Express JS Interview Questions

Express.js is a web application framework built on top of Node.js that provides features and middleware to create scalable web apps. Express.js is used by developers to build server-side applications, APIs, and web applications. If you are preparing for a job interview, it’s important to have a solid understanding of its fundamental concepts and features. In this article, we will explore 30 interview questions that are commonly asked in Express.js interviews.

Express JS Interview Questions

1. What is middleware in Express.js and why is it useful?

Middleware refers to the functions that are executed between the request and response in an Express.js application. It can modify the request and response objects, or even terminate the request-response cycle. Middleware is useful because it allows you to add additional functionality to your application without having to modify the core functionality of Express.js.

2. How does Express.js handle errors?

Express.js provides error handling middleware that can be used to catch and handle errors that occur during the request-response cycle. If an error occurs in your code, you can call the next() function with an error object to pass it on to the error handling middleware.

3. What are the benefits of using the Router module in Express.js?

The Router module in Express.js allows you to organize your application into smaller, modular routes. This can make your code more maintainable and easier to read. Additionally, using the Router module can help you avoid conflicts between different parts of your application that might be using the same route.

4. What is the difference between res.send() and res.json() in Express.js?

res.send() sends a string or buffer as the response body, while res.json() sends a JSON-encoded response. If you pass an object or array to res.send(), it will automatically set the Content-Type header to application/json and send a JSON-encoded response.

5. What is the purpose of the next() function in Express.js?

The next() function in Express.js is used to pass control to the next middleware function in the pipeline. Middleware functions can modify the request and response objects, perform asynchronous operations, or terminate the request by sending a response to the client. The next() function allows middleware functions to delegate control to the next middleware function, ensuring that all middleware functions are executed in the order they were registered.

6. What is the difference between app.use() and app.get() in Express.js?

app.use() is used to specify middleware that should be executed for all HTTP methods, while app.get() is used to specify middleware that should only be executed for GET requests.

7. How can you handle authentication and authorization in Express.js?

There are several ways to handle authentication and authorization in Express.js, such as using session-based authentication or token-based authentication with JSON Web Tokens (JWTs). You can also use middleware like Passport.js to handle authentication and authorization.

8. What is the difference between synchronous and asynchronous functions in Express.js?

Synchronous functions in Express.js will block the event loop until they complete, while asynchronous functions will allow other code to execute while they are running. Asynchronous functions in Express.js are typically preferred, as they allow your application to handle more requests at the same time.

9. What is a Promise in Express.js?

A Promise in Express.js is a way to handle asynchronous operations. It represents the eventual completion or failure of an asynchronous operation and allows you to chain multiple asynchronous operations together in a readable way.

10. What are the benefits of using a templating engine in Express.js?

Using a templating engine like EJS or Pug in Express.js allows you to separate your application logic from your HTML markup, making your code more maintainable and easier to read. Templating engines also allow you to reuse HTML markup across multiple pages, reducing duplication and making your code more efficient.

11. What is the purpose of the express.Router class in Express.js?

The express.Router class is a middleware function that provides a way to organize routes into modular handlers that can be mounted to a specific path in the application. This can be useful for creating multiple sets of routes that can be reused or maintained independently.

The express.Router class is used to create a new router object that can be used to define routes using methods like router.get(), router.post(), router.put(), etc. These routes can then be mounted to a specific path in the application using app.use(path, router). By organizing routes in this way, it can make the code easier to read, maintain, and test.

12. How can you handle file uploads in Express.js?

Express.js provides the multer middleware that can be used to handle file uploads. multer is a Node.js middleware that can be used to handle multipart/form-data requests, which are commonly used for file uploads. multer can be configured to accept different types of files, limit the size of files, and rename files before saving them to disk.

Once the file has been uploaded, it can be accessed in the request object and processed like any other data. multer is a flexible and powerful library for handling file uploads in Express.js.

13. What is the difference between app.route() and app.get() in Express.js?

app.route() is a method in Express.js that allows developers to chain multiple HTTP request handlers for a single path. It can be used to define multiple routes for the same URL path, but with different HTTP verbs, such as GET, POST, PUT, etc. app.get() is a method in Express.js that creates a new route for the GET HTTP verb. It is typically used to handle requests for a specific URL path.

In other words, app.get() is a shortcut for app.route().get(). Both app.route() and app.get() can be used to define routes in an Express.js application, but app.route() provides more flexibility and control over how routes are handled.

