Top 45 Express.js Interview Questions

Express.js Interview Questions and Answers: Beginners Level

This section covers the basics of Express.js, including routing, middleware, and basic request/response handling. It's perfect for those starting their journey and preparing for Express.js interview questions and answers related to foundational concepts.

1. What is Express.js?

Express.js is a minimal and flexible Node.js web application framework that provides robust features for web and mobile applications. It's used to build single-page, multi-page, and hybrid web apps with ease.

2. How do you install Express.js?

You can install Express.js using npm (Node Package Manager). First, make sure you have Node.js installed. Then, run the following command in your terminal:

This installs Express in your project’s node_modules directory and updates the package.json file. If you want to install it globally (less common), use:

3. How do you create a basic Express server?

To create a basic Express server, follow these steps:

• Initialize your project (if not already done):

• Install Express:

• Create a server file (e.g., server.js) with the following code:

const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('Hello World!');
});
app.listen(3000, () => {
console.log('Server is running on http://localhost:3000');
});

• Run the server:

Your server will now respond with “Hello World!” when you visit http://localhost:3000.

4. What is routing in Express.js?

Routing in Express.js refers to how an application defines endpoints (URLs) and handles client requests using specific HTTP methods, such as GET, POST, PUT, and DELETE. Each route in Express is associated with a path and a method. When a request matches both, the corresponding handler function is executed.

Example:

const express = require('express');
const app = express();
app.get('/home', (req, res) => {
res.send('Welcome to the Home Page');
});
app.post('/submit', (req, res) => {
res.send('Form submitted');
});

In this example:

• GET /home responds with a welcome message.
• POST /submit handles form submissions.

Routing helps define the structure and flow of your application’s API or website.

5. What is middleware in Express.js?

Middleware in Express.js is a function that runs during the request-response cycle. They have access to the request (req) and response (res) objects, as well as the next() function, which passes control to the next middleware or route handler.

Middleware is used for tasks like:

• Logging
• Authentication
• Parsing JSON or form data
• Error handling

Example:

const express = require('express');
const app = express();
// Simple middleware
app.use((req, res, next) => {
console.log(`Request made to: ${req.url}`);
next();
});
app.get('/', (req, res) => {
res.send('Hello World');
});

In this example, the middleware logs every request URL before sending a response. Middleware helps modularize and manage application behavior efficiently.

Accelerate your career as a skilled MERN Stack Developer by enrolling in a unique Full Stack Developer - MERN Stack Master's program. Get complete development and testing knowledge on the latest technologies by opting for the MERN Stack Developer Course. Contact us TODAY!

6. How do you use middleware in Express.js?

You can use middleware in Express.js by calling the app.use() method or passing the middleware function directly into route definitions. Middleware can run before a request reaches the route handler or even after a response is sent (e.g., error logging).

Example 1: Application-Level Middleware

const express = require('express');
const app = express();
// Middleware for all routes
app.use((req, res, next) => {
console.log('Middleware executed');
next(); // Pass control to the next middleware or route
});

7. What is the role of next() in middleware?

In Express.js, the next() function is used within middleware to pass control to the next middleware function or route handler in the request-response cycle. If next() is not called, the request will hang and never reach its intended destination, such as a final response or route handler.

8. How do you serve static files in Express?

To serve static files in Express (like images, CSS, or JavaScript), you use the built-in express.static middleware.

• Create a folder (e.g., public) with your static assets.
• Use express.static to make that folder accessible.

Example:

const express = require('express');
const app = express();
// Serve static files from the 'public' folder
app.use(express.static('public'));
app.listen(3000, () => {
console.log('Server running on http://localhost:3000');
});

Now, if you have a file public/logo.png, it can be accessed at:

http://localhost:3000/logo.png

9. What is req.params in Express.js?

In Express.js, req.params is an object that contains route parameters (also known as URL parameters). These are dynamic parts of a route path, usually used to capture values from the URL.

Example:

app.get('/user/:id', (req, res) => {
res.send(`User ID is ${req.params.id}`);
});

If the user visits http://localhost:3000/user/42, the server responds with:

User ID is 42

Here:

• :id is a route parameter
• req.params.id retrieves its value

req.params helps handle URLs with dynamic segments like IDs, usernames, or slugs.

