Top Node.js Interview Questions to Ask Before Hiring a Node.js Developer

NodeJS emerged recently in the tech landscape as an effective JavaScript framework to develop lightning-speed server apps. With companies racing to design robust digital product solutions, NodeJS talent has never been more important.

There is however one thing, that you cannot use the same Node.JS interview questions for each candidate. Regardless of whether you’re going after a fresh-faced junior developer or a battle-worn senior engineer, your interview strategy needs to reflect that.

This will serve as a guide on how to perform effective NodeJS interviews. We handpicked some crucial Node.JS Interview questions and answers designed to drill down to the bottom of a candidate’s knowledge — to help you find the ideal person for your team.

Key Competency Areas for Node.js Developers

When evaluating candidates, focus on these crucial areas:-

Foundation Knowledge: Do they know how to navigate NodeJS Development core concepts and handle the asynchronous code like a pro?

API Expertise: Look at how they can craft and maintain strong REST services

Database Mastery: Test them on their SQL and NoSQL database handling

Problem-Solving: Review how they approach bugs and handle error cases

Speed & Scale: See how well they execute code fast and efficiently

Security Mindset: Make sure that they fully understand Node’s security fundamentals

Node.js Interview Questions by Experience Level

Junior-Level Node.js Developer Questions

Q1: What is Node.js and why is it beneficial to use it?

Expected Answer: Node.js is a cross-platform, open-source JavaScript runtime environment using Chrome V8 JavaScript engine. It is like an improved Swiss knife for developers where you can assure consistent code style for both the frontend and backend executing pure JavaScript code.

It works on the non-blocking JavaScript code, an event-driven programming model that makes the operations easy and fast.

Key Elements in Their Answer:

• They should describe how Node.js is the ideal JavaScript framework and effective data structure to use on the Web server.
• Search for references to Google’s V8 JavaScript engine – it is the heart of Node when it comes to performance.
• Pay attention to how to handle asynchronous operations, which is a key Benefit of Node.js.

Dig Deeper With:

• “Share an example where Node.js was a better choice than other backend and frontend frameworks.”
• “How will you describe the single-thread nature of Node.js and how it manages concurrency?”

Watch Out For:

• Unclear on async operations – this is a fundamental they should understand.
• If they are unable to provide real-life examples, they may not have real-world exposure.

Q2: What is npm and what is its role in Node.js programming?

Expected Answer: This is similar to the App Store, where we find applications for the iPhone; node package manager is the application store for Node.js.

It is like having a toolbox filled with thousands of tools, which means that you have an entire library of building blocks for faster and more efficient NodeJS development services.

Key Elements in Their Answer:

• Knows node package manager is one of the popular NodeJS packages
• Understands the basic usage of npm commands like a boss
• Recognizes how npm enhances the development process

Dig Deeper With:

• “How will you manage packages and update them to the latest NodeJS version with npm?”
• “How does package.json contribute to your projects?”

Watch Out For:

• If they are struggling with the most essential npm commands
• They may require practical training if they are confused about package.json.

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— NodeJS Trends (@NodejsTrends) November 18, 2024

Q3: How does synchronous and asynchronous programming work in Node js?

Expected Answer: Consider sync programming as a line at a coffee shop where one order has to be done before the next one begins. Async programming is a concept that is quite similar to a restaurant where multiple orders are being prepared at the same time.

Similarly, Node JS manages this through the callback function, promise, and through the use of the await and several asynchronous function calls. 

Key Elements in Their Answer:

• Good understanding of the concept of blocking and non-blocking code flows that helps to assure consistent code style.
• Knowledge of Node.js functions (callback functions, promises, utility functions, asynchronous programming with asynchronous functions & await).
• Why async is important for app performance

Dig Deeper With:

• ”Describe an example where the use of async programming in Node.js was beneficial.”
• “What strategies have you used to manage errors in async code, particularly with the callback function?”

Watch Out For:

• If they cannot distinguish between synchronous and asynchronous operations that is a common skill deficit.
• A lack of understanding of how to handle errors in the asynchronous flows can lead to problems later on.

Q4: Can you explain how an event loop works in the context of Node.js?

Expected Answer: The event loop is the heart of the Node JS – it governs how the async operation is managed through switching between the call stack and the callback queue.

Imagine it as an intelligent assistant that ensures that your app remains highly interactive and doesn’t hang up due to the backlogs.

Key Elements in Their Answer:

• Knowing how non-blocking I/O works in Node.
• Elaborates the relationship between call stack and callback queue
• Explains why the event loop is crucial for handling multiple requests

Dig Deeper With:

• “What is the role of the call stack in the event loop?”
• “How does the event loop handle the asynchronous operations?”

Watch Out For:

• Misconception of event loop = weak Node.js fundamentals
• Don’t know how to explain task management? They may have difficulties understanding practical examples

Mid-Level Node.js Developer Questions

Q1: Explain the difference between require and import in Node.js.

