JavaScript is a popular programming language used in web development, with its versatility and ease of use making it a popular choice for developers. For anyone looking to start a career in web development, or for anyone who has an interest in JavaScript, it is essential to be familiar with the language’s most frequently asked interview questions. In this article, we will go through the top 50 JavaScript interview questions and answers for 2023, providing a comprehensive guide to help you prepare for your next interview.

What are JavaScript questions asked in interview?

1. What is JavaScript, and what are its features?

JavaScript is a scripting language that is used to create dynamic and interactive web pages. Its main features include object-oriented programming, functional programming, and event-driven programming.

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

Var, let, and const are all used for variable declarations in JavaScript. Var is a function scope variable, while let and const are block-scoped variables. Let allows for reassignment of values, while const does not.

3. What is the difference between null and undefined?

Null is an explicitly set value, while undefined is a value that is automatically assigned when a variable is declared but has not been assigned a value.

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

== is a comparison operator that compares two values for equality, while === is a strict equality operator that compares the values and their data types.

5. What are the data types in JavaScript?

The data types in JavaScript include number, string, boolean, null, undefined, object, and symbol.

6. What is hoisting?

Hoisting is a behavior in JavaScript where variables and functions are moved to the top of their scope, making them available for use before they are declared.

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

Synchronous code is executed in a single thread, where one operation must complete before the next operation can start. Asynchronous code can execute multiple operations simultaneously, allowing for faster and more efficient code.

8. What are closures in JavaScript?

Closures are functions that can access variables from their outer function, even after the outer function has completed.

9. What is the difference between call and apply?

Both call and apply are methods used to call a function with a specified object as the value of this. The difference is that call takes arguments as a comma-separated list, while apply takes arguments as an array.

10. What is the event loop in JavaScript?

The event loop is a mechanism in JavaScript that manages the execution of asynchronous code, allowing for non-blocking I/O operations.

11. What is the difference between the window and document objects?

The window object is the top-level object in the browser, representing the browser window, while the document object is a sub-object of the window object, representing the HTML document currently loaded in the window.

12. What is the difference between local and global scope?

Local scope refers to variables and functions that are defined within a function and can only be accessed within that function. Global scope refers to variables and functions that are defined outside of a function and can be accessed from anywhere in the code.

13. What is the difference between an object and an array?

An object is a collection of key-value pairs, while an array is an ordered collection of values.

14. What is the difference between map and forEach?

Both map and forEach are methods used to iterate over arrays. The difference is that map returns a new array with the results of the function applied to each element, while forEach only executes the function for each element.

15. What is the difference between splice and slice?

Splice is a method used to remove and add elements to an array, while slice is a method used to create a new array from a portion of an existing array.

16.What is JSON, and how can it be parsed in JavaScript?

JSON stands for JavaScript Object Notation, and it is a lightweight format for storing and exchanging data. JSON is often used for data transfer between client and server in web applications. It is based on a subset of JavaScript syntax and is easy to read and write. Yes, JSON can be parsed in JavaScript using the JSON.parse() method.

17. What is the purpose of the “use strict” directive in JavaScript?

The “use strict” directive is a way to enforce stricter parsing and error handling in JavaScript. It enables a stricter mode of operation that disallows certain features that are prone to errors, such as the use of undeclared variables, duplicate property names in objects, and assigning values to read-only properties.

18. What is a promise in JavaScript?

A promise is an object that represents the eventual completion or failure of an asynchronous operation and its resulting value. Promises enable writing asynchronous code that is more readable and easier to reason about.

19. What is the difference between promise and callback?

Callbacks are functions that are passed as arguments to other functions and are executed after a certain operation is completed. Promises, on the other hand, are objects that represent the eventual completion or failure of an asynchronous operation. Promises provide a more structured way of handling asynchronous operations and are generally easier to read and maintain than callbacks.

20. What is the difference between async and defer attributes in script tags?

The async and defer attributes in script tags are used to control the loading and execution of external JavaScript files. The defer attribute tells the browser to download the script file while continuing to parse the HTML document, then execute the script when the HTML document has finished parsing. The async attribute tells the browser to download and execute the script file asynchronously, without waiting for the HTML document to finish parsing.

21. What are some common methods used for manipulating the DOM?

Some common methods used for manipulating the DOM (Document Object Model) include getElementById, querySelector, createElement, appendChild, and innerHTML.

22. What is event bubbling, and how can it be prevented?

Event bubbling is a behavior in which an event that is triggered on an element is propagated up through its ancestors in the DOM tree. This can sometimes cause unexpected behavior if event handlers are defined on multiple elements in the DOM tree. To prevent event bubbling, you can use the event.stopPropagation() method.

