40+ Node.js Frequently Asked Interview Questions [2023]

Node.js is a crucial part of the tech stack of many big companies, such as PayPal, Trello, Walmart, and NASA. 

According to ZipRecruiter,  a Node.js engineer in the US earns an average of $116,495 a year or approximately $56.01 an hour. 

Interview prep is important if you are already a skilled Node JS engineer looking for a new role. 

The following are some of the most common Node.js questions you will likely encounter in your next interview. 

What is Node.js?

Node.js is a lightweight but powerful framework used for creating server-side web applications. This cross-platform JavaScript runtime environment is built on Google’s V8 JavaScript engine. Its design is similar to that of Ruby’s Event Machine and Python’s Twisted. 

Why use Node.js?

• Allows developers to use JavaScript as a backend language. Node.js allows users to run JavaScript code outside of a browser, and developers can use JavaScript on both the front end and back end of an application. 
• Node.js is Lightweight. Through its asynchronous event-driven I/O model, Node.js ensures that no node directly handles I/O, making it efficient and lightweight. 
• Simplifies developers’ work through Node Package Manager (NPM). Developers don’t have to create everything from scratch, as NPM has a collection of small open-source software modules you can download and use during development.

Why is Node.js single-threaded?

Node.js runs JavaScript code as a single thread. It thus means that it executes this JavaScript code at a time. However, through the libuv library, Node.js runs various threads that handle various tasks, such as reading files from a disk and network requests. 

How does Node.js handle concurrency?

Concurrency is a situation whereby an application handles multiple tasks simultaneously. Node.js uses event loops where asynchronous code is pushed to the end of the event queue. Even though JavaScript is single-threaded, the event loop allows Node.js to perform non-blocking I/O operations by offloading I/O tasks to the system kernel. 

What is NPM?

It is the world’s largest software registry, with over 800,000 code packages. Node Package Manager is automatically installed when you install Node.js. 

What is an event loop?

Despite JavaScript being single-threaded, an event loop allows Node.js to perform non-blocking I/O operations. Whenever possible, Node.js offloads operations to the system kernel. 

What are event emitters?

These are objects in Node.js that trigger an event after sending a message that a certain action has been completed. JavaScript devs can thus write code that listens to events from an emitter. 

What is WASI?

WebAssembly System Interface (WASI) API implements the WebAssembly System Interface. WASI uses POSIX-like functions to give sandboxed web applications access to the underlying operating system. 

What is REPL?

Read-Eval-Print-Loop (REPL), in Node.js,  is an interactive shell that processes Node.js expressions. The easy-to-use command line captures code inputs from a user, interprets, prints out the results, and loops it until the user signals an exit. 

What is the control flow?

It is how we control the logic and flow of code in our applications. If statements can control the flow of the code to a specific direction based on the state of variables and outputs from functions. 

What is callback hell?

Also known as the pyramid of doom, callback hell is a situation whereby multiple callbacks are nested within a function. Callback hell is associated with asynchronous programming and makes the code hard to understand and maintain. 

What is middleware in Node.js?

Middleware is a function that has all access for responding to an object, requesting an object, and moving to the next middleware in the app’s request-response cycle. Middleware acts as a connection between the applications, data, and users. 

What are Node.js streams?

Streams are a collection of data comparable with arrays and strings. However, these streams don’t have to fit in memory and might be available simultaneously. 

What is piping?

Piping is the process of connecting multiple streams. For instance, you can pipe the read and write streams to enable the transfer of data from one file to another. 

What are Node.js buffers?

Buffer modules offer a way to handle binary data streams in a Node.js setting. In Node.js, Buffer is a global object; you don’t have to import using the ‘require’ keyword. 

You can create an empty Buffer of the length of 15 using this syntax;

var buf = Buffer.alloc(15);

What is event-driven programming?

Before we learn what event-driven programming is, we must understand these concepts;

• Event-message, and notification
• Producer-event source, publisher, and sender
• Consumer-event sink, subscriber, receiver, and handler
• Message queue-event queue

Event-driven programming is a concept where code is written to respond to events. For instance, we can have an automated system where sensors detect when a certain temperature is reached. 

Such systems will have event handlers that respond to events and ensure the appropriate action is taken. 

What is a test pyramid?

The test pyramid shows the proportion of unit tests, integration tests, and end-to-end tests needed to develop an application fully. The test pyramid aids in developing a robust testing suite and determines the frequency and order of assessments. 

What is an error-first callback?

Error-first callback pass errors and data. An error object is the first thing you pass to such functions and the second thing is the associated data. Error-first callback allows you to pass an error object, check if anything is wrong and then handle it. If no issue is detected, you go ahead with subsequent arguments. 

What are LTS releases?

Long-term support (LTS) is a software release that gets extended support from the developers. The extended support can be 2-5 years, unlike regular programs, which only get 6 months to 1-year support. 