10. How do you send JSON responses in Express?

In Express.js, you can send a JSON response using the res.json() method. It automatically sets the Content-Type header to application/json and converts the JavaScript object into a JSON string.

Example:

app.get('/api/user', (req, res) => {
res.json({ id: 1, name: 'Alice', role: 'Admin' });
});

This method is commonly used in APIs to send structured data to the frontend or clients.

11. What are the HTTP methods supported by Express.js?

Express.js supports all standard HTTP methods, allowing you to build full-featured RESTful APIs. These methods define the type of action a client wants to perform on a resource.

Common HTTP Methods in Express:

12. How do you handle a 404 route in Express.js?

To handle a 404 (Not Found) route in Express.js, you add a middleware function at the end of all route definitions. This function catches any request that doesn’t match an existing route.

Example:

app.use((req, res) => {
res.status(404).send('404 - Page Not Found');
});

13. Can you chain route handlers?

Yes, you can chain multiple route handlers in Express.js for a single route. This is useful when you want to break up logic into reusable middleware functions, such as authentication, validation, and final response.

Example:

function logRequest(req, res, next) {
console.log('Request received');
next();
}
function sendResponse(req, res) {
res.send('Route handler executed');
}
app.get('/example', logRequest, sendResponse);

14. What is the use of express.Router()?

The express.Router() function creates a modular, mountable route handler in Express. It allows you to organize routes into separate files or groups, enhancing code readability and maintainability, particularly in larger applications.

Example: Creating a Router

// userRoutes.js
const express = require('express');
const router = express.Router();
router.get('/profile', (req, res) => {
res.send('User Profile');
});
module.exports = router;

Example: Using the Router in Main app

// app.js
const express = require('express');
const app = express();
const userRoutes = require('./userRoutes');
app.use('/user', userRoutes);

Now, visiting /user/profile will hit the route defined in the userRoutes module.

15. How do you return HTML in a response?

To return HTML in a response using Express.js, you can use the res.send() method and pass a string containing your HTML content.

Example:

app.get('/', (req, res) => {
res.send('<h1>Welcome to Express</h1><p>This is a simple HTML response.</p>');
});

This sends a raw HTML string to the browser. Express automatically sets the Content-Type header to text/html.

Express.js Interview Questions and Answers: Intermediate Level

Dive deeper into key middleware and advanced routing Express.js interview questions. This level of Express.js interview questions and answers will help you master the more complex topics that interviewers often focus on for candidates with some experience in Express.js development.

16. How do you create route-specific middleware?

To create route-specific middleware in Express.js, you pass the middleware function directly into the route definition before the final route handler. This middleware will only run for that particular route.

Example:

function logTime(req, res, next) {
console.log('Time:', new Date().toISOString());
next();
}
app.get('/dashboard', logTime, (req, res) => {
res.send('Dashboard Accessed');
});

In this example:

• logTime middleware runs only when the /dashboard route is hit.
• It logs the timestamp, then passes control to the route handler.

17. What are the arguments of middleware functions in Express.js?

In Express.js, a middleware function typically takes three arguments, and in the case of error-handling middleware, it takes four.

Standard Middleware:

function middleware(req, res, next) {
// req: Request object
// res: Response object
// next: Function to pass control
}

Error-Handling Middleware:

function errorHandler(err, req, res, next) {
// err: The error object
// req: Request object
// res: Response object
// next: To pass to another error handler
}

18. What is the difference between app.use() and app.get()?

The primary difference between app.use() and app.get() in Express.js lies in their usage and timing. The other differences are as follows:

19. How do you group routes using express.Router()?

You can group routes in Express.js using the express module.Router() to organize your code into modules. This is especially useful for separating different sections of your app, like user-related routes, admin routes, or product routes.

• Create a router module (userRoutes.js)
• Use the router in your main app file (app.js)

20. How do you handle query parameters in Express?

In Express.js, you can handle query parameters using the req.query object. Query parameters are key-value pairs that appear after the ? in a URL.

