Master Node.js (with Express) Interview Questions

Junior-Level Node.js Interview Questions

Here are some junior-level interview questions for Node.js with Express:

Question 01: What is Node.js, and how does it work?

Answer: Node.js is a JavaScript runtime environment that allows you to run JavaScript on the server side. It uses the V8 engine for executing code and features a non-blocking, event-driven architecture for handling I/O operations efficiently.

Node.js processes tasks through a single-threaded event loop, managing multiple operations concurrently. This design is ideal for scalable network applications.

const http = require('http');
const server = http.createServer((req, res) => {
  res.end('Hello World');
});
server.listen(3000);

Question 02: Explain the role of the 'package.json' file in a Node.js project.

Answer: The package.json file in a Node.js project manages project metadata, dependencies, and scripts. It specifies the project's name, version, and author, and lists dependencies needed for the project.

It also defines custom scripts for tasks like testing or starting the application, which can be run with npm run. For example:

{
  "name": "my-node-app",
  "version": "1.0.0",
  "scripts": {
    "start": "node index.js",
    "test": "mocha"
  },
  "dependencies": {
    "express": "^4.17.1"
  }
}        

Question 03: What is Express.js and how does it relate to Node.js?

Answer: Express.js is a web framework for Node.js that simplifies building server-side applications. It provides tools for routing, middleware, and handling HTTP requests and responses.

Express.js builds on Node.js by offering a more user-friendly API for creating web servers and APIs.

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

app.get('/', (req, res) => res.send('Hello World'));
app.listen(3000);

Question 04: How do you install Express.js in a Node.js project?

Answer: You can install Express.js using npm (Node Package Manager) with the following command:

npm install express        

Question 05: How do you define a route in Express.js?

Answer: In Express.js, routes are defined using methods that correspond to HTTP methods. For example:

app.get('/path', (req, res) => {
  res.send('GET request to the homepage');
});
        

Question 06: What is middleware in Express.js? Provide an example.

Answer: Middleware functions in Express.js are functions that have access to the request object (req), the response object (res), and the next middleware function in the application's request-response cycle. Middleware can perform tasks such as executing code, modifying the request and response objects, ending the request-response cycle, and calling the next middleware function. For example:

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

app.use((req, res, next) => {
  console.log('Request URL:', req.originalUrl);
  next();
});

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

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

Question 07: How can you handle POST requests in Express.js?

Answer: To handle POST requests in Express.js, you use the app.post() method and typically parse the incoming request body using middleware such as body-parser or the built-in express.json(). For example:

const express = require('express');
const app = express();
app.use(express.json());

app.post('/submit', (req, res) => {
  res.send(`Received data: ${JSON.stringify(req.body)}`);
});

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

Question 08: What is npm, and how is it used in Node.js?

Answer: npm (Node Package Manager) is a package manager for JavaScript. It manages packages and dependencies for Node.js projects. npm commands like npm install are used to add packages, npm update updates them, and npm uninstall removes them. The package.json file lists project dependencies and scripts for development tasks.

Question 09: What will be the output of this Node.js code?

let obj = { a: 1, b: 2 };
delete obj.b;
console.log(obj);        

Answer: The result will be:

{ a: 1 }        

Question 10: How do you serve static files in an Express application?

Answer: To serve static files in an Express application, you use the built-in middleware express.static(). This middleware serves static assets such as HTML, CSS, JavaScript, and images from a specified directory. For example:

const express = require('express');
const app = express();
app.use(express.static('public'));

app.get('/', (req, res) => {
  res.send('Welcome to the homepage!');
});

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

Here are some mid-level interview questions for Node.js with Express:

Question 01: Explain the concept of middleware chaining in Express.js.

Answer: Middleware chaining in Express.js refers to the process of executing multiple middleware functions in sequence. Each middleware function performs specific tasks such as logging requests, handling authentication, or parsing request bodies. Middleware functions must call the next() function to pass control to the next middleware function in the chain. This approach allows for modular and maintainable code.

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

// Middleware function 1
app.use((req, res, next) => {
  console.log('Middleware 1');
  next(); // Passes control to the next middleware function
});

// Middleware function 2
app.use((req, res, next) => {
  console.log('Middleware 2');
  next(); // Passes control to the next middleware function
});

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

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

Question 02: How do you handle error middleware in Express.js?

Answer: Error-handling middleware in Express.js has four parameters: err, req, res, and next. This middleware catches and handles errors that occur in the application. It should be defined after all other middleware and route handlers. For example:

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

Question 03: What are some common HTTP methods, and how are they used in Express.js routes?

