Advanced JavaScript Interview Questions (with answers) That Matter

There are a hundred guides on JavaScript Interview Questions. They have 20,50,100 questions and they’re an excellent way to practice the basics.

We have only 10 advanced JavaScript interview questions.

The reasoning is quite simple. We’re a job board and we know that no one’s going to ask you the basics. The questions we list are questions that recruiters are actually asking in interviews.

These questions, in some shape or form, will make up a pretty big part of your JS interview. For each question, we’ve included three sections.

The first will tell you why the interviewer is asking you the question, the second section is an example answer, and the last section is an explanation of what makes that answer good.

If your experience does not match up exactly with our answer, refer to the third section to see what makes for a good answer. Then, just weave it in with your personal experiences.

10 Advanced JavaScript Interview Questions

How can closures lead to memory leaks in JavaScript and how can you avoid this issue?

Why is this question asked?

Understanding closures is fundamental to mastering JavaScript, and memory management is a key aspect of any programming language.

The interviewer is trying to test comprehension of the concept of closures, how JavaScript's garbage collection works, and how a misunderstanding or misuse of these concepts can lead to memory leaks - issues that persistently consume memory and cause applications to run slower over time or even crash.

Example answer:

Closures are a powerful feature of JavaScript, allowing a function to 'remember' its lexical scope even when it is executed outside that scope.

However, they can inadvertently lead to memory leaks if not handled properly. A common scenario where this happens is when closures hold on to large objects in memory, but those objects are no longer needed, yet due to the closure, they cannot be garbage collected.

Take, for example, a scenario where we have an object that contains a method that uses a closure. When this object is no longer needed, we'd expect that it can be garbage collected.

However, if there is a reference to the closure somewhere else in the application, the object can't be garbage collected, because, from the perspective of the garbage collector, the object is still 'reachable' via the closure.

This will lead to memory leaks, as memory that is not needed is not released.

We can prevent this by carefully managing our use of closures and diligently dereferencing objects that are no longer needed.

For example, if we are done with an object, we can manually null out the properties that are holding large amounts of memory. This will make the large objects unreachable and hence can be garbage collected.

Also, using tools to profile memory usage in your applications can help identify and fix memory leaks.

For instance, in the developer tools for Chrome, we have Profiles (or Memory in newer versions) that you can use to take a Heap snapshot. This will show you what objects are currently in memory, how much memory they're taking, and what references are keeping them there.

Why is this answer good?

• The answer accurately describes what closures are and how they can cause memory leaks in JavaScript applications, demonstrating a good understanding of JavaScript.

• The answer provides a practical example, making it easier to understand the concept and its implications.

• It suggests how to avoid memory leaks caused by closures, showing the candidate's ability to apply their knowledge to prevent real-world issues.

• The use of developer tools to monitor and fix memory leaks shows the candidate's awareness of the tools available and how to use them in debugging and performance optimization, which is critical for a senior developer role.

• The concise explanation and clear communication show that the candidate can make complex concepts understandable, a valuable skill in team and cross-functional work.

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Can you explain the JavaScript event loop and how it influences concurrency and the execution of asynchronous code?

Why is this question asked?

JavaScript is single-threaded, meaning it can execute only one task at a time. Understanding how it handles multiple tasks, therefore, is crucial to building efficient applications.

The event loop is the mechanism that allows JavaScript to perform non-blocking operations, despite being single-threaded. It's the secret sauce behind JavaScript's asynchronous nature.

Asking this question during an interview allows interviewers to gauge your understanding of this core JavaScript concept and its influence on asynchronous coding patterns.

A clear understanding of the event loop and concurrency model in JavaScript is essential for writing efficient and performant code, especially in Node.js and browser-based applications that heavily rely on I/O operations.

Example answer:

The event loop works in close conjunction with the call stack and the task queue. The call stack is where JavaScript executes functions from - it processes the top function in the stack, and subsequent ones wait their turn.