Example:

URL

http://localhost:3000/search?term=express&limit=5

Route

app.get('/search', (req, res) => {
const term = req.query.term;
const limit = req.query.limit;
res.send(`Searching for "${term}" with limit ${limit}`);
});

21. What is error-handling middleware in Express.js?

Error-handling middleware in Express.js is a special type of middleware designed to catch and process errors that occur during the request-response cycle. It has four parameters: err, req, res, and next.

Syntax:

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

22. How do you define multiple routes with the same path?

In Express.js, you can define multiple routes with the same path by chaining HTTP methods using app.route() or by stacking middleware functions.

Method 1: Using app.route()

This groups all HTTP methods for a single path.

app.route('/product')
.get((req, res) => {
res.send('Get product');
})
.post((req, res) => {
res.send('Create product');
})
.put((req, res) => {
res.send('Update product');
});

Method 2: Separate Definitions

You can also define them separately:

app.get('/product', (req, res) => res.send('GET product'));
app.post('/product', (req, res) => res.send('POST product'));

23. How do you redirect a request in Express?

In Express.js, you can redirect a request using the res.redirect() method. This sends a 302 (Found) status code by default and tells the browser to navigate to a different URL.

24. How do you conditionally apply middleware?

To conditionally apply middleware in Express.js, you place your logic inside the middleware function itself or use custom logic to attach middleware only for specific routes.

Method 1: Inside Middleware Function

function conditionalMiddleware(req, res, next) {
if (req.path === '/secure') {
console.log('Secure route hit');
// Perform auth or other logic here
}
next();
}
app.use(conditionalMiddleware);

25. How to handle form data in Express.js?

To handle form data in Express.js, you need to use middleware to parse incoming request bodies. For simple HTML form submissions (application/x-www-form-urlencoded), Express has built-in support using express.urlencoded().

• Enable middleware
• Handle the form POST route

Launch your career as a Java developer with Simplilearn's comprehensive Full Stack Java Developer Masters Program. Learn front-end and back-end skills, along with real-world project experience, and gain the certification that top employers trust. Start your journey today!

26. What are some common third-party middleware in Express.js?

• morgan – Logging requests

const morgan = require('morgan');
app.use(morgan('dev'));

Logs HTTP requests in the console—great for debugging.

• cors – Enable Cross-Origin Resource Sharing

const cors = require('cors');
app.use(cors());

Allows your Express app to accept requests from different origins (like APIs consumed by frontend apps).

• helmet – Secures HTTP headers

const helmet = require('helmet');
app.use(helmet());

Protects your app from common web vulnerabilities by setting secure HTTP headers.

• express-session – Session management

const session = require('express-session');
app.use(session({ secret: 'my-secret', resave: false, saveUninitialized: true }));

Stores session data on the server for user authentication and tracking.

27. What is the purpose of express.json()?

The express.json() middleware in Express.js is used to parse incoming JSON payloads from HTTP request bodies. It makes the JSON data available in req.body, allowing your application to work with it easily.

When your frontend sends a POST or PUT request with a JSON body like this:

{
"name": "Alice",
"email": "alice@example.com"
}

You can extract the data using express.json():

const express = require('express');
const app = express();
app.use(express.json()); // Enable JSON parsing
app.post('/register', (req, res) => {
const { name, email } = req.body;
res.send(`Welcome, ${name}!`);
});

28. How do you catch all unhandled routes in Express?

To catch all unhandled routes in Express.js, you define a "catch-all" middleware at the end of your route definitions. This middleware will run only if no other route matches the incoming request.

Example:

app.use((req, res) => {
res.status(404).send('404 - Page Not Found');
});

29. Can you handle multiple middleware in a route?

Yes, you can handle multiple middleware functions in a single route in Express.js. Middleware functions are executed in order, and each must call next() to pass control to the next one.

