
44. What do you mean by strict mode in JavaScript and characteristics of JavaScript strict mode?
45. What are some advantages of using External JavaScript?
46. Mention some advantages of JavaScript.
47. What is the use of a constructor function in JavaScript?
48. What is recursion in a programming language?
49. What is memoization?
50. What is the Temporal Dead Zone?

DOM Manipulation

1. What is the DOM (Document Object Model)?
2. How do you select elements in the DOM using JavaScript?
3. What is the difference between innerHTML and innerText?
4. What is event bubbling and event capturing?
5. What is the difference between addEventListener and onclick?
6. Which method is used to retrieve a character from a certain index?
7. What do you mean by BOM (Browser Object Model)?
8. What is the distinction between client-side and server-side JavaScript?
9. Describe the difference between a cookie, sessionStorage, and localStorage in browsers.
10. Describe the difference between , , and .
11. Explain event delegation in JavaScript.
12. What is the difference between mouseenter and mouseover events in JavaScript and browsers?
13. What is the difference between event.preventDefault() and event.stopPropagation()?
14. How do you prevent the default behavior of an event?
15. What is the difference between document.querySelector() and document.getElementById()?
16. How do you add, remove, and modify HTML elements using JavaScript?
17. What are event listeners and how are they used?
18. Explain the event phases in a browser.
19. What is the difference between innerHTML and textContent?
20. How do you manipulate CSS styles using JavaScript?

Functions

1. What is a first-class function?
2. What is a higher-order function?
3. Explain Higher Order Functions in JavaScript.
4. What is a callback function?
5. Why do we use callbacks?
6. What is a pure function?
7. What is a currying function?
8. What is a thunk function?
9. What are asynchronous thunks?
10. What is a decorator in JavaScript?
11. What is a proper tail call in JavaScript?
12. What's a typical use case for anonymous functions in JavaScript?
13. What is recursion and how is it used in JavaScript?
14. What are default parameters and how are they used?
15. Explain why the following doesn't work as an IIFE: function foo(){}();. What needs to be changed to properly make it an IIFE?
16. What are the various ways to create objects in JavaScript?
17. Explain the difference between dot notation and bracket notation for accessing object properties.
18. What are the different methods for iterating over an array?
19. How do you add, remove, and update elements in an array?
20. What are the different ways to copy an object or an array?
21. Explain the difference between shallow copy and deep copy.
22. What are the advantages of using the spread operator with arrays and objects?
23. How do you check if an object has a specific property?
24. Explain the concept of destructuring assignment for objects and arrays.
25. What is Object.freeze() for?
26. What is Object.seal() for?
27. What is Object.preventExtensions() for?

Arrays and Objects

1. What is the purpose of the Array.prototype.slice method?
2. What is the purpose of the Array.prototype.splice method?
3. What is the difference between slice and splice?
4. How do you compare Object and Map?
5. What is the difference between Object.keys and Object.getOwnPropertyNames?
6. What are object prototypes?
7. What is the prototype design pattern?
8. In JavaScript, how many different methods can you make an object?
9. What is Object Destructuring?
10. What are Sets and Maps and how are they used?
11. What are the differences between Map/Set and WeakMap/WeakSet in JavaScript?
12. How do you convert a Set to an array in JavaScript?
13. What is the difference between a Map object and a plain object in JavaScript?
14. How do Sets and Maps handle equality checks for objects?