When an asynchronous operation occurs, such as a setTimeout or a promise, rather than blocking the stack and waiting for this operation to complete, JavaScript offloads these tasks.

For instance, a setTimeout function call is managed by the web API, which is provided by the browser. Once the delay is over, the corresponding callback is moved to the task queue. Here's where the event loop kicks in.

The primary job of the event loop is to continuously monitor the call stack and task queue. If the call stack is empty, it takes the first callback from the task queue and pushes it onto the call stack for execution.

This process allows JavaScript to handle concurrent operations efficiently without blocking the main thread, ensuring a smooth user experience, especially in scenarios involving UI updates and I/O operations. Understanding this is essential when dealing with promises, async/await, or any kind of callback operations.

Why is this answer good?

• The answer provides a detailed explanation of the event loop and its relationship with the call stack and the task queue

• The candidate correctly explains how JavaScript can perform non-blocking operations and handle concurrency, despite being single-threaded.

• The answer includes practical examples (like setTimeout) to illustrate the process, which helps make the complex concept more accessible.

• The response underscores the importance of understanding the event loop when working with asynchronous code in JavaScript, underlining the candidate's ability to connect core concepts with practical coding situations.

• The clarity of the explanation and the confident delivery suggest that the candidate can effectively communicate complex topics, an important trait for team collaboration and leadership roles.

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How does prototypical inheritance in JavaScript differ from classical inheritance, and what unique advantages does it offer?

Why is this question asked?

JavaScript's object model is based on prototypal inheritance, which sets it apart from languages that utilize classical inheritance, such as Java or C++.

This question helps interviewers understand your knowledge of JavaScript's object model and their ability to compare it with other paradigms.

Example answer:

In classical inheritance, we use classes to define blueprints for objects, and these classes are used to instantiate objects. Hierarchies and inheritance are achieved by defining subclasses that inherit properties and methods from a parent class.

On the contrary, JavaScript utilizes prototypal inheritance, a concept that many find liberating and more flexible compared to classical inheritance.

In JavaScript, we don't have classes (at least not in the traditional sense, ES6 classes are syntactical sugar), but we have objects. When one object needs to inherit properties and methods from another, it does so by referencing the other object as its prototype. Each object has an internal link to its prototype, from which it can inherit properties.

One of the main advantages of prototypal inheritance is its dynamism.

You can add or remove properties from the prototype at runtime and all objects that delegate to that prototype will immediately have access to the changes. This is something you can't do with classical inheritance, which requires the structure to be defined upfront in the class declaration.

Prototypal inheritance also offers a few unique features such as differential inheritance, where an object can directly inherit from another object, and multiple inheritance is simpler to achieve through a process known as "mixins".

So, while both classical and prototypal inheritance have their merits, JavaScript's prototypal inheritance model provides a flexible, dynamic mechanism for object construction and inheritance, which can result in more natural and easier-to-understand code when used correctly.

Why is this answer good?

• The answer succinctly explains the difference between classical and prototypal inheritance, demonstrating the candidate's understanding of different object models.

• The candidate provides concrete advantages of prototypal inheritance, showing their ability to not only understand these concepts but also to apply them in practice.

• The response gives an understanding of how prototypal inheritance contributes to JavaScript's flexibility, a crucial factor for advanced JavaScript development.

• The answer indicates an awareness of ES6 class syntax being a syntactic sugar, which displays a deeper understanding of JavaScript's inner workings.

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Explain how the 'this' keyword works in JavaScript, particularly in different contexts like object methods, simple function calls, constructors, and event handlers.

Why is this question asked?

The this keyword in JavaScript is a fundamental concept that any proficient JavaScript developer needs to understand.

However, its behavior can vary greatly depending on the context in which it's used.

The interviewer is trying to assess your understanding of the this keyword in various scenarios - whether in object methods, simple function calls, constructors, or event handlers.

Example answer:

The this keyword in JavaScript can be a bit tricky as it behaves differently based on the context in which it's used.