23. What is the difference between get and post methods in HTTP?

The get and post methods are both used for HTTP requests, but they have different purposes. The get method is used to retrieve data from the server, while the post method is used to submit data to the server. Get requests are generally used for data retrieval, while post requests are used for submitting data to the server, such as form data.

24. What is a cookie, and how can it be created and read using JavaScript?

A cookie is a small text file that is stored on a user’s computer by a website. Cookies are often used for session management and user tracking. Cookies can be created and read using JavaScript by setting the document.cookie property to a string value containing the name, value, and expiration date of the cookie.

25. What is a closure, and how can it be used in JavaScript?

A closure is a function that has access to variables in its outer scope, even after the outer function has completed. Closures can be used for encapsulation and data privacy, allowing variables to be hidden from the global scope and accessed only through the closure function.

26. What is the difference between arrow functions and regular functions in JavaScript?

Arrow functions are a shorthand syntax for writing function expressions in JavaScript. They have a more concise syntax and do not have their own this keyword, instead inheriting the this keyword from their surrounding scope. Regular

functions, on the other hand, have a longer syntax and have their own this keyword, which can be useful in certain situations.

27. What is a higher-order function in JavaScript?

A higher-order function is a function that takes one or more functions as arguments or returns a function as its result. Higher-order functions are useful for creating abstractions and can make code more modular and reusable.

28. What is the difference between let and var in JavaScript?

The let and var keywords are used for variable declaration in JavaScript. The main difference between them is in their scoping behavior. Variables declared with var have function scope, while variables declared with let have block scope. This means that variables declared with var are accessible within the function in which they are declared, while variables declared with let are only accessible within the block in which they are declared.

29. What is a generator function in JavaScript?

A generator function is a special type of function in JavaScript that can be paused and resumed during its execution. Generator functions are defined using the function* syntax and use the yield keyword to pause their execution and return a value. They are useful for generating sequences of values on demand and can be used for asynchronous programming.

30. What is a web worker in JavaScript?

A web worker is a JavaScript script that runs in the background of a web page, independently of the main page thread. Web workers are useful for performing time-consuming operations, such as large data processing or complex calculations, without blocking the main page thread and causing the page to become unresponsive.

31. What is a closure, and how can it be used in JavaScript?

A closure is a function that has access to variables in its outer scope, even after the outer function has completed. Closures can be used for encapsulation and data privacy, allowing variables to be hidden from the global scope and accessed only through the closure function.

32. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code is executed in a single sequence, with each line of code executed one after the other. Asynchronous code, on the other hand, does not block the main thread of execution and allows other code to run while waiting for a response from an external source, such as a server or a file system. Asynchronous code is useful for creating responsive web applications and for handling time-consuming operations, such as data processing and file I/O.

33. What is the difference between null and undefined in JavaScript?

Null and undefined are both JavaScript values that represent the absence of a value. Null is an explicit value that represents a deliberate non-value, while undefined is a value that is used to represent the absence of a value that has not been defined. Undefined is also the default value for function arguments that have not been provided.

34. What is the difference between a function declaration and a function expression in JavaScript?

A function declaration is a statement that declares a function and assigns it to a variable or a property. A function expression, on the other hand, is an expression that creates a function and assigns it to a variable or a property. The main difference between them is in their hoisting behavior. Function declarations are hoisted to the top of their scope and can be called from anywhere in the scope, while function expressions are not hoisted and can only be called after they have been defined.

35. What is the difference between an object and an array in JavaScript?

An object is a collection of key-value pairs, where the keys are strings and the values can be any data type. An array, on the other hand, is an ordered collection of values, where the values can be any data type. Arrays are useful for storing and manipulating collections of data, while objects are useful for storing and accessing data with named keys.

36. What is the difference between a class and a function in JavaScript?

A class is a template for creating objects that share the same properties and methods. In JavaScript, classes are defined using the class keyword and can have a constructor method that is called when an object is created. Classes provide a way to create reusable code and are a key feature of object-oriented programming.

37. What is the prototype chain in JavaScript?

The prototype chain is a mechanism in JavaScript that allows objects to inherit properties and methods from other objects. Each object in JavaScript has a prototype, which is a reference to another object. When a property or method is accessed on an object, JavaScript first looks for it on the object itself. If it is not found, JavaScript looks for it on the object’s prototype. This process continues up the prototype chain until the property or method is found or until the end of the chain is reached.

38. What is the event loop in JavaScript?

The event loop is a mechanism in JavaScript that allows for non-blocking I/O operations and asynchronous programming. The event loop continuously checks for events in the event queue and processes them one by one, allowing JavaScript to handle multiple tasks at the same time without blocking the main thread of execution.