Example: Multiple Middleware in One Route

function logRequest(req, res, next) {
console.log('Request received at:', new Date());
next();
}
function checkAuth(req, res, next) {
if (req.headers['x-auth']) {
next();
} else {
res.status(401).send('Unauthorized');
}
}
function sendResponse(req, res) {
res.send('Protected route accessed');
}
app.get('/secure', logRequest, checkAuth, sendResponse);

30. How does Express handle async errors?

By default, Express doesn't automatically catch errors thrown in asynchronous functions or promises. To properly handle async errors, you must either:

Use try-catch inside your async route handler:

app.get('/data', async (req, res, next) => {
try {
const data = await fetchData();
res.json(data);
} catch (err) {
next(err); // Passes error to error-handling middleware
}
});

Use a wrapper utility (custom or third-party, like express-async-handler):

const asyncHandler = fn => (req, res, next) => {
Promise.resolve(fn(req, res, next)).catch(next);
};
app.get('/safe', asyncHandler(async (req, res) => {
const result = await riskyCall();
res.send(result);
}));

Express.js Interview Questions and Answers: Advanced Level

Here, we explore the advanced features of Express.js, including optimized routing, performance improvements, and error handling strategies. Prepare for Express.js interview questions and answers that test your expertise in building scalable, production-ready applications.

31. How do you write a custom logging middleware with a timestamp?

You can create a custom logging middleware in Express.js to log requests with a timestamp by using console.log() or any other logging library (like winston or morgan). Here’s an example of how to log each request with the current timestamp:

const express = require('express');
const app = express();
// Custom logging middleware with timestamp
app.use((req, res, next) => {
const currentDate = new Date().toISOString();
console.log(`[${currentDate}] ${req.method} ${req.url}`);
next();  // Pass control to the next middleware or route
});
app.get('/', (req, res) => {
res.send('Hello World');
});
app.listen(3000, () => {
console.log('Server is running on http://localhost:3000');
});

32. How does middleware order affect Express app behavior?

In Express.js, the order of middleware is critical because Express executes middleware functions in the order they are defined. This impacts the request-response cycle and the flow of control through your application.

• Middleware runs in sequence: Once a request is received, Express processes middleware from top to bottom, so the order in which you define middleware functions directly impacts how the request is handled.
• Early middleware can block further processing: If an early middleware does not call next(), it can prevent later middleware or route handlers from being executed, effectively "terminating" the request.

33. How do you return a JSON error for an unknown route?

To return a JSON error for an unknown route in Express.js, you can define a catch-all route using app.use() at the end of all your routes. This will catch any request that doesn’t match a defined route and return a JSON error message.

Example:

const express = require('express');
const app = express();
// Define some routes
app.get('/home', (req, res) => {
res.send('Welcome to the Home Page');
});
// Catch-all route for unknown paths
app.use((req, res) => {
res.status(404).json({ error: 'Route not found' });
});
app.listen(3000, () => {
console.log('Server running on http://localhost:3000');
});

34. How do you load routes dynamically in Express?

To dynamically load routes in Express.js, you can use the fs (File System) module to read the files in a specific directory and load them into your application. This is useful for organizing routes in separate files and automatically loading them into your app.

Steps to Load Routes Dynamically:

1. Create a routes directory with separate route files.

For example, create files such as userRoutes.js and productRoutes.js.

2. Use fs to read the files and load them:

const express = require('express');
const app = express();
const fs = require('fs');
const path = require('path');
// Automatically load all routes from the 'routes' folder
fs.readdirSync('./routes').forEach((file) => {
if (file.endsWith('.js')) {
const route = require(`./routes/${file}`);
app.use(`/${file.replace('.js', '')}`, route); // Dynamically set the base path
}
});
app.listen(3000, () => {
console.log('Server running on http://localhost:3000');
});

3. Define a sample route file (routes/userRoutes.js):

const express = require('express');
const router = express.Router();
router.get('/', (req, res) => {
res.send('User Route');
});
router.get('/profile', (req, res) => {
res.send('User Profile');
});
module.exports = router;

If you have multiple route files, such as userRoutes.js and productRoutes.js, in the routes folder, the routes will be automatically loaded and available based on their filenames, e.g., /user and/product.

35. Write an Express.js route with async/await and error handling.

To write an Express.js route with async/await and error handling, you can use try-catch blocks within your asynchronous route handler. This ensures that any error that occurs during the asynchronous operation is properly caught and handled.