Expected Answer: Require is the old-school method of loading modules in Node, while import is the new-age ES6 way of loading modules that have the added benefits of being asynchronous and more efficient.

Now Node supports both, which allows developers to use the one they want.

Key Elements in Their Answer:

• Can they tell whether RequireJS is part of CommonJS while Import is part of ES6 or not?
• Do they know that require is sync while import plays async?
• Knowing which one to use when.

Dig Deeper With:

• “Are there any reasons to use import instead of require in your Node application?”
• “Do you know how to set up Node to use ES6 modules?”

Watch Out For:

• If they don’t know about how Node module systems work, then there is a fundamental gap.
• And if hazy on compatibility issues? May cause problems in real projects

Q2: What are middleware functions in Express.js, and how are they used?

Expected Answer: In the context of software development, middleware can be compared to security checks in an airport as they analyze, change, and regulate the traffic of both incoming requests and responses.

This first-class function can record activity, check user passes, or authenticate data before it gets to the intended recipient.

Key Elements in Their Answer:

• Understands how middleware affects the request/response process
• Can provide real-life examples such as authentication checks or logging
• Understands how to combine them in layers

Dig Deeper With:

• “Is it possible to add middleware to apply an authentication checkpoint with it?”
• “Can middleware functions be asynchronous and if so, then how are they handled?”

Watch Out For:

• Not able to provide use cases of middleware examples.
• Not sure what to do with the middleware flow? That is the fundamental difference between Express and other NodeJS frameworks.

Q3: Explain how you would approach error management in a Node application.

Expected Answer: Proper handling of operational and programmer errors is like having a safety net so you should use try/catch blocks, promise error catching with .catch() or use async/await and logging of the errors.

Express apps get a special error handling center, while process.on(‘uncaughtException’) is the final backup.

Key Elements in Their Answer:

• Good knowledge about how to handle an async error
• An understanding of the error handling middleware in NodeJS & Express
• Knowledge about Node’s global error catchers

Dig Deeper With:

• Why do we need to have centralized error handling in Express?
• “How can you monitor live applications for errors?”

Watch Out For:

• If they are confused about the async error handle, they might face difficulty in advanced Node.js
• If they don’t know about global error, they may not have practical experience.

Q4: What is clustering in Node.js, and when would you use it?

Expected Answer: Let’s consider Node as a one-man band – it can only perform one action at a time. Clustering means you can build a full orchestra, meaning you can start multiple instances(workers) to divide the workload.

This enhances the performance as it utilizes all those CPU cores and lets your app grow to unprecedented levels.

Key Elements in Their Answer:

• Good understanding of clustering in the context of multi-core scaling.
• Describes how the cluster module generates and oversees the worker processes.
• Understanding of frequent high-concurrency scenarios for clustering

Dig Deeper With:

• “What’s the difference between clustering and load balancing?”
• “What are the drawbacks of using the cluster module of Node.js?”

Watch Out For:

• They are unsure about the significance of clustering for performance.
• Do not know cluster modules.

Q5: How does Node.js handle file I/O, and what are the common Node Object methods for reading and writing files?

Expected Answer:  The (File System) fs module is the one-stop shop for JS file system operations in Node. For simple operations, you have async methods such as readFile() and writeFile() while for large files, you have streaming options such as createReadStream().

However, be cautious with the sync variants since they can freeze your event loop if you are not careful.

Key Elements in Their Answer:

• Understanding of the fs module and its sync/asynchronous function calls.
• Knowledge of how to use NodeJS streams when dealing with large files to save raw memory allocation and space
• Knowledge of the possible pitfalls related to synchronous file I/O

Dig Deeper With:

• “How are fs.readFile() and fs.createReadStream() different?”
•  ”How to address asynchronous file system errors?”

Watch Out For:

• There is a concern about managing large volumes of data if they lack streaming knowledge.
• Do not understand the difference between synchronous and asynchronous file I/O.

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Senior Node.js Developer Questions

Q1: How would you design a scalable Node.js application architecture?

Expected Answer: To build a enterprise-level NodeJS application, remember to do it in a modular way. Microservices and serverless functions enable scaling specific services, while load balancing and clustering enable the use of those multi-core CPUs.

Remember the NodeJS Performance Optimization techniques – caching, event queue, and API gateways are great for managing high loads.

Key Elements in Their Answer:

• Can distinguish when it is better to use a monolithic app and when it is better to use a microservices approach.
• Understands how clustering, load balancing, and serverless on NodeJS Development can be used for scaling up
• Integrates caching, queuing, and API gateway to enhance performance.

Dig Deeper With:

• ”What are the pros and cons of microservices architecture?”
• “What strategy do you propose for a NodeJS application in the cloud?”

Watch Out For:

• Does not know common scalability patterns
• There is no discussion of clustering, load balancing, or serverless

Q2: How do you monitor and optimize the performance of a Node.js application in production?

Expected Answer: To make a Node.js app work seamlessly in production, one has to have a proper monitoring and optimizing plan. There are tools like New Relic, Prometheus, and ELK stack that offer real-time metrics, error tracking, and logging.