39. What is the difference between a for loop and a forEach loop in JavaScript?

A for loop is a traditional loop that iterates over an array or an object a specified number of times. A forEach loop, on the other hand, is a method on the Array object that iterates over each element in an array and applies a function to each element. The main difference between them is in their syntax and their use case. For loops are more flexible and can be used to iterate over any data structure, while forEach loops are more concise and are specifically designed for iterating over arrays.

40. What is a promise in JavaScript?

A promise is a special object in JavaScript that represents the eventual completion or failure of an asynchronous operation and its resulting value. Promises are used for asynchronous programming and provide a way to handle asynchronous operations in a more structured and organized way.

41. What is a callback function in JavaScript?

A callback function is a function that is passed as an argument to another function and is called when a certain event or condition occurs. Callback functions are commonly used in asynchronous programming, where they are passed as a parameter to a function that performs an asynchronous operation and is called when the operation is complete.

42. What is the difference between a shallow copy and a deep copy in JavaScript?

A shallow copy creates a new object that references the same memory as the original object. Any changes made to the copied object will also affect the original object. A deep copy, on the other hand, creates a new object that is completely separate from the original object and has its own memory. Changes made to the copied object will not affect the original object.

43. What is the difference between a primitive data type and a complex data type in JavaScript?

A primitive data type is a basic data type in JavaScript that is not an object and has no methods. The primitive data types in JavaScript are string, number, boolean, null, and undefined. Complex data types, on the other hand, are objects and have methods and properties. The complex data types in JavaScript are object, array, and function.

44. What is a constructor function in JavaScript?

A constructor function is a special type of function in JavaScript that is used to create objects with a specific set of properties and methods. Constructor functions are defined using the function keyword and are called with the new keyword. When a constructor function is called with the new keyword, a new object is created and the this keyword is bound to the new object.

45. What is the difference between a private and a public method in JavaScript?

A private method is a method that is only accessible within the scope of the object in

which it is defined. It cannot be accessed or called from outside the object. A public method, on the other hand, is a method that can be accessed and called from outside the object. Public methods are typically used to expose the functionality of an object to the outside world, while keeping the implementation details hidden.

46. What is a closure in JavaScript?

A closure is a feature in JavaScript that allows a function to access variables from its outer lexical scope, even after the outer function has returned. Closures are created when a function is defined inside another function and references variables from the outer function’s scope. This allows the inner function to “remember” the values of the outer variables, even if the outer function has already finished executing.

47. What is a generator function in JavaScript?

A generator function is a special type of function in JavaScript that can be paused and resumed during execution. Generator functions are defined using the function* syntax and use the yield keyword to pause execution and return a value to the calling code. When a generator function is called, it returns an iterator object that can be used to iterate over the values generated by the function.

48. What is the difference between == and === in JavaScript?

The == operator in JavaScript compares two values for equality, but it performs type coercion if the values have different types. This means that the values are converted to a common type before they are compared. The === operator, on the other hand, compares two values for equality without performing type coercion. It checks both the value and the type of the values to determine if they are equal.

49. What is the difference between let and var in JavaScript?

The let and var keywords are used to declare variables in JavaScript, but they have some important differences. The main difference is that let creates a block-scoped variable, while var creates a function-scoped variable. This means that a variable declared with let is only accessible within the block in which it is defined, while a variable declared with var is accessible within the entire function in which it is defined.

50. What is the difference between an arrow function and a regular function in JavaScript?

An arrow function is a new type of function introduced in ES6 that provides a more concise syntax for defining functions. Arrow functions are defined using the => operator and do not have their own this keyword, which means they inherit the this value from the surrounding context. Regular functions, on the other hand, have their own this keyword and are defined using the function keyword. They are more flexible and can be used in a wider range of contexts, but they require more verbose syntax.

FAQ

What are JavaScript questions asked in interview?

Here are some common JavaScript questions that you may be asked during a JavaScript interview:

1. What is JavaScript and what are its key features?
2. What is the difference between == and === in JavaScript?
3. What is the DOM and how does it relate to JavaScript?
4. How do you declare a variable in JavaScript?
5. What is hoisting in JavaScript and how does it work?
6. What is a closure in JavaScript and how does it work?
7. What is event bubbling and how does it work in JavaScript?
8. What is the difference between let and var in JavaScript?
9. What is a callback function in JavaScript and how does it work?
10. What is the difference between synchronous and asynchronous code in JavaScript?
11. What is the difference between null and undefined in JavaScript?
12. What is the difference between a for loop and a for…in loop in JavaScript?
13. What is the difference between a function declaration and a function expression in JavaScript?
14. What is a promise in JavaScript and how does it work?
15. What is the difference between map, filter, and reduce in JavaScript?

