Table of Contents

Top Express JS Interview Questions (2025)

Express.JS Interview Questions
Table of Contents

In today’s competitive job market, securing a backend developer position often hinges on demonstrating expertise in popular frameworks like Express.js.  As a minimalist and flexible framework built on top of Node.js, Express.js has become a cornerstone for building web applications and APIs.

This article serves as your comprehensive guide to mastering Express.js, covering fundamental concepts, advanced techniques, and common interview questions. We’ll delve deep into the intricacies of Express.js, equipping you with the knowledge and confidence to ace your next technical interview.

Express.js Basic Questions

This section focuses on simple questions to evaluate a candidate’s basic understanding of Express.js. These questions are designed to check their familiarity with key concepts and features.

1) What is Express.js?

Express.js is a minimal and flexible Node.js web application framework. It provides a robust set of features for building web and mobile applications. Express.js simplifies the process of developing web servers and APIs by offering a streamlined approach to handling routing, requests, and responses.   

Express.js is often compared to other Node.js frameworks like Koa and Hapi. Koa, developed by the same team behind Express.js, aims for an even more minimalist approach. Hapi focuses on configuration-driven development and provides a more structured framework. Express.js strikes a balance between simplicity and features, making it a popular choice for many developers.   

2) What are the main features of Express.js?

Express.js offers several key features that make it a powerful framework:

  • Middleware Support: Express.js uses middleware to process requests in a modular way. Middleware functions can access the request object, response object, and the next() function in the application’s request-response cycle.   
  • Routing: Express.js provides a robust routing mechanism to define endpoints and handle different HTTP requests (GET, POST, PUT, DELETE, etc.).   
  • Template Engines Integration: Express.js supports various template engines like EJS, Pug, and Handlebars, allowing you to generate dynamic HTML content easily.   
  • Error Handling: Express.js includes built-in middleware for handling errors and provides a mechanism to define custom error-handling middleware. 

3) How do you install Express.js?

You can install Express.js using npm (Node Package Manager):   

Bash

npm install express

To set up a basic Express.js server:

JavaScript

const express = require(‘express’);

const app = express();

const port = 3000;

app.get(‘/’, (req, res) => {

  res.send(‘Hello from Express!’);

});

app.listen(port, () => {

  console.log(`Server listening on port ${port}`);

});

4) What is middleware in Express.js?

Middleware functions are functions that have access to the request object ( req ), the response object ( res ), and the next() function in the application’s request-response cycle. They are executed in the order they are defined.   

There are different types of middleware in Express.js:

  • Application-level middleware: Binds to the app object using app.use().
  • Router-level middleware: Similar to application-level middleware, but it’s bound to an instance of express.Router().
  • Error-handling middleware: Used for handling errors that occur during request processing. It takes four arguments: (err, req, res, next).   
  • Built-in middleware: Express.js has some built-in middleware functions, like express.static for serving static files.

5) Explain routing in Express.js.

Routing in Express.js refers to how an application responds to a client request to a particular endpoint, which is a URI (or path) and a specific HTTP request method (GET, POST, and so on).   

Each route can have one or more handler functions, which are executed when the route is matched.   

JavaScript

app.get(‘/’, (req, res) => {

  res.send(‘This is the home page.’);

});

app.post(‘/user’, (req, res) => {

  res.send(‘Create a new user’);

});

6. How can you serve static files in Express.js?

Express.js provides the express.static built-in middleware function for serving static files such as images, CSS, and JavaScript.

JavaScript

app.use(express.static(‘public’));

This code tells Express.js to serve static files from the ‘public’ directory in the root directory of your application.

7) What is a request object in Express.js?

The req object represents the HTTP request and has properties for the request query string, parameters, body, HTTP headers, and so on.   

req.params: Contains route parameters.

req.query: Contains query string parameters.

req.body: Contains the request body.

8) What is a response object in Express.js?

The res object represents the HTTP response that an Express app sends when it gets an HTTP request.

Some common methods of the res object include:

res.send(): Sends a response of various types.

res.json(): Sends a JSON response.

res.status(): Sets the HTTP status code of the response.

Once you’ve covered the basics, it’s time to move to intermediate questions that test practical knowledge and problem-solving skills.

Express.js Intermediate Questions

Here, we will discuss intermediate-level questions that help assess a candidate’s ability to handle real-world scenarios using Express.js. These questions focus on topics like middleware and routing.

9) How do you handle errors in Express.js?

Express.js has a built-in error handler that takes care of any errors that might be encountered in the middleware chain. You can also define your own error-handling middleware functions. These functions have four arguments: (err, req, res, next).   

To trigger error-handling middleware, you use the next(err) method.

JavaScript

app.get(‘/’, (req, res, next) => {

  // Simulate an error

  const err = new Error(‘Something went wrong’);

  next(err);

});

// Error handling middleware

app.use((err, req, res, next) => {

  console.error(err.stack);

  res.status(500).send(‘Something broke!’);

});

10) How does the Express routing system work internally?