In object methods, this refers to the object that the method belongs to. If you define a method on an object and use this within that method, this will refer to the object itself.

In simple function calls, this doesn't refer to the object that the function belongs to. Instead, it's bound to the global object (window in a browser environment, global in Node.js).

However, in "strict mode", this will be undefined instead of the global object, as a way to prevent accidental modifications to the global environment.

In the context of constructors, this refers to the newly created instance when a constructor function is called with the new keyword. Essentially, this allows us to assign properties to the object being created.

Event handlers have a unique behavior with this. In event listeners, this is set to the HTML element that the event was attached to. This can be very useful for manipulating the DOM elements upon event triggers.

It's worth noting that the value of this can also be explicitly set with function methods like call, apply, and bind. These methods allow us to dictate what this should refer to, providing additional flexibility.

The behavior of this in JavaScript is complex and understanding its quirks in different contexts is vital to avoiding bugs and effectively leveraging the power of JavaScript.

Why is this answer good?

• The answer clearly explains how this works in different contexts, showing a solid understanding of this fundamental JavaScript concept.

• The candidate provides examples to illustrate how this behaves in each scenario, making the explanation more concrete and easier to understand.

• The mention of "strict mode" and methods like call, apply, and bind shows a deep understanding of JavaScript and its mechanisms to control the context of this.

• The response demonstrates an understanding of both browser-based and Node.js environments, indicating broad experience with JavaScript in different settings.

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Discuss the importance of immutable data structures in JavaScript and how it improves application performance, particularly in large-scale applications.

Why is this question asked?

JavaScript is a language often used in highly interactive web applications, and understanding how to manage data within these applications is critical.

The concept of immutability is particularly important in managing application state in large-scale JavaScript applications.

Immutability can lead to performance optimizations and make the application more predictable, easier to understand, and less prone to bugs.

The interviewer is trying to understand how well you understand these concepts.

Example answer:

Immutable data structures are crucial in JavaScript for a few key reasons. Immutability means once a variable is created, it can't be changed.

To modify it, we have to create a new variable. This might seem inefficient at first, but it comes with several benefits, particularly in large-scale applications.

One of the main benefits of immutability is that it leads to more predictable code. Since data isn't changed, we don't have to worry about unintended side effects altering the state of our application in unexpected ways.

This makes our application easier to reason about and debug, which is particularly beneficial in large-scale applications with many moving parts.

Moreover, immutability can improve performance.

For example, libraries like React can make use of immutability to optimize the re-rendering process. By comparing the new state with the old state, React can quickly identify whether a component needs to be re-rendered.

If the state is immutable and the current state is not a new instance, React knows that it doesn't need to re-render that component, as nothing has changed. This is much faster than checking every property of a mutable object for changes.

Immutable data structures also play a crucial role in functional programming in JavaScript, where immutability is a core concept. With immutable data, functions become pure, leading to more predictable and testable code.

While JavaScript doesn't enforce immutability by default, we can create immutable objects using methods like Object.freeze(). Moreover, there are libraries available, such as Immutable.js, which provide immutable versions of JavaScript's native data structures.

So, in essence, while managing immutable data might require a bit more effort initially, the benefits it provides in terms of predictability, simplicity, and performance optimization, particularly in large-scale applications, are worth the trade-off.

Why is this answer good?

• The answer provides a clear definition of what immutable data structures are, demonstrating a solid understanding of the concept.

• The candidate highlights key advantages of immutability, such as predictability, simpler state management, and performance optimization, showing their ability to apply theoretical knowledge in practical scenarios.

• The mention of specific libraries like React and Immutable.js indicates a familiarity with current best practices and tools in JavaScript development.

• The response ties immutability to broader programming concepts like functional programming, demonstrating the candidate's holistic understanding of JavaScript programming.

• The candidate communicates complex ideas in a clear, understandable manner, showcasing excellent communication skills, a valuable asset for any team environment.

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Can you explain the difference between promises and observables in JavaScript? In what scenarios would you prefer one over the other?

