Top Basic and Advanced Node.js Interview Questions

Node.js interview questions are essential for preparing for a software developer interview. Node.js is used by top companies such as Netflix, LinkedIn, NASA, Trello, PayPal, Walmart, Uber, Twitter, Yahoo, eBay, and GoDaddy because programming in the Node.js environment is faster, easier, and reduces development time and overall time-to-market.

The demand for Node.js programmers is increasing at an alarming rate. The Node.js interview questions listed below will familiarize you with the types of questions you may face during your interview. In this article, we covered both basic and advanced Node.js interview questions. If you practice these Node.js interview questions, you will undoubtedly ace your next software developer interview.

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This article will look at the Node.js interview questions that you may expect during software developer interviews. 

Here's what we'll cover:

• Basic Node.js Interview Questions and Answers
• Advanced Node.js Interview Questions
• Node.js Programming Interview Questions
• Node.js Interview Questions for Practice
• FAQs on Node.js Interview Questions

Basic Node.js Interview Questions and Answers

Some employers may use a combination of these basic Node.js interview questions depending on the job type. So, ensure that you know the important topics ahead of time.

Q1. What is Node.js? Where can you use Node.js? 

This is one of the most frequently asked Node.js interview questions. Node.js is a popular web application framework popular for server-side scripting to build scalable programs. It is built on Google Chrome's V8 JavaScript Engine. Nide.js comes under MIT license and is free to use. It extends JavaScript API to offer usual server-side functionalities. It is generally for the following purposes:

• Large-scale application development, including video streaming sites, single-page applications, and other real-time web applications.
• It is ideal for data-intensive applications.
• Network applications
• General-purpose applications
• Distributed systems

Q2. What makes Node.js better than other frameworks?

You must consider the following factors while answering such Node.js interview questions:

• No buffering of data as it outputs the data in chunks.
• Node.js offers simple development because of its non-blocking I/O. Unlike other frameworks where software developers use thread management, Node.js has an event-based model that provides short response time and concurrent processing.  
• Node.js runs on a chrome v8 engine written in C++. It offers high performance with constant improvement.  
• The use of JS in both the front-end and backend facilitates faster development.  
• Node.js has ample libraries, and you do not need to reinvent the wheel. Moreover, Node.js is built on Google Chrome's V8 JavaScript Engine, making its library very fast in code execution.
• Its event mechanism makes it highly scalable and helps the server to respond in a non-blocking way.
• Its extensive open source community produces excellent modules that add additional capabilities to Node.js applications.

Q3. Is Node.js single-threaded?

Node.js employs a single-threaded model that supports async processing. Async processing enables the application to perform better and makes it more scalable under web loads. Thus, Node.js has a single-threaded model with event looping.

Q4. What is an event loop?

In Node.js, an event loop is responsible for all the asynchronous callbacks in an application. It is an endless loop that awaits tasks, executes them, and sleeps till it receives the next tasks. 

It executes tasks from the event queue only when there is no ongoing task, i.e., the call stack is empty. It is the event loop that allows Node.js to have non-blocking I/O. You must know that the event loop allows you to use callbacks and promises.

Q5. What is Callback in Node.js? 

A function that is called after a given task is a callback function. It allows other code to run meanwhile and prevents any blocking. Node.js heavily relies on callback as it is an asynchronous platform. All Node APIs are written to support callbacks.

6. What is Callback hell? How can you avoid them?

A problematic phenomenon for a JavaScript developer when he tries to execute multiple asynchronous operations consecutively is termed callback hell. An asynchronous function is one that requires some external activity completion before processing a result. These asynchronous functions require a callback function to tackle errors and process the result.

Q7. What are Promises in Node.js? 

A promise in Node.js is a placeholder for a value that will be available in the future. Promises in Node.js allow asynchronous methods to return values like synchronous methods wherein the asynchronous method returns a promise for the value in the future instead of the final value. They have separate callbacks that would be executed for success/ failure and handling timeouts. Promises fall in the following three different states:

• The initial or the pending state. 
• The fulfilled state represents a successful operation.
• The rejected state of a promise represents a failed operation.

Q8. Why would you prefer promises over callbacks?

You must answer such Node.js interview questions with a relevant yet precise reason explaining why you would choose one over the other. The reasons for selecting promises over callbacks are as follows:

• Promises easily handle multiple asynchronous operations providing better error handling than callbacks and helping in avoiding the undesired callback hell situation. 
• Built-in error handling
• The coupling is low.
• Improved readability

Q9. What are the types of API functions?

The API functions in Node.js are of two types, namely:

• Asynchronous, Non-blocking functions
• Synchronous, Blocking functions.

Blocking functions execute synchronously. Every other code is blocked from running until an awaiting I/O event occurs.

Non-blocking functions execute asynchronously, i.e., multiple I/O calls can be performed without the program's execution being halted. 

Q10. What is REPL? Why is it used?

REPL is the abbreviation for Reading Eval Print Loop. It represents a computer environment similar to a Windows console or Unix/Linux shell wherein a command is entered, and the system reverts with an output. It's an easy-to-use command-line tool for processing Node.js expressions.

You can also elaborate on REPL while answering this Node.js interview question. 

• Read: It reads the input, resolves it into JavaScript data structure, and stores it in memory.
• Eval: It accepts and evaluates the data structure.
• Print: It prints the result.
• Loop: It loops the command, i.e., it repeats the above process till you press ctrl-c twice to terminate.

These were the most commonly asked Node.js interview questions. Scroll through the following section for some advanced Node.js interview questions for your tech interview prep. 