Answer: Common HTTP methods include GET, POST, PUT, DELETE, PATCH, and OPTIONS. In Express.js, these methods are used to define routes that handle specific types of requests:

app.get('/', (req, res) => res.send('GET request'));
app.post('/', (req, res) => res.send('POST request'));
app.put('/user', (req, res) => res.send('PUT request'));
app.delete('/user', (req, res) => res.send('DELETE request'));
        

Question 04: How do you manage environment variables in a Node.js application?

Answer: Environment variables in Node.js are managed using the .env file and the dotenv package. The .env file contains key-value pairs of environment-specific settings, which are loaded into process.env by calling dotenv.config().

require('dotenv').config();
const port = process.env.PORT || 3000;
app.listen(port, () => {
  console.log(`Server is running on port ${port}`);
});

Question 05: How can you handle file uploads in an Express application?

Answer: File uploads in Express applications are managed using the multer middleware. It processes multipart/form-data, which is used for file uploads. For example:

const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded successfully!');
});

Question 06: What is the purpose of the next function in Express middleware?

Answer: In Express.js, the next function is used to pass control from one middleware function to the next in the stack. By calling next(), you ensure that the request moves forward to the subsequent middleware or route handler, allowing you to chain multiple middleware functions for tasks like authentication, logging, or request parsing.

If next is not called, the request will hang and not proceed through the middleware chain, potentially causing the application to become unresponsive. It’s crucial for the flow of requests and responses in an Express application, ensuring that all intended middleware functions and route handlers are executed in sequence.

Question 07: What will be the output of the following Node.js code?

const x = [10, 20, 30];
const y = x.slice(1, 2);
console.log(y);

        

Answer: The output will be:

[20]    

Question 08: How do you implement session management in an Express application?

Answer: Session management in Express can be implemented using the express-session middleware. This middleware allows you to create and manage user sessions, storing session data on the server-side. For example:

const session = require('express-session');

app.use(session({
  secret: 'secret-key',
  resave: false,
  saveUninitialized: true,
  cookie: { secure: true }
}));

app.get('/', (req, res) => {
  if (req.session.views) {
    req.session.views++;
    res.send(`Number of views: ${req.session.views}`);
  } else {
    req.session.views = 1;
    res.send('Welcome to the homepage!');
  }
});        

Question 09: What are some common security practices for Express applications?

Answer: Common security practices for Express applications include:

• Use HTTPS: Encrypt data between the server and client.
• Validate and Sanitize Inputs: Prevent XSS and SQL injection attacks.
• Use Environment Variables: Store sensitive data securely.
• Implement Rate Limiting: Protect against DDoS attacks.
• Secure Cookies: Set httpOnly, secure, and sameSite attributes.
• Use Security Headers: Implement security headers with helmet middleware.
• Protect Against Common Vulnerabilities: Mitigate XSS, CSRF, and SQL injection.

Question 10: How can you optimize the performance of a Node.js (with Express) application?

Answer: To optimize the performance of a Node.js (with Express) application, you can implement techniques such as caching, using a load balancer, optimizing database queries, using compression middleware, minimizing middleware usage, and ensuring non-blocking code. Additionally, profiling and monitoring the application can help identify and address performance bottlenecks.

Here are some expert-level interview questions for Node.js with Express:

Question 01: Explain the event loop in Node.js and how it handles asynchronous operations.

Answer: The event loop is a core concept in Node.js that allows it to handle asynchronous operations in a non-blocking manner. It is a single-threaded loop that continuously checks for events, processes them, and delegates I/O operations to the system's kernel whenever possible. The event loop has several phases, including timers, pending callbacks, idle, poll, check, and close callbacks.

When an asynchronous operation (such as I/O or timers) completes, its callback is placed in the event queue, and the event loop picks it up and executes it. This mechanism allows Node.js to handle many concurrent operations efficiently without blocking the execution of the program.

Question 02: How do you implement authentication and authorization in an Express application?

Answer: Answer: Authentication and authorization in an Express application can be implemented using middleware such as passport. passport provides various strategies for authentication, including local, OAuth, and JWT. You can set up routes to handle login, registration, and protected resources, using middleware to verify user credentials and permissions.

Question 03: What are streams in Node.js, and how do they improve performance?

