Node.js Interview Prep Guide: Key Questions and Answers

Node.js has revolutionized server-side development with its non-blocking, event-driven architecture. It’s widely used in building scalable web applications, APIs, and real-time services. Whether you’re preparing for your first backend developer interview or looking to transition into full-stack development, having a solid grasp of commonly asked Node JS interview questions can give you the competitive edge.

In this blog, we’ll walk through some essential Node.js interview questions and answers that are frequently asked by top tech companies. These questions cover basic to advanced concepts and are designed to help you understand the "why" behind each topic—not just the "what."

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1. What is Node.js and how does it work?

Node.js is an open-source, cross-platform JavaScript runtime environment that runs on the V8 engine developed by Google. It allows developers to run JavaScript code outside the browser, typically on the server side.

How it works:
Node.js uses an event-driven, non-blocking I/O model which makes it lightweight and efficient. It uses a single-threaded event loop to handle multiple client requests without creating multiple threads.

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2. What is the difference between synchronous and asynchronous programming in Node.js?

• Synchronous: Tasks are executed one after another. Each task waits for the previous one to complete.

• Asynchronous: Tasks can be executed in parallel. It doesn’t block the execution and allows other tasks to run while waiting for I/O operations to complete.

Example of asynchronous code:

javascriptCopyEditfs.readFile('file.txt', (err, data) => {
  if (err) throw err;
  console.log(data.toString());
});

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3. What is the Event Loop in Node.js?

The Event Loop is at the heart of Node.js. It allows Node.js to perform non-blocking I/O operations by offloading tasks to the system kernel and continuing execution without waiting.

The event loop constantly checks the call stack and task queue. If the stack is empty, it pushes tasks from the queue to the stack for execution.

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4. What is the role of the package.json file?

The package.json file is a configuration file located in the root of a Node.js project. It contains metadata about the project like name, version, dependencies, scripts, and more.

Key benefits:

• Manages project dependencies

• Runs scripts (e.g., start, test, build)

• Shares project metadata when publishing to npm

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5. What is the difference between require() and import?

• require() is the CommonJS module syntax used in Node.js (ES5).

• import is the ES6 module syntax, now supported in newer versions of Node.js with type: "module" in package.json.

Example:

javascriptCopyEdit// CommonJS
const fs = require('fs');

// ES6
import fs from 'fs';

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6. What are middleware functions in Node.js (with Express)?

Middleware functions are functions that have access to the request (req), response (res), and the next middleware function in the application’s request-response cycle.

They’re used for:

• Logging

• Authentication

• Error handling

• Modifying requests or responses

Example:

javascriptCopyEditapp.use((req, res, next) => {
  console.log('Request received');
  next();
});

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7. How does Node.js handle concurrency with a single thread?

Node.js uses asynchronous callbacks, the event loop, and non-blocking I/O to handle multiple concurrent requests on a single thread. While traditional servers use multi-threading, Node.js relies on these patterns to ensure efficiency and scalability without blocking the main thread.

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8. What is the difference between process.nextTick() and setImmediate()?

Both are used to schedule callbacks in the event loop, but they operate at different phases.

• process.nextTick(): Executes the callback before the next event loop tick.

• setImmediate(): Executes the callback on the next iteration of the event loop.

Use nextTick for microtasks and setImmediate for macrotasks.

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9. What are streams in Node.js?

Streams are objects that let you read data from a source or write data to a destination in a continuous manner. They're useful for handling large files efficiently.

Types of streams:

• Readable

• Writable

• Duplex (both read and write)

• Transform (modify data while reading/writing)

Example:

javascriptCopyEditconst fs = require('fs');
const stream = fs.createReadStream('file.txt');
stream.on('data', chunk => console.log(chunk.toString()));

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10. How is error handling done in Node.js?

Error handling is typically done using:

• try-catch for synchronous code

• Callback error pattern: (err, result) => {} in async code

• Promises and async/await with try-catch

Example:

javascriptCopyEditasync function fetchData() {
  try {
    const data = await fetchSomeData();
    console.log(data);
  } catch (error) {
    console.error('Error:', error);
  }
}

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11. What is clustering in Node.js?

Node.js runs in a single thread but can handle clustering using the cluster module, which allows you to spawn child processes that share the same server port. This helps in utilizing multiple CPU cores.

Example:

javascriptCopyEditconst cluster = require('cluster');
if (cluster.isMaster) {
  cluster.fork();
} else {
  // worker logic
}

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Final Thoughts

Preparing for a backend or full-stack role? These Node JS interview questions are your go-to resource. They cover the most frequently tested topics that recruiters and interviewers expect candidates to know. Focus on understanding concepts like the event loop, async behavior, Express middleware, and error handling.

Whether you’re a junior developer learning the ropes or an experienced engineer brushing up before your next interview, revisiting these Node JS interview questions will help sharpen your problem-solving skills and boost your confidence.