Top 100 Node.js interview questions and answers in 2025

Below are the main benefits of using Node.js:

• The APIs in Node.js are asynchronous which means that they do not cause blockers. This happens due to the Node.js server not waiting for the return of data once an API has been called. It moves on to the next API. Instead, there is a notification mechanism that gets the response from the prior API call. Thus, we can also say that Node.js is event-driven.
• Node.js is powered by the Google Chrome V8 JavaScript Engine. This makes code execution in the Node.js library super fast.
• Node.js is based on a single-thread model with event looping. The event mechanism in the Node.js library makes it highly scalable because it prevents blockers on the server as compared to other servers that can only handle limited requests. In Node.js, the single thread program can service copious requests compared to something like Apache HTTP Server.
• Because of the event mechanism in Node.js, there is no buffering. Instead, the data output occurs in chunks.

/npm directory stores the globally installed dependencies. While these dependencies can be used in the Command Line Interface or CLI function of all Node.js libraries, they cannot be directly imported in the Node application with the require() command. If you want to globally install a dependency, you can use the -g flag.

REPL stands for Read Eval Print Loop. REPL performs tasks related to reading, executing, printing, and looping. The server contains several ad-hoc Java statements that REPL can help execute. With REPL, the JavaScript can enter the shell post directly and reliably perform tasks like debugging, testing, and experimenting.

Node.js is a JavaScript Runtime Environment that is useful in developing server-side applications.

Yes, Node.js is cross-platform and we can run it on Windows, Linux, Unix, and macOS.

It is different from other JavaScript environments because it is asynchronous and event-driven.

Node.js is open-source and cross-platform.

Node.js developers can build various applications, including web applications, chat applications, real-time applications, streaming applications, APIs, and desktop applications, etc.

Node.js is based on JavaScript and it uses the V8 engine developed by Google. It is used for building server-side applications.

Yes, it is possible to run Node.js on Windows.

I/O stands for input/output, which helps write and read files and network operations.

Node.js supports two categories of data type - primitive and non-primitive.

Asynchronous non-blocking and synchronous blocking are the two different types of API functions that Node.js supports.

Google uses V8 for Node.js because it is faster and more efficient. It compiles the JavaScript code directly into machine code.

.js extension is used to save Node.js files.

Npm stands for the Node package manager. It is used for installing, updating, and uninstalling packages in your application. It helps to manage dependencies in Node.js applications.

Node.js is not compatible with CPU-intensive applications.

Node red is a visual programming tool for Node.js that is used to wire hardware devices and online services as part of IoT applications.

An operational error occurs naturally and is part of the application flow, while programming errors are referred to as bugs that are caused by developers.

Unit testing in Node.js is a process of testing individual units of code.

Blocking code is code that cannot be executed until the current code is completely executed.

No technology comes without a few disadvantages. Node.js also has a few drawbacks. The main drawback is that Node.js responses can be greatly blocked if an intensive CPU computation is used.

In such cases, the multiple thread options are better but they are sluggish in performance. Moreover, if a relational database is used with Node.js, it behaves strangely, preventing favorable outcomes that the users can be sure of. Since Node.js does not support multiple threads, it is better suited for lightweight applications but not large-scale or heavy applications

Some examples of async functions are setTimeout(), setInterval(), and process.nextTick()

The V8 library provides Node.js with the JavaScript engine.

JavaScript is a programming language, whereas Node.js is an interpretation and environment for JavaScript. Node.js is used for performing non-blocking operations of any operating system. On the other hand, JavaScript is used for comprehensive application development.

It is fast, scalable, open-source, and asynchronous.

Yes, Node.js developers can develop a variety of applications, including network applications.

The tasks include - intensive CPU computation, I/O operations, GUI programming, and database operations can be done asynchronously with the help of an event loop.

The different types of security implementations within Node.js include error handling, authentications and authorization, data sanitization, encryption, and logging and monitoring.

Present in the root directory of a Node application/module, package.json defines the properties of a package including dependencies, metadata, and configuration options.

Node.js is single-threaded.

In JavaScript, a Promise object can have three states:

Pending: The initial state of the promise before it is resolved or rejected.

Fulfilled: The state of a promise representing a successful operation. This is also sometimes called "resolved."

Rejected: The state of a promise representing a failed operation. To create a Promise object, you must pass a function (often called an executor function) to the Promise constructor.
This function takes two arguments: resolve and reject. These are functions that you call to either fulfill or reject the promise.

