Top 30 JavaScript Interview Questions You Must Prepare in 2020

JavaScript is the most popular programming language in software development. It is known among software engineers as ‘the language of the web’ and is essential for building and supporting any web-based project. JS used to be a front-end language – now, however, it is extensively used to support server-side operations as well.

Thanks to its wide range of applications, JavaScript boasts a wide job market. Currently, there are over 56,000 openings in the US alone. If you want to make a career as a JavaScript developer, you will not be short of opportunities, but the competition for these positions will be fierce. 

What to prepare for if an HR or a tech team lead wants to interview you? Here is an extensive list of sample interview questions – it will help you ace the talk and get a lucrative job in JavaScript development.

Top 30 JavaScript Interview Questions You Must Prepare in 2020

Top 30 JavaScript Interview Questions

Depending on who’s interviewing you, the level of depth and technical details of JavaScript interview questions will vary. HR managers usually stick to theoretical textbook questions. CTOs and fellow developers are likely to review your knowledge with practical, real-world based practice problems.

To make sure you’re ready for both, we grouped the most common Javascript interview questions into categories. There will be a short substantial answer next to each question.

JavaScript Theory Interview Questions

Question #1. What do you know about JavaScript?

JavaScript is an object-oriented language that allows tech teams to add interactivity to a static webpage. JS is interpreted and lightweight. The language is supported by most browser clients.

Question #2. What are some of the language’s core features?

JavaScript is designed to support network-based applications. It is based on Java and is integrated directly into the language. Following the original founding principle of Java – ‘write once, run everywhere’, JS is cross-platform.

Question #3. Which data types does JS support?

JavaScript supports seven data types, namely symbols, numbers, objects, classes,  strings, booleans, null, and undefined data.

Question #4. How is JavaScript different from Java?

JavaScript and Java are designed with different goals in mind. Java is a full-on programming language, designed to build software from scratch. JS, on the other hand, is a complementary language, closely intertwined with HTML. Although the former offers a couple of server-side applications, JavaScript is universally considered a client-side-scripting language.

Question #5. Is JavaScript case-sensitive?

Yes, it is. Developers should keep case-sensitivity in mind when working with keywords, functions, variables, and other data identifiers. It’s crucial to follow consistent capitalization rules.

Question #6. Is JS or ASP faster?

JavaScript is faster than ASP because it doesn’t need to interact with the webserver to run.

Question #7. Why would you recommend teams to use JavaScript?

The main advantages of the programming language are:

  • Low level of server interaction. JS enables an off-server user validation, reducing the server load. Thus, a team can count on considerable performance improvements after migrating to JavaScript. 
  • Rich interfaces. JavaScript gives front-end teams additional design tools – you can add sliders to the page, use drag-and-drop interfaces, and so on. 
  • Higher interactivity level. JS enables the creation of responsive interfaces that change when a user hovers over them with a mouse or clicks on a given element.

JavaScript Structure Interview Questions

Question #8. How to create a JavaScript object?

To create an object in JavaScript, developers use the object literal. The end code should look the following way:

var emp = {
user: "John",
age: 25

Question #9. What is an isNaN function in JavaScript?

The function is ‘true’ if the input value is not numeric.

Question #10. What is a negative infinity in JS?

The official definition of negative_infinity is a negative number divided by zero. In simpler terms, it refers to a number that’s lower than other numbers. The value equals that of the infinity property of a global object.

Question #11. How do you define a JavaScript name function?To declare a name function, you need to use the ‘named()’ function keyword. Here’s how declaring a name function looks in the real-world context:

function named(){
// insert your code

Question #12. How do I break a string statement into several lines?To separate a string statement in JavaScript, mark the first line of the code with a ‘\’. Here’s a broader example that will help you understand how JS code separation works:

document.write("Break the code \down into separate lines")

Question #13. What is the function of close() in JavaScript?
Close() is a method JS developers use to close a window they are currently working in. To specify which object the command is related to, a programmer needs to specify it as window.close().