Intermediate-Level Questions

Advanced Concepts

1. What is the prototype chain in JavaScript?
2. What is the difference between prototypal inheritance and classical inheritance?
3. Difference between prototypal and classical inheritance.
4. What is the difference between Object.create and the new keyword?
5. What is the difference between Object.freeze and Object.seal?
6. What is the difference between deep copy and shallow copy?
7. What is the role of deferred scripts in JavaScript?
8. What has to be done in order to put Lexical Scoping into practice?
9. Explain the concept of lexical scoping.
10. How can closures be used to create private variables?
11. What are the potential pitfalls of using closures?
12. Explain the difference between global scope, function scope, and block scope.
13. Explain how this works in JavaScript.
14. Explain the different ways the this keyword can be bound.
15. What are the common pitfalls of using the this keyword?
16. Explain the concept of this binding in event handlers.
17. What is the DOM and how is it structured?
18. What's the difference between an "attribute" and a "property" in the DOM?
19. How do you add, remove, and modify HTML elements using JavaScript?
20. What are event listeners and how are they used?

Asynchronous JavaScript

1. What is the event loop in JavaScript runtimes?
2. Explain the difference between synchronous and asynchronous functions in JavaScript.
3. What are Promises and how do they work?
4. Explain the different states of a Promise.
5. What are the pros and cons of using Promises instead of callbacks in JavaScript?
6. What is the use of Promise.all()?
7. How is Promise.all() different from Promise.allSettled()?
8. What is async/await and how does it simplify asynchronous code?
9. How do you handle errors in asynchronous operations?
10. Explain the concept of a microtask queue.
11. What is the difference between setTimeout(), setImmediate(), and process.nextTick()?
12. Explain how prototypal inheritance works in JavaScript.
13. What is the prototype chain and how does it work?
14. Explain the difference between classical inheritance and prototypal inheritance.
15. Explain the concept of inheritance in ES2015 classes.
16. What is the purpose of the new keyword?
17. How do you create a constructor function?
18. What are the differences between JavaScript ES2015 classes and ES5 function constructors?
19. What advantage is there for using the JavaScript arrow syntax for a method in a constructor?
20. Why might you want to create static class members in JavaScript?

ES6+ Features

1. What are template literals in JavaScript?
2. What is destructuring assignment in JavaScript?
3. What are default parameters in JavaScript?
4. What are JavaScript modules?
5. What are JavaScript generators?
6. What are generator functions?
7. What are classes in JavaScript?
8. What are Symbols used for in JavaScript?
9. What are proxies in JavaScript used for?
10. What are iterators and generators in JavaScript and what are they used for?
11. Explain the difference between mutable and immutable objects in JavaScript.
12. What is the difference between a Map object and a plain object in JavaScript?
13. What are the differences between Map/Set and WeakMap/WeakSet in JavaScript?
14. Why might you want to create static class members in JavaScript?
15. What are JavaScript object getters and setters for?
16. What are JavaScript object property flags and descriptors?
17. How do you reliably determine whether an object is empty?
18. What are the benefits of using a module bundler?
19. Explain the concept of tree shaking in module bundling.
20. What are the metadata fields of a module?

Error Handling

1. What is the purpose of the Error object in JavaScript?
2. What are the different types of errors in JavaScript?
3. How do you handle errors using try...catch blocks?
4. What is the purpose of the finally block?
5. How can you create custom error objects?
6. Explain the concept of error propagation in JavaScript.
7. What are some best practices for handling sensitive data in JavaScript?
8. Explain the concept of Content Security Policy (CSP) and how it enhances security.
9. What are some common security headers and their purpose?
10. How can you prevent clickjacking attacks?
11. Explain the concept of input validation and its importance in security.
12. What are some tools and techniques for identifying security vulnerabilities in JavaScript code?
13. How can you implement secure authentication and authorization in JavaScript applications?
14. Explain the same-origin policy with regards to JavaScript.
15. What is use strict in JavaScript for?
16. What tools and techniques do you use for debugging JavaScript code?
17. How does JavaScript garbage collection work?
18. Explain what a single-page app is and how to make it SEO-friendly.
19. How can you share code between JavaScript files?
20. How do you organize your code?