One of the main applications where Node.js can be used is building real-time web applications. Apart from this, distributed systems for sub-programming collections can also use Node.js. It can be used in general applications as well as complex network applications. One can also use Node.js to create, read, write, or close server files.

Yes, all browsers support AJAX.

We use req.connection.remote address to get the IP address.

To install the body-parser module for Node.js, follow these steps:

• Open a command prompt or terminal window.
• Navigate to the directory where your Node.js project is located.
• Run the command “npm install body-parser” to install the body-parser module and add it to your project's dependencies.

Exit codes are a collection of specific codes whose function is to complete a specific process. Examples of exit codes are fatal error, unused, internal JavaScript evaluation failure, etc.

Several factors can cause server latency and prevent scalability in Node.js. Some of the most common ones include:

• Blocking I/O: Blocking I/O operations can cause the server to become unresponsive while waiting for I/O operations to complete. This can be especially problematic in Node.js, which is designed to handle many concurrent connections. To avoid this, Node.js provides non-blocking I/O operations.
• Inefficient code: Inefficient code can cause unnecessary processing and slow down the server. This can be caused by poor algorithmic choices, excessive use of synchronous operations, or inefficient data structures.
• Insufficient hardware resources: Insufficient hardware resources, such as CPU, memory, or network bandwidth, can cause the server to become overloaded and unresponsive. This can be especially problematic in high traffic scenarios.
• Improper configuration: Improperly configured servers can cause performance issues. This can be caused by incorrect network settings, improper load balancing, or other misconfigurations.

Node.js uses the Google V8 JavaScript engine to convert JS code to C++.

Event programming is programming paradigm that uses events to trigger actions. An event can be generated by the user, by the system, or by the program itself.

Ajax is a client-side technology used to make web pages more interactive and dynamic, while Node.js is a server-side technology used to build scalable, high-performance web applications.

Non-blocking in Node.js means that the program can continue to execute other code even while waiting for I/O operations to complete.

Dependencies are present in the package.json file.

A control function manages and manipulates the flow of asynchronous code execution.

Node.js is designed to handle asynchronous I/O operations, which means that multiple I/O operations can be executed simultaneously without blocking the execution of other code. However, managing the flow of asynchronous code can be challenging, especially when multiple operations need to be executed in a particular order.

Control functions provide a solution to this problem by allowing developers to define the order in which asynchronous operations should be executed. They can be used to perform tasks such as error handling, callback management, and flow control.

Modularization in Node.js provides scalability when developing complex applications. The modularization option can be used to execute the import of objects, classes, functions, modules, and external files.

The call-back function is used to execute a function after a certain event has occurred.

Because of the event mechanism in Node.js, a callback function is called every time an event starts. This prevents blocking in Node.js.

There are three layers in the application architecture - API, service, and integration layers.

Two input arguments for asynchronous queue are concurrency value and task function.

No. Node.js applications do not buffer data.

The Boolean data type in Node.js can have one of two values: true or false.

It is possible to run external processes with Node.js. This can be done with the help of the child_process module.

It is possible to avoid callback hells by using promises; they can help to make the code more readable and easier to debug. You can also avoid callback hells using async/wait, libraries, and modularization.

The fs module is used to create and manipulate files. It also provides an API for interacting with the file system.

The os module provides a set of tools for interacting with the operating system. It provides an API for getting information about the system including memory, processor, file system, and network interfaces.

Duplex streams are both readable and writable. This means that they can be used to read data from a source and write data to a destination.

A transform stream is a type of stream that can be used to modify or transform the data as it is being read or written.

There are a total of 18 Node object methods available. These methods can be used to create, manipulate, and delete objects.

HTTP status code 504 indicates that the server is unable to process the request. This can be due to several reasons, such as an overloaded server or a network issue.

Routing is a process where you associate HTTP request to a URL path or specific routes. When a request matches one of these routes, a corresponding handling function is used. Routing is a robust mechanism that enables you to define how an application handles incoming requests.

A file can be opened in Node.js using the fs.open() method. This method takes two arguments, the path of the file and the flags.

Angular is a front-end web application framework. Node.js is a back-end runtime environment.

Node.js prevents bottlenecks and aids programmers in easily writing the code because of the single-thread model. Internally, there are several POSIX threads for different I/O operations like File, DNS, etc.