Question #14. Why would a developer want to use ‘use strict’ before writing JavaScript code?
Use strict helps developers detect code errors more precisely. By applying it to the source code, development teams can improve the readability of the code at runtime.

To elaborate, mention that use strict helps avoid variable doubling and accidental globals, facilitates debugging, along with eliminating ‘this’ coercions.

Question #15. What are JavaScript global variables?

Global variables are those that don’t have a defined scope and apply throughout the entire code. By omitting a var variable when creating objects, a developer will create a global. Generally, developers are advised against using global variables since they complicate the debugging process.

Question #16. How is ‘this’ keyword used in JavaScript?

The keyword refers to a component it was used to define. There’s no strict meaning for this – when it comes to methods, the keyword will refer to the owner object. In a function, however, this will stand for a global object.

Question #17. How to use a prompt box in JS?

The prompt box facilitates entering text in JavaScript – essentially, it’s a dedicated field for typing in text symbols.

Question #18. Is there any difference between ViewState and SessionState?

Yes, there is. ViewState refers to a page used during the session. SessionState, on the other hand, refers to a defined data fragment that will be pinpointed through the entire web application.

Question #19. Explain the term ‘callback’

In JavaScript, callback refers to a function that’s transferred to a different argument as an option. The execution of the callback is tied to the execution of a preceding function, as the name suggests.

Question #20. State a few JavaScript naming conventions

Naming conventions are the rules applied to naming variables in the programming language. Here are some examples:

  • Don’t reuse keyword names for variable names;
  • Don’t start the name of the variable with a numeric value;
  • Pay attention to case sensitivity – Test and test are different variables in JS.

Question #21. How to create JavaScript cookies?

You can create cookies by assigning strings to document.cookie objects. Take a look at the example below:

document.cookie = “key1 = value1; expires = enterdate”

Question #22. What is a closure in JavaScript?

A closure is a function that can access variables in three different scopes. For one thing, it reaches the variables that are included in its scope. Also, it can access global variables, as well as those within the scope of the enclosing function.

Intermediate and Advanced JavaScript Interview Questions

In case you are interviewing for a position of a middle and senior Js developer, looking through advanced Javascript interview questions will be helpful before a job interview. We collected some advanced questions below:

Question #23. Tell the difference between a property and an attribute

Attributes display the details of an element. Properties, on the other hand, are values assigned to a data type.

Question #24. Which are the ways for HTML code to integrate with JS?

There are three ways to integrate JavaScript code into HTML – an external, internal, and inline one.

Question #25. == and === operators, what is the difference?The main difference between the operators is that == allows comparing variables of different types. If you try to compare a string and a numeric variable, === will, on the other hand, return ‘false’.

Question #26. How to clone an object in JavaScript?

To clone a JS object, use the objclone variable. The values of the object remain the same as these of an original object but point to a different object than obj. When cloning objects, keep in mind that objclone creates shallow copies – thus, nested objects will not be duplicated.

Question #27. What is the difference between ‘call and’ apply’?

Call is a method that calls the function without providing arguments as a single array. The following is the task accomplished by the apply method.

Question #28. What JavaScript characters are called the ‘escaped characters’?

These are the characters that allow developers to use special characters without entirely breaking the application code. Backlash is one of the escape characters that make special characters visible.

Question #29. What is the difference between let and const?

Let and const were introduced as a var replacement in ES6. Essentially, both are different ways to declare variables. Declaring a const variable requires using initializer, while a let once can be declared without one.

Question #30. Why should JS developers use a function book?

The technique is used among developers to prevent name clashes and create a closure around file contents, along with a private namespace for each class and library.


Compiling a definite list of JavaScript-related interview questions is close to impossible since they rely heavily on the type of project and assignments a developer will have to work on. However, familiarizing yourself with sample questions will make it easier to understand which skills and coding competencies are expected from all tiers of JS developers.

Take your time to practice real-world coding scenarios, along with scanning through theoretical definitions since HRs are likely to bring them up.

With enough perseverance and work, you will be able to successfully pass a job interview and become a professional JavaScript developer.

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