50 advanced JavaScript interview questions to ask senior programmers

Have you received a job requisition to hire a senior JavaScript programmer for your team and are unsure how to identify the perfect candidate?

If you have sourced suitable talent with a precise JavaScript developer job description, you must follow a multi-step approach to hiring a skilled developer from your applicant pool.

Programming skills tests will ensure you select proficient candidates to interview by helping you compare the best of your bunch.

All you need to do is send applicants a skills assessment for JavaScript developers. Then you can conduct technical JavaScript and behavioral interviews to learn more about your candidates’ skills, technical knowledge, and experience.

For entry-level positions, check out our list of basic JavaScript interview questions. But if you need advanced JavaScript interview questions to interview senior developers, read on.

Use the 50 questions in this article to determine which of your candidates are the most qualified.

24 advanced JavaScript interview questions about technical definitions

With these 24 advanced JavaScript interview questions, you can learn more about your candidates’ specialized experience. Ask your interviewee some of these thought-provoking JavaScript questions to evaluate their knowledge.

1. Please tell me what equality means in JavaScript.

2. Can you explain what IIFEs refer to in JavaScript?

3. Please explain what the bind() function does in JavaScript.

4. Could you tell us the difference between const and Object.freeze() in JavaScript?

5. Please explain what generators are in JavaScript.

6. Can you explain what hoisting is in JavaScript?

7. Please explain what the prototype design pattern does in JavaScript.

8. Could you tell me what the temporal dead zone is in ES6?

9. What is the primary difference between map() and forEach()?

10. Please explain the difference between null and undefined variables.

11. Can you tell me the difference between WeakMap and ES6 maps?

12. Please explain whether JavaScript is a pass-by-value or pass-by-reference language.

13. Please tell me your best advanced JavaScript soft skill.

14. Please tell us your best advanced technical JavaScript skill.

15. Could you tell us what closures do in JavaScript?

16. Can you tell us what the close() function does in JavaScript?

17. Please tell us what imports are in JavaScript.

18. Please tell us what exports are in JavaScript.

19. Can you explain the use of strict JavaScript directives?

20. Please explain what typecasting means in JavaScript.

21. Please tell us the main primitive data types in JavaScript.

22. Could you tell us what you know about the different pop-up boxes in JavaScript?

23. Can you explain what event bubbling means in JavaScript?

24. Please explain the key differences between the ES5 function and ES6 class constructors.

5 sample answers to advanced JavaScript interview questions about technical definitions

You can assess your candidates’ responses by referring to the following sample answers to essential advanced JavaScript interview questions about technical definitions.

1. Please tell me your best advanced JavaScript soft skill.

Assessing soft skills in interviews, such as communication, interpersonal, and problem-solving, is crucial because it will show you if your candidates can work effectively in teams to solve technical code issues with testers’ support.

Responses to this advanced JavaScript question will also reveal if your applicants’ verbal and written skills match those in your job requisition. 

Candidates may discuss their technical writing skills in JavaScript, which help them share technical project-related information with non-technical stakeholders. They may also explain how they use their written communication skills to produce clear, concise, and well-organized documentation for their code.

Alternatively, when responding to this question, they could mention how their time management skills enable them to efficiently manage their time while working on projects. They might use the following techniques to achieve this:

• Use timebox methods

• Plan with project management software 

• Set up reminders in task management tools

Soft skills are a must-have for developers, so evaluating them with soft-skills tests like our Project Management test is essential.

2. Please tell us your best advanced technical JavaScript skill.

Technical JavaScript skills, such as debugging, knowledge of security practices for writing code, and code reviewing skills, are indispensable for JavaScript developers, helping them create bug-free, secure applications. 

Your interviewees might mention several other technical skills, such as programming object-oriented applications, writing cross-browser code, working with databases, and using client-side libraries. But it’s equally important that candidates know the following benefits of these skills:

• Facilitating code modularity

• Ensuring web pages are compatible with multiple browsers

• Helping developers design flawless applications

No two candidates will mention the same skills. However, debugging is particularly important for developers. Debugging sessions can last more than 100 minutes, so it’s ideal if applicants have debugging abilities alongside their other technical skills.

To find applicants whose technical JavaScript skills align with your job description, use aptitude tests such as our JavaScript (Coding): Debugging test.