Why is this question asked?

Promises and observables are two powerful constructs used for handling asynchronous operations in JavaScript, but they operate quite differently and are better suited to different scenarios.

Understanding the differences between the two, as well as when to use each, is a sign of advanced JavaScript proficiency.

This question allows interviewers to see how much you know about asynchronous programming in JavaScript and their ability to choose the appropriate tool for a given situation.

Example answer:

A Promise represents a single eventual value from an asynchronous operation.

When a Promise is created, it kicks off the asynchronous operation immediately and is either resolved with a value or rejected with an error at some point in the future. Once a Promise is resolved or rejected, it cannot be reused — its state is immutable.

Observables, on the other hand, which are a part of the RxJS library, represent a stream of values over time, rather than a single eventual value. An Observable doesn’t start producing values until you subscribe to it.

This makes Observables lazy, whereas Promises are eager. Unlike Promises, Observables can be cancelled (unsubscribed from), can be retried, and can provide multiple values.

Choosing between Promises and Observables really depends on the specific needs of the situation.

For simple, one-time asynchronous operations, such as a single HTTP request, a Promise is usually sufficient. Promises have a simpler API and are built into JavaScript, so they can be used without any additional libraries.

For more complex asynchronous patterns, however, Observables are the better choice.

If you're dealing with a series of values over time — for example, user interface events, websocket messages, or even animations — the additional control and functionality that Observables provide can be invaluable.

Also, Observables provide powerful operators (like map, filter, reduce, retry, etc) which make complex asynchronous code more readable and easier to manage.

Why is this answer good?

• The candidate provides clear definitions of both Promises and Observables, demonstrating a solid understanding of these core JavaScript concepts.

• They correctly identify the key differences between the two, such as the single vs multiple values and eager vs lazy execution, showcasing a deep understanding of asynchronous programming in JavaScript.

• The answer explains when to use each construct, tying the theoretical knowledge to practical use cases.

• The mention of Observables' operators shows familiarity with the RxJS library and advanced asynchronous programming techniques.

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Describe hoisting in JavaScript. Does it apply to both variable and function declarations? What potential issues can arise from hoisting?

Why is this question asked?

Hoisting is a key concept in JavaScript that can often lead to confusion and unexpected results if not understood properly.

Understanding hoisting is crucial for writing predictable JavaScript code. This question is all about testing your knowledge of how JavaScript handles variable and function declarations, and their understanding of the potential pitfalls associated with hoisting.

Example answer:

Hoisting in JavaScript is a process during which the JavaScript interpreter, before executing the code, moves variable and function declarations to the top of their containing scope — which could be a function scope or the global scope.

This is why it's called "hoisting" - it's as if these declarations are being lifted or "hoisted" to the top of the code.

Now, it's essential to understand that hoisting applies to both variable declarations and function declarations, but with nuances. With variables declared using var, only the declaration is hoisted, not the initialization.

In simpler terms, if you declare and assign a value to a variable, JavaScript will hoist the declaration to the top, but the assignment stays where you've placed it in the code.

For function declarations, the entire function definition gets hoisted, not just the name. Therefore, you can call a function before its declaration in the code, and it will work perfectly fine because the entire function has been hoisted.

However, this concept does not apply to variables declared using let and const, or to function expressions and arrow functions stored in variables. They have a different behavior known as "Temporal Dead Zone", but that's a topic for another discussion.

While hoisting can be beneficial, it can also lead to confusing bugs if not properly understood.

For example, referencing a variable before its declaration will result in undefined rather than a reference error, which might lead to unexpected results. Similarly, if you aren't aware that function declarations are hoisted, you might accidentally invoke a function before you've logically declared it in your code.

To avoid such issues, one common recommendation is to always declare variables at the top of their scope and to declare functions before they're used. Alternatively, using let and const for variable declarations can help, as they have different scoping rules and are not hoisted in the same way as var.

Why is this answer good?