Example:

const express = require('express');
const app = express();
// Simulating an async function that fetches data
async function fetchData() {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve('Data fetched successfully');
}, 1000);
});
}
// Async route with error handling
app.get('/data', async (req, res, next) => {
try {
const data = await fetchData();
res.json({ message: data });
} catch (error) {
// Pass the error to the next middleware
next(error);
}
});
// Error-handling middleware
app.use((err, req, res, next) => {
console.error(err.stack);
res.status(500).json({ error: 'Something went wrong!' });
});
app.listen(3000, () => {
console.log('Server running on http://localhost:3000');
});

36. How do you protect routes using middleware?

To protect routes using middleware in Express.js, you can create a custom authentication middleware that checks if the user is authorized before allowing access to certain routes. If the user is not authorized, you can send a 403 Forbidden or 401 Unauthorized response.

37. How can you implement role-based access control in Express.js?

To implement Role-Based Access Control (RBAC) in Express.js, you need to create middleware that checks the user's role before granting access to a particular route. This middleware typically checks the user's role (stored in the request object or decoded from a token) and either grants or denies access based on the role.

Steps to Implement Role-Based Access Control:

• Define user roles: For example, "admin", "user", "manager", etc.
• Create middleware to check the user's role: The middleware will check if the user has the required role.
• Apply middleware to protect routes: Only users with the required role will be able to access certain routes.

38. How do you test Express routes using Supertest?

To test Express routes using Supertest, you can simulate HTTP requests to your Express application and check the responses. Supertest provides a simple API for making requests and verifying the behavior of your routes.

Steps to Test Express Routes Using Supertest:

• Install Supertest: You need to install supertest and Mocha (or any other testing framework, such as Jest or Chai).
• Write the test: Use Supertest to test your Express routes by sending requests and validating the responses.
• Run the Tests: Run your tests using Mocha or your preferred testing framework.

39. How do you modularize your Express app for scalability?

To modularize your Express app for scalability, you can separate different aspects of your application (like routes, controllers, models, and middleware) into other files or modules. This keeps the code organized, making it easier to maintain and scale as your application grows.

Directory Structure Example:

/my-express-app
/controllers
userController.js
/models
userModel.js
/routes
userRoutes.js
/middleware
authMiddleware.js
errorHandler.js
/config
dbConfig.js
app.js

40. How do you prevent DDoS in Express?

To prevent Distributed Denial-of-Service (DDoS) attacks in an Express application, you can implement various techniques and use middleware to help mitigate the effects of DDoS. While a full solution might require infrastructure (e.g., load balancers, CDN services), there are steps you can take directly within your Express application.

1. Rate Limiting

One of the simplest ways to prevent DDoS attacks is by limiting the number of requests a client can make within a specific time period (e.g., 100 requests per minute).

You can use the express-rate-limit middleware to achieve this.

• Install express-rate-limit: npm install express-rate-limit

Example:

const express = require('express');
const rateLimit = require('express-rate-limit');
const app = express();
// Set up rate limiting
const limiter = rateLimit({
windowMs: 15 * 60 * 1000, // 15 minutes
max: 100, // limit each IP to 100 requests per windowMs
message: 'Too many requests from this IP, please try again after 15 minutes'
});
// Apply rate limiting to all routes
app.use(limiter);
app.get('/', (req, res) => {
res.send('Hello World!');
});
app.listen(3000, () => {
console.log('Server is running on http://localhost:3000');
});

2. Use a Web Application Firewall (WAF)

A WAF can be used to block malicious requests, including DDoS attacks. Some cloud services (like AWS, Cloudflare, and Azure) provide WAFs to mitigate attacks before they reach your server.

• Cloudflare offers DDoS protection as part of its CDN services.
• AWS WAF can be set up to detect and mitigate large-scale DDoS attacks.

3. Blocking Suspicious IPs

You can blacklist IPs that send a large number of requests within a short period, especially if rate-limiting is insufficient.

4. Use CAPTCHA for Sensitive Routes

To prevent automated requests (often used in DDoS attacks), you can implement a CAPTCHA (e.g., reCAPTCHA) on specific forms or API endpoints.