3. Please tell us the main primitive data types in JavaScript.

Responses to this question should mention that JavaScript has many value types that take different forms, like immutable values. The perfect candidates for your role will know that primitive values are not objects and have no properties or methods.

Look out for candidates who can name the following primitive data types:

• Number

• Boolean

• String

• Symbol

• Null

• Undefined

More importantly, see whether your applicants can explain what these primitive values mean and provide an example of each one.

For instance, Booleans are data types that can only be true or false. Null values represent the intentional absence of object values, and symbols represent unique identifiers.

Test candidates’ knowledge further by asking them about projects in which they’ve used these data types and why they were important.

4. Please explain what generators are in JavaScript.

A generator is a process that yields multiple values in JavaScript and works with a generator function to define an infinite data structure. The function returns iterable generator objects, which are objects that have a Symbol.iterator function. The best responses will explain that generators enable developers to write individual functions.

If candidates can also mention three advantages of JavaScript generators, they might be the ideal hire you’ve been looking for. Some advantages they might mention include the following:

• The ability to generate infinite value sequences

• The opportunity to create asynchronous code

• A way to reduce memory usage

• An option for iterating values one at a time

These benefits help JavaScript developers write code that is more efficient and easier to read. With generators, they can also simplify complex code and produce modular syntax that can be tested more easily.

5. Can you explain what hoisting is in JavaScript?

Candidates should understand that hoisting is a JavaScript behavior that enables developers to move function and variable declarations to the top of their scopes.

This process occurs during compilation and before execution. If developers didn’t have this method, they would have to declare functions in a specific order. 

Expert answers will be more detailed and explain that developers can use variables and functions before declaring them with this method. They will also know the drawbacks of hoisting, including the possibility of unintentionally introducing bugs into an application. This issue is more likely to occur if the developer doesn’t understand how to use hoisting methods.

If applicants also know that hoisting moves the declaration, not the assignments, which can lead to unexpected behavior, they may be able to help your team handle application errors efficiently.

16 advanced JavaScript interview questions about processes

These 16 advanced JavaScript interview questions will help you determine whether your applicants can complete crucial JavaScript processes. Ask them some of these questions to review their approaches.

1. Could you tell us when you should use arrow functions in ES6?

2. Can you name one typical case where you would use anonymous functions?

3. Could you tell us two features of generators and when we should use them?

4. Please tell me how you would check if a variable is null or undefined.

5. Please explain how you would deep freeze an object in JavaScript.

6. Can you explain how you would empty a JavaScript array?

7. Please tell us what you know about the document loading procedure in JavaScript.

8. Could you explain how to eliminate value duplicates in an array?

9. Please explain how you would temporarily remove content from a site using JavaScript.

10. Can you tell us how you would import all file exports as objects?

11. How would you dynamically add properties to JavaScript objects?

12. How would you dynamically delete properties from JavaScript objects?

13. Please explain how you would change the page title in JavaScript.

14. How does attention to detail help you debug JavaScript code?

15. How do problem-solving skills help you troubleshoot JavaScript coding issues?

16. How would you reduce the loading time of a web page that uses JavaScript?

5 sample answers to advanced JavaScript interview questions about processes

If you’re looking for an easy way to check which candidates in your applicant pool have knowledge of processes, the sample answers below will help. Compare candidates’ responses with the sample answers in this section after the interview.

1. Could you tell us two features of generators and when we should use them?

Expert candidates who have used JavaScript generators should be able to describe two generator features or functions. They should know that JavaScript generators enable developers to:

• Pause and resume function execution whenever they want with the yield keyword, which facilitates the function-writing process and ensures they generate values over time.

• Create iterator objects that make sequence iteration and working with large datasets easier. This also makes implementing custom iteration behavior simpler for developers.

Candidates should also be aware of the situations in which developers should use JavaScript generators. For example, generators are ideal if developers need to create asynchronous code, manage the code control flow, build complex state machines, or implement the lazy evaluation method. 

Generators help programmers define infinite data structures for implementing simpler algorithms and create faster and more responsive programs for end users.

2. How does attention to detail help you debug JavaScript code?

In response to this question, your interviewees shouldn’t just mention that attention to detail helps developers spot subtle errors in code. They should understand that it also enables them to examine variable names, syntax, and function calls to ensure the code works as intended.