• The answer provides a comprehensive definition of hoisting, explaining the mechanics behind it in a clear and concise way.

• The candidate accurately describes how hoisting affects both variables and functions, demonstrating a deep understanding of the topic.

• The response highlights potential pitfalls associated with hoisting and offers practical advice on avoiding such issues, showing the candidate's ability to apply theoretical knowledge to real-world coding practices.

• The mention of "Temporal Dead Zone" and the different behavior of let and const demonstrates the candidate's advanced knowledge of JavaScript.

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Could you explain how Just-In-Time (JIT) Compilation works in modern JavaScript engines, such as V8 or SpiderMonkey, and the difference between it and traditional interpretation or ahead-of-time compilation?

Why is this question asked?

Understanding Just-In-Time (JIT) compilation provides insight into how modern JavaScript engines, like V8 and SpiderMonkey, execute code efficiently.

It's a complex topic that requires you to have a very good understanding of JavaScript execution.

This question is asked to evaluate your understanding of the inner workings of JavaScript engines, which is important for optimizing code performance and debugging complex issues.

Example answer:

Just-In-Time (JIT) Compilation is a fascinating technique employed by modern JavaScript engines like V8 (used in Chrome and Node.js) and SpiderMonkey (used in Firefox) to execute JavaScript code more efficiently.

In traditional interpretation, the JavaScript engine reads and executes the code line-by-line, which is simple but can be slow for larger scripts or repeated executions.

In contrast, Ahead-Of-Time (AOT) compilation, used by languages like C++, involves compiling the entire code into machine language before execution. This approach is generally faster at runtime, but it lacks flexibility and can be inefficient for scripts that have portions of code that never get executed.

JIT compilation in JavaScript engines strikes a balance between these two methods. It combines the flexibility of interpretation with the performance benefits of compilation.

Instead of compiling the entire code at once, a JIT compiler compiles the code "just in time" for it to be executed, hence the name.

When a script runs for the first time, it's initially interpreted line-by-line. But as the engine identifies "hot" parts of the code that are executed frequently, it compiles those parts into machine code for faster execution in the future.

Modern JavaScript engines like V8 take JIT compilation even further with techniques like inline caching and optimizing compilers.

For instance, V8 uses two compilers — a baseline compiler for quick and simple compilation, and an optimizing compiler that recompiles "hot" code, performing optimizations like inlining (replacing function calls with the function's content) and dead code elimination.

However, these optimizations come with their complexities.

For example, the optimizing compiler makes certain assumptions about the code. If those assumptions are invalidated later (a process known as deoptimization or "bailout"), V8 has to revert the optimized code back to the baseline version, which can impact performance.

Understanding these nuances can help us write more performant JavaScript code.

Why is this answer good?

• The answer provides a clear and concise explanation of JIT Compilation, demonstrating a deep understanding of JavaScript engines.

• The candidate draws a comprehensive comparison between traditional interpretation, AOT compilation, and JIT compilation, showcasing their broad knowledge of different code execution methods.

• The mention of advanced techniques like inline caching and deoptimization shows familiarity with optimization strategies in modern JavaScript engines.

• The candidate relates the understanding of JIT Compilation back to writing more performant JavaScript, emphasizing the practical implications of this knowledge.

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Tell me about the most challenging JavaScript project you have ever worked on. What made it challenging and how did you overcome those challenges?

Why is this question asked?

This question allows interviewers to understand the depth of your practical experience, your problem-solving skills, and your ability to handle complex tasks.

It's an opportunity for you to demonstrate how you apply their technical knowledge to real-world scenarios, how you deal with pressure, and how you collaborate with others to resolve challenges.

Example answer:

One of the most challenging projects I've worked on was an enterprise-scale, real-time collaboration tool built in JavaScript. This tool allowed multiple users to work on a shared document simultaneously, much like Google Docs.

The biggest challenge was managing the complexities of real-time, bidirectional communication between multiple clients and the server while ensuring data consistency.