Answer: Answer: Streams in Node.js are objects that allow you to read or write data in a continuous flow. They improve performance by processing data piece-by-piece (chunks), rather than loading the entire data into memory at once. Streams are useful for handling large files, network communications, and other data-intensive operations. For example:

const fs = require('fs');

// Reading a file using streams
const readableStream = fs.createReadStream('largefile.txt');
readableStream.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});

// Writing to a file using streams
const writableStream = fs.createWriteStream('output.txt');
writableStream.write('This is some data.\n');
writableStream.end('This is the end of the data.');

        

Question 04: How do you handle real-time communication in a Node.js application?

Answer: Real-time communication in a Node.js application is commonly handled using Socket.IO, a library that enables bidirectional and event-based communication between clients and servers. Socket.IO works on top of WebSockets, but it also provides fallbacks for older browsers that do not support WebSockets.

To implement real-time communication, you first need to install Socket.IO and set it up on both the server and client sides. The server establishes a WebSocket connection, listens for events from clients, and can emit events to clients. The client, typically a web browser, connects to the server and communicates through events as well.

Question 05: What is clustering in Node.js, and how does it help with scalability?

Answer: Clustering in Node.js allows you to create multiple instances (workers) of your application that can run on multiple CPU cores. This helps with scalability by distributing incoming requests across the workers, effectively utilizing the available CPU resources and improving the application's throughput. The cluster module in Node.js can be used to set up clustering.

Question 06: How do you use async/await in Node.js to handle asynchronous code?

Answer: async/await is a syntax in JavaScript that allows you to write asynchronous code in a synchronous manner. Functions declared with the async keyword return a promise, and the await keyword can be used to pause the execution of the async function until the promise is resolved. For example:

const fetch = require('node-fetch');

async function fetchData() {
  try {
    const response = await fetch('https://api.example.com/data');
    const data = await response.json();
    console.log(data);
  } catch (error) {
    console.error('Error fetching data:', error);
  }
}

fetchData();

Question 07: What are some advanced routing techniques in Express.js?

Answer: Advanced routing techniques in Express.js include using route parameters, query parameters, and nested routes. You can also use express.Router() to create modular and mountable route handlers. For example:

const express = require('express');
const router = express.Router();

router.get('/users/:userId',(req,res)=>{
const userId = req.params.userId;
res.send(`User ID: ${userId}`);
});

app.use('/api',router);

Question 08: How do you manage database connections in a Node.js (with Express) application?

Answer: Database connections in a Node.js (with Express) application can be managed using connection pooling and ORM libraries such as sequelize for SQL databases or mongoose for MongoDB. Connection pooling helps manage multiple database connections efficiently, reducing the overhead of establishing and closing connections repeatedly.

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/mydatabase', {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
db.once('open', () => {
  console.log('Connected to the database');
});

Question 09: Explain the use of process management tools like PM2 in a Node.js application.

Answer: Process management tools like PM2 are essential for maintaining Node.js applications in production. PM2 ensures continuous operation by automatically restarting applications if they crash, minimizing downtime. It provides features for starting, stopping, and monitoring processes, as well as real-time performance tracking.

PM2 enhances scalability through load balancing and cluster mode, running multiple instances of an application to distribute traffic and fully utilize system resources. It also simplifies log management and can generate startup scripts to ensure applications start on system boot, making it a comprehensive tool for managing Node.js applications in production.

Question 10: What are some strategies for ensuring the security of a Node.js (with Express) application?

Answer: Strategies for ensuring the security of a Node.js (with Express) application include using HTTPS, validating and sanitizing user inputs, implementing authentication and authorization, protecting against common web vulnerabilities (XSS, CSRF, SQL injection), using security headers with helmet, managing environment variables securely, and keeping dependencies up to date. Additionally, performing regular security audits and code reviews can help identify and address potential security issues.

To excel in a Node.js technical interview, it's crucial to have a strong grasp of the language's core concepts. This includes a deep understanding of syntax and semantics, data types, and control structures. Additionally, mastering Node.js approach to error handling is essential for writing robust and reliable code. Understanding concurrency and parallelism can set you apart, as these skills are highly valued in many programming languages.

• Core Language Concepts: Syntax, semantics, data types (built-in and composite), control structures, and error handling.
• Concurrency and Parallelism: Creating and managing threads, using communication mechanisms like channels and locks, and understanding synchronization primitives.
• Standard Library and Packages: Familiarity with the language's standard library and commonly used packages, covering basic to advanced functionality.
• Practical Experience: Building and contributing to projects, solving real-world problems, and showcasing hands-on experience with the language.
• Testing and Debugging: Writing unit, integration, and performance tests, and using debugging tools and techniques specific to the language.