Express.js maintains a routing table that maps routes to their corresponding handler functions. When a request comes in, Express.js matches the request path and method against the routes in the table.

Routes are matched in the order they are defined. It’s important to define more specific routes before more general routes. Express.js uses pattern matching to match routes with parameters.   

11) What are different ways to define middleware in Express?

You can define middleware at three levels:

  • Application-level middleware: Applied to all requests using app.use().
  • Router-level middleware: Attached to a specific router instance using router.use().
  • Built-in middleware: Express.js provides built-in middleware for common tasks like serving static files.   

12) Explain the role of next() in Express middleware.

The next() function is used to pass control to the next middleware function in the chain. If next() is not called, the request-response cycle will be halted.

JavaScript

const myLogger = (req, res, next) => {

  console.log(‘LOGGED’);

  next();

};

app.use(myLogger);

In this example, myLogger middleware logs a message and then calls next() to pass control to the next middleware or route handler.

13) How do you define routes with parameters in Express?

You can define routes with parameters using the colon : followed by the parameter name.

JavaScript

app.get(‘/users/:id’, (req, res) => {

  const userId = req.params.id;

  res.send(`User ID: ${userId}`);

});

14) What is app.use() method in Express?

The app.use() method is used to mount middleware functions or routers at a specific path. Middleware functions mounted using app.use() will be executed for all requests that match the specified path.

15) What is a RESTful API, and how can you build one using Express?

A RESTful API (Representational State Transfer) is an architectural style for designing networked applications. It uses various HTTP methods (GET, POST, PUT, DELETE) to interact with resources.   

Express.js makes it easy to build RESTful APIs. Here’s an example of CRUD (Create, Read, Update, Delete) operations:

JavaScript

// Get all users

app.get(‘/users’, (req, res) => { … }); 

// Get a single user by ID

app.get(‘/users/:id’, (req, res) => { … }); 

// Create a new user

app.post(‘/users’, (req, res) => { … }); 

// Update a user by ID

app.put(‘/users/:id’, (req, res) => { … }); 

// Delete a user by ID

app.delete(‘/users/:id’, (req, res) => { … });

16) How can you handle form data and file uploads in Express?

Form Data: You can use the body-parser middleware to parse incoming request bodies in a middleware before your handlers, available under the req.body property.   

File Uploads: The multer middleware is commonly used for handling file uploads. It processes multipart/form-data, which is used for uploading files.

17) How do you manage sessions in Express?

The express-session middleware enables session management in Express.js. It allows you to store user data on the server-side and associate it with a unique session ID. This ID is typically stored in a cookie on the client-side.

18) How can you use templates in Express.js?

Template engines allow you to generate dynamic HTML content by combining templates with data. Express.js supports various template engines, including EJS, Pug, and Handlebars.   

You can use the res.render() method to render a template.

JavaScript

app.set(‘view engine’, ‘ejs’); // Set EJS as the template engine

app.get(‘/’, (req, res) => {

  res.render(‘index’, { name: ‘John Doe’ });

});

19) How do you implement authentication in Express.js?

Passport.js is a popular authentication middleware for Node.js. It provides various authentication strategies (e.g., local, OAuth, JWT).   

Express.js applications can implement both session-based and token-based authentication:

  • Session-based: User data is stored in a session on the server.   
  • Token-based: A unique token is issued to the user upon successful authentication. This token is then used to authenticate subsequent requests.   

Now that you have an idea of intermediate questions, let’s dive into advanced questions to test in-depth expertise.

Express.js Advanced Questions

This section includes advanced questions to evaluate a candidate’s expertise in Express.js. These questions help you understand how well they can handle complex challenges.

20) How do you secure an Express.js application?

Securing an Express.js application involves several best practices:

  • HTTPS: Always use HTTPS to encrypt communication between the client and server.
  • Helmet.js: This middleware helps secure your app by setting various HTTP headers.   
  • Input Validation: Validate all user input to prevent vulnerabilities like Cross-Site Scripting (XSS) and SQL injection.
  • Authentication and Authorization: Implement proper authentication and authorization mechanisms to control access to resources.
  • Cross-Site Request Forgery (CSRF) Protection: Use CSRF tokens to prevent CSRF attacks.   

21) What is CORS and how do you handle it in Express?

CORS (Cross-Origin Resource Sharing) is a mechanism that allows restricted resources on a web page to be requested from another domain outside the domain from which the first resource was served.   

You can enable CORS in your Express.js app using the cors middleware.

JavaScript

const cors = require(‘cors’);

app.use(cors());

22) How do you optimize performance in Express applications?

  • Caching: Implement caching strategies using tools like Redis or in-memory caches to reduce database queries and improve response times.
  • Load Balancing: Distribute incoming traffic across multiple servers to handle increased load.
  • Code Optimization: Write efficient code and use asynchronous operations whenever possible.   

23) Explain how you can scale an Express.js application.

Horizontal Scaling: Increase the number of server instances to handle more traffic. You can use tools like PM2 for process management and clustering.   

Vertical Scaling: Increase the resources (CPU, memory) of your server.

24) How do you use the Express router?