What is a URL module?

A URL module will split up (parse) a web address into readable parts. You need to include the require() method to use the URL module. 

What is libuv?

Libuv is a cross-platform library written in C programming language, and it provides an asynchronous, event-driven programming style. Libuv provides an event loop and callback-based notifications of I/O. 

What are stubs?

Test stubs are spies/ functions with pre-programmed behavior. Stubs can wrap up existing functions or can be anonymous. 

You can use stubs when;

• Some tests are failing due to network connectivity errors and not because of code errors.
• You want to control a method’s behavior and force the code down a specific path. 

What is package.json?

Package.json is the heart of a Node.js project. This file records important metadata about a project which is needed before publishing to NPM. Package.json defines a project’s functional attributes that NPM uses to run scripts, install dependencies and identify the entry point to the package. 

What is the .json file package?

JSON is the short form for JavaScript Object Notation. It is a lightweight format for transporting and storing data and is often used when data is sent from a web server to a web page. 

A package.json file is written in JSON format. 

What is the Express.js package?

Express.js is a Node.js framework with robust features for creating mobile and web applications. 

You can install this minimalistic web framework using this command;

npm install express --save

Mention some frameworks used in Node.js.

• Express.js
• Koa.js
• Meteor.js
• Socket.io
• Nest.js
• Sails.js
• Total.js
• Hapi.js

Mention some timing features of Node.js

• setTimeout/clearTimeout (): It is a function that schedules code to execute after the assigned time (timed in milliseconds)
• process.nextTick(): It is a function that schedules a callback function which is required to be invoked in the event loop’s next iteration
• clearImmediate/setImmediate(): Code is executed after the existing event loop cycle
• setInterval/clearInterval(): This function is used to execute a block of code several times

Mention some commonly used libraries in Node.js

• Lodash
• Async
• Express.js
• Karma 
• Axios
• Grunt
• Molecular
• Cloudinary
• Mocha 
• PM2

What command is used to import external libraries?

“require” command. For instance, “var http=require (“HTTP”)” will load the HTTP library. 

What are callbacks?

These are functions that are called after a certain task is complete. Such a setting allows the program to run another piece of code and prevent blocking. 

What are the modules?

Modules are reusable JavaScript codes that help developers adhere to the DRY (Don’t Repeat Yourself) programming principle. Such modules also aid in breaking down complex logic into small, simple, and manageable chunks. 

How would you define the term I/O?

I/O is the short form for input/output. It refers to the interaction of the program with the system’s network and disk. Talking to databases, reading/writing data from/to a disk, and making HTTP requests are examples of I/O operations. 

Which database is commonly used with Node.js?

MongoDB

Describe Node.js exit codes

Exit codes are a specific group of codes that finish off processes. 

Some of the most common Node.js exit codes are;

• Handler Run-time failure
• Internal JavaScript Evaluation Failure
• Uncaught fatal exception
• Fatal Error
• Internal Exception
• Unused

What is a reactor pattern in Node.js?

The reactor pattern in Node.js is used to avoid blocking I/O operations. A reactor pattern has a handler associated with Input/Output operations.

A reactor pattern follows this pattern;

• Application makes requests
• Event demultiplexer gathers these requests and forms Event Queues 
• Libuv, an asynchronous IO library, runs the event demultiplexer.

Explain asynchronous and non-blocking APIs in Node.js

When a message is sent through the asynchronous architecture. The server stores the information, and it will be notified when the task is completed. 

Non-blocking APIs respond immediately with whatever data is available. However, it does not block executions but keeps running as per the requests. 

Explain the V8 engine in Node.Js

V8 JavaScript was developed for Chromium and Google Chrome web browsers to improve JavaScript execution on web browsers. Instead of using an interpreter, the V8 translates JS code to more efficient code to achieve faster JavaScript execution speeds. 

Differentiate between spawn() and fork()

• Spawn() creates a new function and launches it using the command line. This function creates a node module on the processor, after which, Node.js invokes it when the child processes return data. 
• Fork() is an instance of an already existing spawn() method. The spawning process ensures that there will always be more than one active node that handles tasks at any given time. 

Difference between synchronous and asynchronous functions

In a synchronous function, code executes in a particular sequence based on the instructions given by the program. 

In an asynchronous function, code can execute in parallel. The next operation can occur in such a setting even if another process is being processed. 

How to manage packages in Node.js projects?

Various package installers come with configurations to manage packages in Node.js projects. Most of these packages use npm or yarn. Luckily, both provide access to almost all JavaScript libraries with extended functionalities. 

Explain the control flow function 

This is a piece of code that runs between various asynchronous function calls.

Conclusion

We have covered most of the questions you will likely encounter in a Node.js interview. However, you must also ensure that you possess Node.js technical skills. Good luck as you prepare for the next interview.

You may also explore some NodeJS frameworks to speed up web and API development.