As for the optimization part, use caching, code profiling, and efficient SQL queries. Never underestimate the value of load testing – it is the only way to reveal performance issues and streamline test-driven development with NodeJS to make sure your application will work fine under a heavy load.

Key Elements in Their Answer:

• Awareness of tools for instance New Relic, Datadog, or Prometheus
• Familiarity with various methods such as caching, code profiling, and query optimization.
• Understanding load testing as a key method of detecting and solving performance concerns

Dig Deeper With:

• “Which tools have you used to monitor Node.js applications in production?”
• “What methods do you use to detect and fix memory leaks in a Node.js application?”

Watch Out For:

• Inadequate exposure to real-time production monitoring applications
• There is no explanation of the necessity of load testing for performance tuning.

Q3: How does Node.js handle concurrency, and how would you manage high-load scenarios?

Expected Answer: Node.js uses a single thread with an event queue to handle multiple requests at a time; this is good and bad when it comes to concurrency. The event loop is what keeps the show on the road, but this is not always an effective approach for CPU-intensive operations.

This is where clustering, worker single thread, and offloading come into play – they allow you to split workloads and stay interactive even under load.

Key Elements in Their Answer:

• The concept of event loop and how Node.js applies asynchronous I/O for concurrency
• Awareness of how to utilize clustering and multiple threads to address issues with high concurrency
• Capability of recognizing and delegating computationally expensive operations to other services or workers

Dig Deeper With:

• “In your opinion, how does the single-thread model of Node.js impact concurrency?”
• “When would you prefer using worker threads over the cluster module?”

Watch Out For:

• Lack of understanding of concurrency management in Node.js
• No discussion on worker threads or offloading for purely computational tasks.

Q4: What is the role of process.nextTick() and setImmediate() in Node.js, and how do they differ?

Expected Answer: 

Process.nextTick() and setImmediate() are two of the most valuable functions in the Node.js event loop itself. While the former runs the callback function quickly after the current operation is done, the latter defers them to the next event loop cycle.

These first-class functions enable you to control flow function, precision, and priority of the operations within the event loop.

Key Elements in Their Answer:

• Knowledge of the event loop phases concerning nextTick and setImmediate
• Sharing real-life NodeJS examples for attached functions about when they should be applied
• Comparison of nextTick and setImmediate in terms of timing and priority

Dig Deeper With:

• “When should you ever use process.nextTick() over setImmediate()?”
• “How does the setTimeout() function relate to process.nextTick() and setImmediate() in terms of timing within the Node.js event loop?”

Watch Out For:

• Uncertainty concerning event loop timer and the purpose of these first-class functions
• There are no real-life samples to illustrate their application

Q5: Describe how you would implement secure coding practices in a Node.js application.

Expected Answer: To secure the Node.js app we need to consider the following measures: First, store the sensitive information such as API keys in environment variables to enhance security. Then, sanitize all user input to protect against injection attacks.

Use JWT tokens for authentication, always use HTTPS for communication, and set the CSP headers to prevent XSS attacks. Therefore, understanding security best practices is vital in developing secure Node.js applications.

Key Elements in Their Answer:

• Reference to the use of variables for storing sensitive information.
• Knowledge of input validation and sanitization to avoid injection attacks
• Familiarity with security standards such as HTTPS, JWT authentication, and CSP headers.

Dig Deeper With:

• “If you are developing a Node.js MVC app how would you prevent SQL injection?”
• “Which tools would you use to perform security scanning in Node.js?”

Watch Out For:

• There is no reference to the secure storage of confidential information
• Lack of knowledge about typical threats and countermeasures

Q6: Explain how you would implement a rate limiter in Node.js API functions to prevent abuse.

Expected Answer: It is vital to secure your Node.js APIs from abuse, and rate limiting is a great way to do that. Thus, if you log the requests of clients within a specific timeframe and set a certain rate for them, you will be able to avoid the abuse of previous API call functions and decrease the server load.

Express rate limiting or the use of Redis for a distributed solution can help in integrating rate limiting easily.

Key Elements in Their Answer:

• Awareness of the term rate limiting as a security and performance mechanism
• Knowledge of libraries or tools for rate limiting like express-rate-limit and Redis.
• Understanding of the situations when rate limiting should be applied, such as for public API functions.

Dig Deeper With:

• “How do you handle rate limiting in a clustered Node.js application?”
• “Why is Redis widely used in the implementation of distributed rate limiting?”

Watch Out For:

• Insufficient information on Rate-limiting tools and libraries
• There is no discussion of distributed rate-limiting strategies.

Chintan Gor

CTO, eSparkBiz

Enthusiastic for web app development, Chintan Gor has zeal in experimenting with his knowledge of Node, Angular & React in various aspects of development. He keeps on updating his technical know-how thus pinning his name among the topmost CTO's in India. His contribution is penned down by him through various technical blogs on trending tech. He is associated with eSparkBiz from the past 14+ years where one can get premium services.