The express.Router() function creates a new router object. This allows you to modularize your routes by separating them into different files.

JavaScript

const router = express.Router();

router.get(‘/’, (req, res) => {

  res.send(‘Hello from the router!’);

});

app.use(‘/api’, router);

25) How can you handle rate limiting in an Express app?

Rate limiting helps prevent abuse and overload by throttling the number of requests from a particular IP address within a given time frame.   

You can use the express-rate-limit middleware to implement rate limiting.

JavaScript

const rateLimit = require(‘express-rate-limit’);

const limiter = rateLimit({

  windowMs: 15 * 60 * 1000, // 15 minutes

  max: 100, // limit each IP to 100 requests per windowMs

});

app.use(limiter);

26) What is the purpose of process.env in Express?

process.env provides access to environment variables. You can use it to configure your application based on the environment (development, production).

JavaScript

const port = process.env.PORT || 3000;

27) How can you debug an Express.js application?

  • console.log: Use console.log to print messages and inspect variables.
  • Node Inspect: Use the built-in Node.js debugger.   
  • Morgan Middleware: Use the morgan middleware to log HTTP requests and responses.

28) How do you deploy an Express.js app?

You can deploy your Express.js app to various cloud platforms like Heroku, AWS, Google Cloud, and Azure. These platforms provide tools and services for deploying and managing Node.js applications.

29) What are the best practices for error handling in production?

  • Centralized Logging: Use a logging service like Winston or Sentry to capture and analyze errors.
  • Graceful Shutdown: Handle unexpected errors and shut down the application gracefully to prevent data loss.
  • Error Monitoring: Monitor your application for errors and performance issues.   

30) How do you implement WebSockets in Express.js?

WebSockets enable real-time, two-way communication between the client and server. You can use Socket.IO with Express.js to build real-time applications like chat apps and online games.   

After reviewing advanced questions, it’s important to focus on effective interview preparation tips to identify the right candidate.

Express.js Interview Preparation Tips

In this section, we will cover practical tips to help you prepare for Express.js interviews. These tips will make the hiring process smoother and more effective.

Tips for Express.js Interview Preparation

1) Understand the Node.js Ecosystem: Familiarize yourself with Node.js modules and libraries commonly used with Express.js, such as body-parser, cookie-parser, express-session, and multer.

2) Master JavaScript ES6+: Have a strong grasp of modern JavaScript features, including arrow functions, template literals, destructuring, and async/await.

3) Build Practical Projects: Create a simple API or web application using Express.js to gain hands-on experience. This will help you solidify your understanding of the concepts and demonstrate your practical skills.

4) Database Integration: Learn how to integrate Express.js with databases like MongoDB or MySQL. Understand how to perform CRUD operations and handle database interactions efficiently.

5) Testing: Understand how to test Express.js applications using tools like Mocha, Chai, and Jest. Write unit tests and integration tests to ensure the quality of your code.

6) Security Best Practices: Be aware of common security vulnerabilities and how to prevent them in Express.js applications. Understand concepts like input validation, authentication, authorization, and CSRF protection.

7) Performance Optimization: Learn about techniques for optimizing the performance of Express.js applications, such as caching, load balancing, and code optimization.

8) Deployment: Familiarize yourself with different deployment platforms and strategies for Express.js applications.

9) Practice: Practice answering common Express.js interview questions using iScalePro. This will help you articulate your knowledge clearly and confidently.

With these tips in mind, you’re now ready to conclude the interview process and choose the best candidate for the role.

Conclusion

This comprehensive guide has covered a wide range of Express.js concepts, from basic to advanced, to prepare you for your next technical interview. By mastering these topics, building practical projects, and practicing your interview skills, you’ll be well-equipped to impress potential employers and secure your desired backend development role. Remember to showcase your understanding of Express.js’s core principles, middleware, routing, security best practices, and ability to build efficient and scalable applications.

Express.js Interview FAQs

1) What is Express.js and how does it relate to Node.js?

Express.js is a minimal and flexible web application framework for Node.js, designed to build web applications and APIs. It simplifies server development by providing robust features like routing, middleware support, and template engines, enhancing Node.js’s capabilities.

2) What is middleware in Express.js?

Middleware in Express.js refers to functions that have access to the request, response, and the next middleware function in the application’s request-response cycle. They can modify request and response objects, end the request-response cycle, or call the next middleware in the stack.

3) How do you handle errors in Express.js?

Error handling in Express.js involves defining error-handling middleware that takes four arguments: error, request, response, and next. This middleware captures and processes errors, allowing for centralized error management and consistent responses to clients.

4) How do you serve static files in Express.js?

Express.js serves static files using the ‘express.static’ middleware. By specifying a directory, Express can serve static assets like HTML files, images, and stylesheets, making them accessible to clients.

5) What is routing in Express.js?

Routing in Express.js defines how an application responds to client requests to specific endpoints, using HTTP methods and paths. It allows developers to specify callback functions that execute when a route is matched, facilitating organized and modular code.

Click below to simplify hiring 👇

Scroll to Top