Node.Js Interview Questions

Node.js is an open-source runtime environment that allows JavaScript to run on the server side, enabling the development of fast and scalable applications. In interviews, you may encounter questions about Node.js fundamentals, architecture, and application development processes.

1. What is Node.js and Why Use It?

Node.js is a runtime built on Chrome’s V8 JavaScript engine that uses an event-driven, asynchronous, non-blocking I/O model. It enables building fast and scalable network applications using JavaScript on the server side.

Common use cases include:

• Real-time applications: chat apps, live notifications
• API services: RESTful and GraphQL APIs
• Single Page Applications (SPA): dynamic, fast-loading web apps

Tip: Node.js’s non-blocking architecture offers performance benefits for high-traffic apps but should be used cautiously for CPU-intensive tasks.

2. Node.js Single-Threaded Architecture and Concurrency

Although Node.js runs on a single thread, it handles concurrency through asynchronous, non-blocking I/O operations supported by the event loop and thread pool. Time-consuming I/O operations are offloaded to the thread pool while the main thread continues executing other tasks without blocking.

Tip: Efficient use of the thread pool is key for CPU-heavy tasks to maintain performance.

3. What is the Event Loop?

The event loop is Node.js’s mechanism for managing asynchronous operations by queueing events and executing them non-blockingly. This allows the main thread to handle other operations while waiting for I/O tasks to complete.

Example:

console.log('Start');

setTimeout(() => {
 console.log('Timeout');
}, 0);

console.log('End');

Output:

Start
End
Timeout

Tip: Even with zero milliseconds delay, setTimeout executes after synchronous code due to the event loop.

4. Difference Between Blocking and Non-Blocking I/O

• Blocking I/O: Operations block the thread until completion (synchronous).
• Non-Blocking I/O: Operations start and return immediately; completion triggers a callback (asynchronous).

Example - Blocking I/O:

const fs = require('fs');
const data = fs.readFileSync('/path/to/file.txt', 'utf8');
console.log(data);
console.log('File read complete.');

Example - Non-Blocking I/O:

const fs = require('fs');
fs.readFile('/path/to/file.txt', 'utf8', (err, data) => {
 if (err) throw err;
 console.log(data);
});
console.log('File read started.');

Tip: Use non-blocking I/O for scalable, high-performance apps; manage errors and callbacks carefully.

5. Callback, Promise, and Async/Await Concepts

• Callback: A function called after an asynchronous operation completes; nested callbacks may cause "callback hell."
• Promise: An object representing eventual completion/failure of an async operation; supports chaining with .then and .catch.
• Async/Await: Syntactic sugar over promises for more readable, synchronous-looking asynchronous code.

Example:

• Callback:

function operation(callback) {
 setTimeout(() => {
   console.log('Operation completed.');
   callback();
 }, 1000);
}

operation(() => {
 console.log('Next operations.');
});

• Promise:

function operation() {
 return new Promise((resolve, reject) => {
   setTimeout(() => {
     console.log('Operation completed.');
     resolve();
   }, 1000);
 });
}

operation().then(() => {
 console.log('Next operations.');
});

• Async/Await:

async function run() {
 await operation();
 console.log('Next operations.');
}

run();

Tip: Async/await improves readability, especially for complex workflows, but always use try/catch for error handling.

6. What is Express.js? How is it Different from Node.js?

Express.js is a minimalist web framework built on top of Node.js’s HTTP module. It simplifies routing, middleware management, and RESTful API development.

Example – Basic Express Server:

const express = require('express');
const app = express();

app.get('/', (req, res) => {
 res.send('Hello World!');
});

app.listen(3000, () => {
 console.log('Server running on port 3000');
});

Tip: Express is fundamental for backend projects. Proper middleware usage is critical for performance and security.

7. What is Middleware and How is it Used?

Middleware functions execute during request-response cycles for tasks like authentication, logging, or error handling.

Example – Simple Logging Middleware:

app.use((req, res, next) => {
 console.log(`Request time: ${new Date().toISOString()} - ${req.method} ${req.url}`);
 next();
});

Tip: Use app.use() for global middleware. Define error-handling middleware separately.

8. Database Integration – MongoDB Example

Typically, Node.js uses the mongoose ODM to model data and perform CRUD operations with MongoDB.

Example – MongoDB Connection:

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/test', {
 useNewUrlParser: true,
 useUnifiedTopology: true
});

const UserSchema = new mongoose.Schema({
 name: String,
 email: String
});

const User = mongoose.model('User', UserSchema);

const newUser = new User({ name: 'Ali', email: 'ali@example.com' });
newUser.save();

Tip: Use .catch() or try/catch blocks to handle connection errors in production.

9. Microservices Architecture and Node.js

Microservices divide applications into small, independent services, each potentially its own Node.js app.

Key Components:

• REST/gRPC APIs
• Message queues (Kafka, RabbitMQ)
• Container orchestration (Docker, Kubernetes)
• API Gateway (Kong, Express Gateway)

Tip: Each microservice should have its own database and be loosely coupled. Node.js’s lightweight nature suits microservice architectures.

10. Node.js Performance Optimization

Techniques to improve performance:

• Use caching (e.g., Redis)
• Use the Cluster module to leverage multi-core CPUs
• Favor asynchronous programming
• Identify and optimize slow queries

Example – Cluster Module:

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
 for (let i = 0; i < numCPUs; i++) {
   cluster.fork();
 }
} else {
 http.createServer((req, res) => {
   res.writeHead(200);
   res.end('Hello World!');
 }).listen(8000);
}

Tip: Use process managers like PM2 to manage Node.js apps in production for auto-restarts, logging, and load balancing.

11. Error Handling in Node.js

• For synchronous code: use try/catch
• For asynchronous code: use try/catch with async/await or .catch()
• For global errors: use process.on('uncaughtException') and process.on('unhandledRejection')

Example – Express Error Middleware:

app.use((err, req, res, next) => {
 console.error(err.stack);
 res.status(500).send('Something went wrong!');
});

Success in Node.js interviews depends not only on technical knowledge but also on your ability to solve real-world problems. This article covers Node.js interview questions, Express.js, event loop, database integration, microservices, and more.

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