• Google reCAPTCHA can be integrated into your application to verify that the request is coming from a real user rather than a bot.

41. Can you implement a global error handler using a class?

Yes, you can implement a global error handler in Express.js using a class. By defining a custom Error Handler Class, you can standardize error handling across your application. This approach enables you to create reusable error-handling logic, making it easier to manage errors at a global level.

Steps to Implement a Global Error Handler Using a Class:

• Create a custom error class to define error types and messages.
• Define an error-handling middleware that uses the custom error class.
• Use the middleware globally to catch errors.

42. What are some alternatives to Express.js?

There are several alternatives to Express.js that provide different features, optimizations, and paradigms. Here are some popular alternatives that you might consider based on your project requirements:

• Koa.js
• Fastify
• Hapi.js
• NestJS
• Sapper/SvelteKit
• Restify
• Meteor.js
• AdonisJS

Did You Know? 🔍

Express.js is the fifth most popular web framework, used by 17% of developers. Over 53% of current Express.js users expressed a desire to continue working with it. (Source: Stack Overflow Developer Survey)

43. How do you handle file uploads in Express.js?

To handle file uploads in Express.js, you can use the multer middleware, which is designed to handle multipart/form-data (the type of form used for file uploads). Multer provides an easy way to store files on the server and access them in your route handlers.

Steps to Handle File Uploads in Express.js:

• Install Multer
• Set Up Multer in Your App
• Create a File Upload Route

44. How do you debug Express.js applications?

Debugging Express.js applications can be accomplished in various ways, ranging from simple console.log statements to more advanced tools such as debuggers and logging libraries. Here’s a breakdown of methods you can use to debug and monitor your Express.js applications efficiently.

Using console.log for Debugging

The most straightforward method for debugging is using console.log to print out values, track function calls, or log errors.

app.get('/user', (req, res) => {
console.log('User route hit');
console.log('Request Params:', req.params);
res.send('User Profile');
});

Using the debug Module

const express = require('express');
const debug = require('debug')('app:server');  // Create a custom namespace
const app = express();
app.get('/', (req, res) => {
debug('Request received at /');
res.send('Hello World');
});
app.listen(3000, () => {
debug('Server started on port 3000');
});

45. How to ensure performance in a high-traffic Express.js app?

Ensuring performance in a high-traffic Express.js app requires optimizing various aspects of your application to handle a large number of concurrent requests efficiently. Here are some best practices and strategies for improving performance and scalability:

• Enable HTTP Caching
• Use a Content Delivery Network (CDN)
• Implement Rate Limiting
• Use Asynchronous Code
• Database Optimization
• Use Clustering and Load Balancing
• Implement GZIP Compression
• Minimize Middleware
• Optimize Static File Serving
• Use Connection Keep-Alive

Boost your career with our Full Stack Developer - MERN Stack Master's program! Gain in-depth expertise in development and testing with the latest technologies. Enroll today and become a skilled MERN Stack Developer!

10 Tips to Prepare for Express JS Interview

1. Understand Express.js basics: Routing, middleware, and request/response handling.
2. Master error handling in Express.js and be familiar with template engines and how they integrate with Express.
3. Learn how to create and manage RESTful APIs using Express.
4. Get comfortable with middleware usage, both built-in and custom.
5. Know how to handle file uploads using multer in Express.
6. Understand how to implement authentication and authorization in Express apps.
7. Learn about query parameters and how to handle them.
8. Familiarize yourself with promises/async/await for asynchronous programming in Express.
9. Practice testing Express routes using tools like Supertest.
10. Get hands-on with database integrations (e.g., MongoDB, PostgreSQL) in Express.

Conclusion

Mastering the key concepts covered in these interview questions on Express.js will be essential for showcasing your expertise in building robust web applications and APIs. Whether you're starting or aiming to deepen your knowledge, understanding routing, middleware, error handling, and more will help you stand out.

If you're looking to take your skills to the next level and become a well-rounded full-stack developer, consider enrolling in the Full Stack (MERN Stack) Developer Master's Program. This comprehensive program covers Express.js, MongoDB, React, and Node.js, equipping you with the practical skills needed to excel in the modern development landscape.