These are just a few examples of the types of questions that you may be asked during a JavaScript interview. It is important to have a strong understanding of the basics of JavaScript and to be able to communicate your ideas clearly and concisely. Remember to practice, be confident, and demonstrate your passion for JavaScript and programming.

How do I ace my JavaScript interview?

Acing a JavaScript interview requires preparation, confidence, and the ability to articulate your thoughts clearly. Here are some tips to help you ace your JavaScript interview:

1. Know your JavaScript: Review the basics of JavaScript and common interview questions. Make sure you understand concepts such as variables, functions, loops, and object-oriented programming.
2. Understand the company and its products: Research the company and its products or services. Understand their mission, values, and culture. This will help you tailor your responses to the interview questions and demonstrate your interest in the company.
3. Practice coding problems: Practice coding problems on websites like LeetCode, HackerRank, or CodeSignal. These websites provide a range of coding challenges that can help you hone your problem-solving skills and become more comfortable with JavaScript.
4. Be ready to discuss your experience: Prepare to discuss your experience with JavaScript, including any projects you have worked on and any frameworks or libraries you have used. Be prepared to give examples of how you solved problems and what you learned from your experiences.
5. Practice communicating your ideas: Practice answering interview questions out loud. This will help you develop clear, concise responses that demonstrate your knowledge and problem-solving skills. Use concrete examples from your past experiences to illustrate your points.
6. Be prepared to ask questions: Prepare a list of questions to ask the interviewer. Ask about the company’s culture, the role’s responsibilities, and the team’s dynamics. This demonstrates your interest in the position and can help you determine if the company is a good fit for you.
7. Stay calm and confident: Remember to take deep breaths, smile, and be yourself. It’s normal to feel nervous during an interview, but try to remain calm and confident. Remember that the interviewer is looking for someone who is not only skilled but also a good fit for the company culture.

By following these tips and preparing thoroughly, you can ace your JavaScript interview and demonstrate your skills and knowledge to the interviewer. Remember to be confident, show your passion for JavaScript and programming, and communicate clearly and concisely. Good luck!

How do you beat the 5 common interview challenges in JavaScript?

Interviews can be challenging, and there are some common challenges that you may face during a JavaScript interview. Here are some strategies for overcoming them:

1. Technical questions: Technical questions can be challenging, especially if you are not familiar with a specific concept or technology. To overcome this challenge, make sure to brush up on your JavaScript skills and review common interview questions. Practice coding problems on websites like LeetCode or HackerRank, and consider joining a coding challenge group to get feedback on your solutions.
2. Time pressure: Interviews often involve time pressure, with interviewers expecting you to provide answers quickly and concisely. To overcome this challenge, practice answering interview questions out loud, focusing on clarity and conciseness. Use concrete examples from your past experiences to illustrate your points.
3. Nerves and anxiety: Nerves and anxiety can be a major challenge during an interview. To overcome this challenge, try practicing relaxation techniques such as deep breathing or visualization. Remember to take breaks between interviews to stretch and reset your mind.
4. Lack of experience: Lack of experience can be a challenge if you are applying for a more advanced position. To overcome this challenge, emphasize your willingness to learn and your passion for JavaScript. Highlight any relevant experience you have and be open to discussing your potential to grow and learn on the job.
5. Communication skills: Communication skills are essential during an interview, as interviewers expect you to articulate your thoughts and ideas clearly. To overcome this challenge, practice communicating your ideas to others. Consider joining a public speaking group or practicing mock interviews with a friend.

By practicing and preparing for these challenges, you can overcome them and present your best self during a JavaScript interview. Remember to stay calm, be yourself, and demonstrate your passion for JavaScript and programming. Good luck!

What are advanced JavaScript interview questions ?

Advanced JavaScript interview questions are designed to assess your in-depth knowledge of the language and your ability to apply it to complex problems. Here are some examples of advanced JavaScript interview questions:

1. What is event delegation in JavaScript and how does it work?
2. Explain the difference between the call, apply, and bind methods in JavaScript.
3. What is the difference between a closure and a module in JavaScript?
4. What is a pure function in JavaScript and why is it important?
5. What is the event loop in JavaScript and how does it work?
6. What is the difference between a shallow copy and a deep copy in JavaScript?
7. What is a higher-order function in JavaScript and how is it used?
8. What are generators in JavaScript and how are they used?
9. What is the difference between a classical and prototypal inheritance in JavaScript?
10. What is a memoization in JavaScript and how is it used?
11. Explain the difference between async/await and promises in JavaScript.
12. What are the different types of design patterns in JavaScript and how are they used?
13. What is the difference between null, undefined, and undeclared in JavaScript?
14. What is the difference between a closure and a scope chain in JavaScript?
15. What are the benefits of using a framework like React or Angular over vanilla JavaScript?