Candidates should also explain that attentiveness makes it more likely that programmers will notice key details in error messages, identify ways to enhance code performance, and troubleshoot issues. These actions will optimize the codebase and the application as a whole. 

This ability also helps JavaScript developers adhere to best practices, such as writing modular code, following established conventions, and using descriptive variables.

Therefore, use an Attention to Detail skills test to determine if applicants can carefully scrutinize code during the debugging process. The test will help you learn if your candidates would be a real asset to your team.

3. How do problem-solving skills help you troubleshoot JavaScript coding issues?

The US Bureau of Labor Statistics emphasizes that alongside being detail-oriented, software developers require advanced problem-solving skills.

Abilities like analytical thinking, research skills, and debugging code through trial and error help them analyze and identify the root causes of code issues and fix user experience problems by breaking down complex code issues into manageable parts.

Candidates should also be able to share some situations in which their problem-solving skills have helped them troubleshoot coding issues. For instance, they may have used this ability to track down bugs in mobile gaming applications or discover software architectural problems.

Since the average annual cost of software bugs is $59.5bn, hiring a professional with the problem-solving skills to find these coding errors is critical. Therefore, it’s important to select the perfect developer by evaluating candidates with a data-first Problem Solving skills test.

4. Please explain how you would temporarily remove content from a site using JavaScript.

Developers can hide specific content on a web page by using the JavaScript style attribute to change an HTML element’s display CSS property to none. This approach might be useful if a client wants to hide certain content temporarily.

Another technique applicants may describe is using the remove() method to remove a specific element from the Document Object Model (DOM). They can also use the removeChild() method to remove a child element from the parent element, enabling them to target specific parts of the page.

Candidates may also mention that if they need to use jQuery to remove content, they can use several methods, including remove(), empty(), and detach().

5. Could you explain how you would eliminate value duplicates in an array?

JavaScript developers can eliminate value duplicates in an array using several methods. Doing so is crucial for improving the code’s performance and enhancing the user experience.

The first option they may consider is using Set, an inbuilt object that stores unique values in arrays. It works with the set() constructor function, which creates an instance of unique values.

Alternatively, developers might implement the filter() method to achieve the same goal. For example, if they want to create a new array that features unique values, they could use the following code snippet:

const arr = [1, 1, 2, 3, 3, 4];

const uniqueArray = arr.filter((value, index) => arr.indexOf(value) === index);

console.log(uniqueArray); // Output [1, 2, 3, 4]

10 advanced JavaScript interview questions about technical and soft skills

If you need to find out more about your applicants’ technical and soft skills related to JavaScript, these advanced JavaScript interview questions about skills are essential. Find the perfect candidate by using these questions to assess their knowledge.

1. Why is a basic understanding of JavaScript programming concepts important? 

2. Please explain why debugging skills are essential for JavaScript programmers.

3. Please tell us why asynchronous programming is a key skill for JavaScript programmers.

4. Why do JavaScript developers need React skills and knowledge?

5. Why do JavaScript developers need Angular skills and knowledge?

6. Please explain why version control is critical for JavaScript developers.

7. Can you explain why testing knowledge is fundamental for JavaScript engineers?

8. Could you explain why data structure skills are vital for JavaScript engineers?

9. Please explain why communication is crucial for JavaScript engineers.

10. Name a few examples of ideal time management strategies for completing JavaScript projects.

5 sample answers to advanced JavaScript interview questions about technical and soft skills

The simplest way to evaluate your candidates’ technical and soft skills is to refer to the sample answers to these advanced JavaScript interview questions. Find a top professional by reviewing their knowledge against these answers.

1. Why do JavaScript developers need Angular skills and knowledge?

JavaScript developers use the Angular framework to build dynamic, responsive interfaces for users. They also use it to manage the state of an application and satisfy the client and company’s expectations.

Angular skills are particularly important if JavaScript developers need to work on large-scale projects because the framework’s modular architecture enables them to break their code into reusable components.

Research shows that Angular’s popularity as a framework has declined – although 15% of developers used it in 2021, only 7.2% of developers used it in 2022. Yet, it remains one of the most widely used frameworks for front-end development, single-page applications, and mobile applications.