Advanced Node.js Interview Questions

Here are a few advanced Node.js interview questions and answers for senior positions. If you're interviewing for a position as a senior software developer, practice these questions to ace the interview.

Q1. What is a buffer class in Node.js?

A buffer class holds raw data just like an array of integers but corresponds to a raw memory allocation outside the V8 heap. These aren't resizable and have several methods specifically for binary data. Pure JavaScript is not compatible with binary data, so a buffer class comes into play. 

Q2. What is piping in Node.js?

This is one of the most common Node.js interview questions to gauge your knowledge of vital topics in Node.js. Piping in Node.js is the mechanism that connects the output of one stream to another, i.e., it connects multiple streams. It retrieves data from one stream and passes the output to another stream.

Q3. What do you understand about node.js streams?

Node.js streams are instances of EventEmitter. You can use them to work with streaming data in Node.js, especially for handling and manipulating streaming large files (such as videos and mp3) over the network. Streams use buffers for temporary storage. There are four main types of streams. 

• Writable streams to write data (e.g. fs.createWriteStream())
• Readable streams to read data (e.g. fs.createReadStream())
• Duplex streams that are readable and writable (e.g. net.Socket)
• Transform streams or duplex streams can modify or transform the data as it is written and read (e.g., zlib.createDeflate())

Q4. What are the modules in Node.js? Mention the different modules in Node.js?

Modules in Node.js applications are similar to JavaScript libraries. They include a set of functions. You have to use the require() function (the parentheses contain the module's name) to add a module in a Node.js application. The following modules provide the basic functionality to a web application: 

• HTTP module includes methods, classes, and events to create an HTTP server in Node.js.
• The URL module contains the methods for URL parsing.
• Util module incorporates utility functions
• Fs module has events, classes, and methods to handle the file I/O operations
• Stream module includes the methods for streaming data
• The Zlib module has the methods to compress or decompress the files used in an application.
• The query string module consists of the methods to work with a query string.

Q5. What is middleware?

A middleware function has access to the response object, request object, and the next middleware function in the application's request-response cycle. A variable named next denotes the next middleware function. Middleware finds usage in between your request and business logic. You can also write your own middleware for a specific use case. The primary tasks that the middleware functions perform are:

• Code execution
• Modify the request and the response objects
• Complete the request-response cycle
• It can invoke the next middleware function in the stack.

Q6. What is a Reactor Pattern in Node.js?

A reactor pattern is a pattern for non-blocking I/O operations generally used in any event-driven architecture. It has two components: 

• A reactor that dispatches the I/O event to appropriate handlers
• Handler actually works on those events

Q7. Why does Google use the V8 engine for Node.js?

This is one of the most crucial Node.js interview questions that an interviewer asks to gauge your understanding of the system design and preferences. Google's v8 is the most evolved and fastest JavaScript and Web assembly engine. It is open-source and thus has a huge community helping develop features and bug-fixing. The V8 engine is portable to almost every machine.

Q8. What are the exit codes of Node.js?

Exit codes of Node.js provide an idea of the reason behind code termination. Some exit codes are as follows: 

• Uncaught fatal exception - (code - 1) when there was an unhandled exception.
• Unused - (code - 2) is reserved by bash.
• Fatal error - (code - 5) when there has been an error in V8.
• Internal exception handler run-time failure (code - 7) when there has been an exception when the bootstrapping function was called.

Q9. How would you enhance Node.js performance through clustering?

Node.js applications do not use multiple core systems. Instead, they run on a single processor. In Node.js, the cluster mode helps startup multiple node.js processes, thus having multiple instances of the event loop. 

When you use a cluster in a Node.js app behind the scenes, there are numerous node.js processes created, but there is also a parent process called the cluster manager. A cluster manager monitors the health of the individual instances of the application.

Q10. What is WASI? What is its significance?

WASI stands for the WebAssembly System Interface. It is a capability-oriented set of APIs built to standardize the sandboxed execution of WebAssembly modules outside of browsers. 

WASI was introduced with the idea in mind that it is possible to use the underlying operating system via a collection of POSIX-like functions and further enable the application to use resources more efficiently and features requiring system-level access.

Node.js Coding Interview Questions

You must practice the following Node.js coding interview questions to gear up for your next tech interview.

1. What steps will you follow to connect a MongoDB database to Node.js?
2. How would you measure the duration of async operations?
3. How would you measure the performance of async operations?
4. What tools will you use to ensure consistent code style?
5. How would you make a post request in Node.js?
6. How would you load HTML in Node.js?
7. Write a code to create an HTTP server in Node.js. 

Sample Node.js Interview Questions for Practice

Here are some sample Node.js technical interview questions for you to practice. Preparing answers to these questions can help you ace your tech interview.

1. What is Package.json?
2.  What is an Event Emitter in Node.js?
3. How is the process.nextTick() different from setImmediate()?
4. What is the significance of module.exports?
5. What is NODE_ENV?
6. Differentiate between fork() and spawn() methods in Node.js.
7. What is a test pyramid?
8. What challenges have you faced working with Node.js, and how did you overcome them?
9. How would you deal with Node.js maintenance problems?
10. What are RESTful Web Services in Node.js?
11. How would you use JSON Web Token (JWT) for authentication in Node.js?
12. What is the difference between Angular and Node.js?
13. How is Node.js different from JS?
14. How are worker threads different from clusters?

This completes our list of important Node.js interview questions for interview preparation. You must check out domain-specific preparation strategies and questions on the interview questions page.

FAQs on Node.js Interview Questions

Q1. Does Netflix include Node.