These are just a few examples of the types of advanced JavaScript interview questions that you may encounter. It is important to have a solid understanding of the language, its advanced concepts, and how to apply them to solve complex problems. Be prepared to demonstrate your knowledge and skills with practical examples from your experience.

What is JavaScript best answer?

JavaScript is a high-level, dynamic, interpreted programming language that is widely used for web development. It is a versatile language that allows developers to create interactive and dynamic web pages, as well as build complex applications for both client-side and server-side programming. JavaScript is also known for its ease of use, as it is a beginner-friendly language with a simple syntax and many built-in features.

JavaScript can be used in conjunction with other web technologies such as HTML and CSS to create dynamic web pages and user interfaces. It can be used to add functionality to web pages, such as validating form input, creating animations, and manipulating the DOM (Document Object Model). JavaScript can also be used on the server-side with platforms such as Node.js to build scalable and high-performance applications.

Overall, JavaScript is a powerful and versatile language that is essential for modern web development. Its popularity has made it a valuable skill for developers and a core component of web development.

How Do I Prepare for a JavaScript Interview?

Preparing for a JavaScript interview can be daunting, but there are several steps you can take to increase your chances of success:

1. Review the job description: Read the job description carefully to understand the skills and experience required for the position. Make a list of the most important skills and knowledge areas, and focus your preparation on those topics.
2. Research the company: Do your research on the company and its products or services. Understand their mission, values, and culture. This will help you tailor your responses to the interview questions and demonstrate your interest in the company.
3. Practice coding: Practice coding problems on websites like LeetCode, HackerRank, or CodeSignal. These websites provide a range of coding challenges that can help you hone your problem-solving skills and become more comfortable with JavaScript.
4. Review JavaScript concepts: Brush up on your knowledge of fundamental JavaScript concepts such as data types, control structures, functions, objects, and prototypes. Make sure you understand how to work with common JavaScript libraries and frameworks.
5. Practice communicating your ideas: Practice answering interview questions out loud. This will help you develop clear, concise responses that demonstrate your knowledge and problem-solving skills. Use concrete examples from your past experiences to illustrate your points.
6. Be prepared to ask questions: Prepare a list of questions to ask the interviewer. Ask about the company’s culture, the role’s responsibilities, and the team’s dynamics. This demonstrates your interest in the position and can help you determine if the company is a good fit for you.
7. Stay calm and confident: Remember to take deep breaths, smile, and be yourself. It’s normal to feel nervous during an interview, but try to remain calm and confident. Remember that the interviewer is looking for someone who is not only skilled but also a good fit for the company culture.

By taking these steps, you can prepare yourself for a successful JavaScript interview. Remember to be yourself, be confident, and show your passion for JavaScript and programming. Good luck!

Which are the most important JavaScript interview questions for 2 years experience?

important JavaScript interview questions that may be asked for someone with 2 years of experience:

1. What is the difference between null and undefined in JavaScript?
2. How does prototypal inheritance work in JavaScript?
3. What are closures in JavaScript and how are they used?
4. How does the this keyword work in JavaScript?
5. What is event bubbling and event capturing in JavaScript?
6. What is the difference between synchronous and asynchronous code execution in JavaScript?
7. What is a callback function in JavaScript and how is it used?
8. What is the difference between var, let, and const in JavaScript?
9. How do you create a new object in JavaScript?
10. What is the difference between == and === in JavaScript?
11. What is the difference between a function declaration and a function expression in JavaScript?
12. How do you handle errors in JavaScript?
13. What is the difference between a for loop and a for…in loop in JavaScript?
14. How do you implement inheritance in JavaScript?
15. What are the different types of data types in JavaScript?
16. How do you debug JavaScript code?
17. What is the difference between a promise and a callback in JavaScript?
18. How does the typeof operator work in JavaScript?
19. How do you handle asynchronous code in JavaScript?
20. How does JSON.stringify() work in JavaScript?

It’s important to note that the specific questions asked in an interview may vary depending on the company and the interviewer. It’s always a good idea to research the company and review the job description to get an idea of the skills and experience they are looking for. Additionally, it’s important to have practical experience working with JavaScript and be able to demonstrate your knowledge through examples and projects.