Therefore, it’s important to assess your candidates’ Angular skills by asking them follow-up questions, such as:

• Name a project in which you used the Angular framework

• Explain the advantages of the Angular framework

• What projects are best suited to Angular?

But before interviewing candidates, make sure you include an Angular skills test in your applicant-screening process. This data-driven method will help you review candidates’ Angular knowledge objectively.

2. Please explain why version control is critical for JavaScript developers.

Version control offers many benefits for JavaScript development. It helps developers monitor and check the history of code changes and revert to or restore a previous version if they have introduced a bug in the code. It can also offer an efficient fix when the codebase has compatibility issues with third-party libraries.

In addition, if issues such as unoptimized code or inefficient algorithms cause performance issues that make the program run slowly, the developer might revert to a previous version to restore the application’s speed.

More than 100 million developers use GitHub, so it’s clear that version control tools are popular across the globe thanks to their ability to help developers complete projects more efficiently. Therefore, you can expect candidates to be aware of these tools. However, the best applicants will mention projects in which they’ve used them.

To ensure your candidates are a good match for your team, give them a Git skills test before the interview and compare their interview responses with their results.

3. Could you explain why data structure skills are vital for JavaScript engineers?

JavaScript engineers require data structure skills to complete specific responsibilities like managing, sorting, and storing data.

Applicants with data structure knowledge will know that they can enhance an application’s performance by minimizing the complexity of data structures. For example, some candidates might understand that using binary search trees instead of arrays can improve the application’s performance in certain situations.

Candidates who are familiar with a wide range of data structures, such as linked lists, stacks, or queues, may be able to optimize your application’s performance and help keep your organization’s clients satisfied.

Make sure you test your applicants’ understanding of data structures with our JavaScript data structures skills test to hire the perfect candidate for your team.

4. Why is a basic understanding of JavaScript programming concepts important?

A basic grasp of JavaScript programming concepts can help your candidates write efficient code because it enables them to understand the key features of the language, such as the following:

• Variables

• JavaScript functions

• Specific data types

• Loops

• Conditional statements

• Control flow

These concepts form the fundamental building blocks of websites and web applications written in JavaScript. Applicants who do not understand them will likely encounter challenges when writing even simple programs in JavaScript.

Your interviewees should also know that familiarity with JavaScript programming concepts can help with debugging. For instance, if developers fully understand how JavaScript variables work, they will know to look for incorrect variable assignments.

5. Why do JavaScript developers need React skills and knowledge?

React skills help candidates solve many difficulties, including debugging issues, slow server-side rendering, and poor application responsiveness. Therefore, applicants with this skill will be able to deal with common JavaScript coding challenges more efficiently. 

However, there are many other advantages of the React library, which 42.6% of developers use. Here are just a few of its top benefits:

• Provides reusable component features that facilitate the creation of complex applications

• Offers a virtual DOM that reduces the need to update the actual DOM directly and minimizes rendering time

• Features a large ecosystem of tools that assists developers with testing and integrating React with other technologies

React skills are clearly essential for JavaScript developers. Ensure you assess them with a React skills test to narrow down your candidate pool and find a professional who can use this library efficiently.

For which candidates and roles should you use advanced JavaScript interview questions?

Advanced JavaScript interview questions are best suited for senior-level developers. These questions are ideal if you must dig deeper into your candidates’ advanced knowledge and ensure they match your senior-level role perfectly.

You should also use them for any applicant you expect to have hands-on experience with JavaScript programming to learn more about their technical JavaScript skills, soft skills like communication and time management, and work style.

Assess developers in your candidate pool with advanced JavaScript interview questions

You’re almost prepared to start evaluating your candidates using these advanced JavaScript interview questions, but there’s just one thing you should do before you begin.

Explore our website to learn more about skills tests, particularly our JavaScript skills tests. TestGorilla is a data-driven solution that’s already helped 9,200 companies replace resumes with a precise hiring method and find the perfect talent for their teams.

Register for a free demo to find out more about our platform. You can also learn about the soft-skills, programming, and cognitive ability tests we offer by checking out our skills test library.

Finding the perfect developers among a large applicant pool is simpler than you think. Use advanced JavaScript interview questions and skills tests to discover a proficient candidate for your team.