So, when Node receives an I/O request, it uses one of these threads for the I/O operation. Once the operation is complete, the result joins the event queue. Because of the event mechanism, the event loop starts after each event, checks the queue, and if Node’s execution stack is free, then the loop adds the queue result to it, thus managing concurrency.

Ctrl + C shortcut is used for terminating processes in Node.js.

A Node-Inspector is a debugging tool that allows developers to inspect and debug the code of an application through a graphical user interface.

The Dgram is an in-built module. It is useful in implementing UDP datagram packets.

You can import the buffer class without the buffer module.

The emit() function is used to fire an event.

In Node.js, a callback function is a function that is passed as an argument to another function and is called back at a later point in time. The purpose of using a callback function is to handle asynchronous operations.

In asynchronous programming, a function that initiates an asynchronous operation returns immediately while the operation continues in the background. When the operation is completed, the callback function is called with the result of the operation.

Yes, the middleware functions can execute code and they can modify the request or response objects.

There are four types of streams in Node.js: readable, writable, duplex, and transform.

Readable: The readable streams are used to read data from a specific source.

Writable: These streams are utilized for writing data to the destination.

Duplex: The duplex streams are used for both reading and writing data.

Transform: The transform stream enable the data to be transformed while it is being read or written. For example, you can use transform stream for data compression or data encryption as it is transmitted over the network.

The difference between setImmediate() and setTimeout() is that setImmediate() will execute the callback function immediately, while setTimeout() will wait for a specified time before executing the callback function.

Task function and concurrency value are the two arguments async.queue takes.

It is a special data type that only takes one value, i.e., null.

The difference between an event and a callback is that an event is a mechanism that signals a change or action that represents a part of the program's behaviour, while a callback is a function that takes data and sends it back to the calling function.

To delete a directory, we use fs. rmdir() method.

Yes, it is private and used internally.

Error-first callbacks are used to pass data and identify if an error has occurred.

Global objects are universal objects that are present in every module or file of the application without requiring explicit import statements.

In Node.js, a child process occurs when a new process is created using the child_process module. This module provides functionality to spawn child processes in a similar way to the fork() system call in Unix.

No, the let keyword can’t be redeclared and should be declared before use.

A control flow function controls the order in which code is executed. Usually, the code runs from the first line to the last line unless the structures (loops and conditions) changes the control flow.

module.exports is a Node.js module that allows you to export objects, functions, and variables from your code. You can use it to create modules that can be imported and used in other Node.js programs.

Libuv is a library that provides an event loop and a thread pool for handling asynchronous operations seamlessly. It helps in handling file systems, child processes, files, DNS, etc.

The express framework is a web application framework that helps you easily create server-side applications.

It is used when you want to disperse request processing over multiple Node.js processors. Mostly this is done to ramp up the ability to handle more requests per second.

Node.js uses cryptography for a variety of purposes, including password hashing, public key authentication, and encrypting network traffic. Some of the crypto modules used in Node.js are crypto, tls, crypto.createHash(), and crypto.createCipher() among others.

The important features of the npm directory are:

• A package.json file that defines the dependencies for your project
• A Node_modules directory that contains the modules for your project
• An .npmignore file that defines which files should be ignored while publishing your module

You can include an HTTP server in your Node module by using the http module. The http module provides an API for creating and interacting with HTTP servers and clients. It allows you to handle incoming HTTP requests and send HTTP responses, as well as control the server's behavior, such as listening on a specific port and setting various options.

The nmp -Ig-command installs the specific package for your project globally through npm which is the Node.js package manager.

You can scale a Node.js application by using a cluster. A cluster is a pool of worker processes that share a single port.

The process of connecting the output of one stream to another is called piping.

Punycode is a way of encoding internationalized domain names that use characters outside of the ASCII range.

The purpose of EventEmitter is to provide a mechanism for event-driven programming. The EventEmitter class provides a set of methods that allow you to emit events, listen for events, and remove event listeners. When an event is emitted, any listeners that are registered for that event will be invoked. This allows developers to write code that can respond to changes in state, user actions, or other events within the application.

The child process module allows you to spawn child processes in Node.js. The child process module provides a way to create and control child processes. A child process is a separate instance of the Node.js process that runs concurrently with the parent process.

The child process module allows you to spawn new child processes, execute commands in a shell, and communicate with child processes using IPC (Inter-Process Communication) channels.

This method returns a character at a particular index in a string.

The Split() function splits the string and returns an array of strings.

The simple way to do this is with help of the request library -