We used WebSockets for real-time communication, and the main difficulty was dealing with concurrency issues and conflict resolution when multiple users made changes to the same part of the document at the same time.

Another challenge was the application's performance. As the number of users and the size of the documents grew, we started experiencing performance bottlenecks.

The application's responsiveness began to degrade, especially when handling large documents with many concurrent users.

To overcome these challenges, we had to deep-dive into advanced JavaScript concepts and use some smart engineering solutions.

To handle the concurrency issues and ensure data consistency, we implemented Operational Transformation (OT).

OT is a complex algorithm used in collaborative software to handle and resolve conflicts in real-time multi-user editing scenarios. It took a while to get it right, but this approach effectively solved our conflict resolution problem.

Addressing the performance issues required us to refactor significant portions of our codebase.

We started by profiling our application to identify the bottlenecks. We then optimized our JavaScript code by employing various techniques such as debouncing and throttling of events, implementing lazy loading, and making better use of JavaScript's asynchronous features.

We also worked on optimizing the server-side code and database queries.

This project was a real test of my problem-solving abilities and my understanding of JavaScript.

But, through a mix of technical skills, creativity, and perseverance, I was able to resolve the challenges and deliver a product that was able to scale efficiently and provide a seamless user experience.

Why is this answer good?

• The candidate provides a detailed example of a complex project, demonstrating the depth of their experience.

• They clearly explain the challenges they faced, showing their ability to identify and articulate problems.

• The candidate describes how they used advanced JavaScript concepts and practical engineering solutions to solve these challenges, demonstrating their technical expertise and problem-solving skills.

• The response illustrates their initiative and determination, traits that are valuable in any professional setting.

• The candidate reflects on what they learned from the project, showing their ability to learn and grow from experience.

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Can you share an example of a project where the use of an advanced JavaScript feature had a significant impact on the project's outcome?

Why is this question asked?

This question is designed to test your understanding of advanced JavaScript features and their ability to utilize them effectively in real-world applications.

The interviewer is essentially trying to know the depth of your knowledge.

Example answer:

Certainly. One of the projects that come to mind is a high-performance, data-intensive web application I was involved in developing. The application needed to handle large amounts of data in real time, presenting a considerable performance challenge.

Our initial implementation was sluggish, especially during peak usage times. The user experience was far from ideal, with a significant lag between user actions and updates on the UI. It was clear we needed to find a way to improve the performance of our application.

We decided to leverage JavaScript's asynchronous features — specifically Promises and the async/await syntax.

Prior to this, we were using traditional callback methods to handle asynchronous operations, which led to callback hell due to the nesting of callback functions.

By shifting to Promises and async/await, we were able to write cleaner, more readable code, which made it easier to reason about and debug.

More importantly, we could handle numerous asynchronous operations concurrently, allowing us to process large amounts of data without blocking the UI and significantly improving the application's responsiveness.

We supplemented this with the use of Web Workers, another advanced feature in JavaScript. Web Workers allowed us to run intensive JavaScript computations in a background thread separate from the main execution thread, thereby freeing up the main thread to remain responsive to user interactions.

The combined use of Promises, async/await, and Web Workers had a transformative effect on our application.

The user experience improved dramatically due to the reduction in latency, and the codebase became more manageable and easier to maintain.

Why is this answer good?

• The candidate presents a real-world scenario where advanced JavaScript features played a crucial role, demonstrating their ability to apply advanced concepts practically.

• The answer illustrates the candidate's problem-solving skills and their ability to leverage appropriate solutions to meet project needs.

• They detail the challenges faced in the project, their approach to overcoming them, and the project's positive outcome, demonstrating the ability to critically analyze and adapt to situations.

• The candidate shows knowledge of different JavaScript features, like Promises, async/await, and Web Workers, reinforcing their understanding of advanced JavaScript concepts.

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Conclusion

Front-end interviews are getting tougher, thanks to the sheer number of applications each job posting gets. This doesn’t mean that it’s impossible to find a great job, though.

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