Advanced-Level Questions

Performance and Optimization

1. What is tree shaking in JavaScript?
2. What is the need for tree shaking?
3. How do you optimize JavaScript performance?
4. What is the difference between debounce and throttle?
5. What is the purpose of the requestAnimationFrame method?
6. What are some common performance bottlenecks in JavaScript applications?
7. Explain the concept of debouncing and throttling.
8. How can you optimize DOM manipulation for better performance?
9. What are some techniques for reducing reflows and repaints?
10. Explain the concept of lazy loading and how it can improve performance.
11. What are Web Workers and how can they be used to improve performance?
12. Explain the concept of caching and how it can be used to improve performance.
13. What are some tools that can be used to measure and analyze JavaScript performance?
14. How can you optimize network requests for better performance?
15. What are the different types of testing in software development?
16. Explain the difference between unit testing, integration testing, and end-to-end testing.
17. What are some popular JavaScript testing frameworks?
18. How do you write unit tests for JavaScript code?
19. Explain the concept of test-driven development (TDD).
20. What are mocks and stubs and how are they used in testing?
21. How can you test asynchronous code in JavaScript?
22. What are some best practices for writing maintainable and effective tests in JavaScript?
23. Explain the concept of code coverage and how it can be used to assess test quality.
24. What are some tools that can be used for JavaScript testing?
25. What are design patterns and why are they useful?
26. Explain the concept of the Singleton pattern.
27. What is the Factory pattern and how is it used?
28. Explain the Observer pattern and its use cases.
29. What is the Module pattern and how does it help with encapsulation?
30. Explain the concept of the Prototype pattern.
31. What is the Decorator pattern and how is it used?
32. Explain the concept of the Strategy pattern.
33. What is the Command pattern and how is it used?
34. Why is extending built-in JavaScript objects not a good idea?
35. What is Cross-Site Scripting (XSS) and how can you prevent it?
36. Explain the concept of Cross-Site Request Forgery (CSRF) and its mitigation techniques.
37. How can you prevent SQL injection vulnerabilities in JavaScript applications?
38. What are some best practices for handling sensitive data in JavaScript?
39. Explain the concept of Content Security Policy (CSP) and how it enhances security.
40. What are some common security headers and their purpose?
41. How can you prevent clickjacking attacks?
42. Explain the concept of input validation and its importance in security.
43. What are some tools and techniques for identifying security vulnerabilities in JavaScript code?
44. How can you implement secure authentication and authorization in JavaScript applications?
45. Explain the same-origin policy with regards to JavaScript.
46. What is use strict in JavaScript for?
47. What tools and techniques do you use for debugging JavaScript code?
48. How does JavaScript garbage collection work?
49. Explain what a single-page app is and how to make it SEO-friendly.
50. How can you share code between JavaScript files?
51. How do you organize your code?
52. What are some of the advantages/disadvantages of writing JavaScript code in a language that compiles to JavaScript?
53. When would you use document.write()?

Coding Challenges

Beginner

1. Write a function to reverse a string.
2. Write a function to check if a string is a palindrome.
3. Write a function to find the factorial of a number.
4. Write a function to find the Fibonacci sequence up to a given number.
5. Write a function to check if a number is prime.

Intermediate

6. Write a function to flatten a nested array.
7. Write a function to implement a debounce function.
8. Write a function to implement a throttle function.
9. Write a function to implement a deep clone of an object.
10. Write a function to implement a memoization function.

Advanced

11. Write a function to implement a polyfill for Array.prototype.map.
12. Write a function to implement a polyfill for Array.prototype.filter.
13. Write a function to implement a polyfill for Array.prototype.reduce.
14. Write a function to implement a polyfill for Promise.all.
15. Write a function to implement a polyfill for Promise.race.

Behavioral and Conceptual Questions

1. How do you debug JavaScript code?
2. Why do we use the word “debugger” in JavaScript?
3. What tools do you use for debugging JavaScript?
4. What are some best practices for writing clean JavaScript code?
5. How do you handle cross-browser compatibility issues?
6. What are some common security issues in JavaScript?