14. How can you implement authentication and authorization in Express.js?

Authentication and authorization are important aspects of web applications that help ensure that only authorized users have access to certain resources or features. There are many ways to implement authentication and authorization in Express.js, including using third-party packages like passport.js and jsonwebtoken, or implementing custom middleware that validates user credentials and checks permissions before allowing access to certain routes or resources.

passport.js is a popular Node.js middleware that provides a framework for implementing authentication strategies, such as OAuth, OpenID, and local authentication. jsonwebtoken is a library that can be used to generate and verify JSON Web Tokens (JWTs), which are commonly used for implementing token-based authentication in web applications.

15. Write a middleware function to log the incoming request method and URL to the console.

Code

function logRequest(req, res, next) {
  console.log(`${req.method} ${req.url}`);
  next();
}

16. Write a middleware function to set the X-Powered-By header to a custom value.

Code

function setPoweredByHeader(value) {
  return function(req, res, next) {
    res.setHeader('X-Powered-By', value);
    next();
  };
}

17. Write a middleware function to authenticate a user using a JWT token.

Code

const jwt = require('jsonwebtoken');

function authenticate(req, res, next) {
  const token = req.headers.authorization.split(' ')[1];
  jwt.verify(token, 'secret', function(err, decoded) {
    if (err) {
      return res.status(401).json({ message: 'Invalid token' });
    } else {
      req.user = decoded.user;
      next();
    }
  });
}

18. Write a route handler to retrieve all documents from a MongoDB collection and return them as a JSON response.

Code

const MongoClient = require('mongodb').MongoClient;

app.get('/documents', function(req, res) {
  MongoClient.connect('mongodb://localhost:27017', function(err, client) {
    if (err) {
      return res.status(500).json({ message: 'Database error' });
    }
    const db = client.db('mydb');
    db.collection('documents').find().toArray(function(err, result) {
      if (err) {
        return res.status(500).json({ message: 'Database error' });
      }
      res.json(result);
      client.close();
    });
  });
});

19. Write a route handler to insert a new document into a MongoDB collection and return its ID as a JSON response.

Code

const MongoClient = require('mongodb').MongoClient;

app.post('/documents', function(req, res) {
  MongoClient.connect('mongodb://localhost:27017', function(err, client) {
    if (err) {
      return res.status(500).json({ message: 'Database error' });
    }
    const db = client.db('mydb');
    db.collection('documents').insertOne(req.body, function(err, result) {
      if (err) {
        return res.status(500).json({ message: 'Database error' });
      }
      res.json({ id: result.insertedId });
      client.close();
    });
  });
});

20. Write a middleware function to serve static files from a specific directory.

Code

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

const app = express();

app.use('/static', express.static(path.join(__dirname, 'public')));

21. How can you implement routing in Express.js?

In Express.js, you can define routes using the app object’s HTTP method functions (app.get(), app.post(), app.put(), app.delete(), etc.). Each HTTP method function takes two arguments: the route path and the callback function that will be executed when the route is requested. The callback function can access the request and response objects and perform any necessary operations before sending a response back to the client.

22. What is the purpose of the body-parser middleware in Express.js?

The body-parser middleware is used to parse incoming request bodies in Express.js. It can handle various types of request bodies, such as JSON, text, and URL-encoded data. By default, Express.js does not parse request bodies, so the body-parser middleware must be used explicitly. Once parsed, the request body is available as the req.body object, which can be accessed in subsequent middleware or route handlers.

23. How can you implement authentication in an Express.js application?

Authentication can be implemented in Express.js using middleware functions that check the user’s credentials and grant access to protected routes. A common approach is to use JSON Web Tokens (JWTs), which are encrypted tokens that contain user information and can be verified on the server-side. The client sends the JWT in the Authorization header of each request, and the server verifies it using a secret key. If the JWT is valid, the user is granted access to the protected resource.

24. What are the advantages of using Express.js over other Node.js frameworks?

Express.js is one of the most popular Node.js frameworks due to its flexibility, simplicity, and performance. Its modular architecture allows developers to choose the components they need and avoid unnecessary overhead. Additionally, its middleware ecosystem provides a vast range of functionalities that can be easily integrated into an application. Finally, Express.js is highly extensible and customizable, allowing developers to build applications that meet their specific requirements.

25. How can you optimize the performance of an Express.js application?

There are several ways to optimize the performance of an Express.js application. One way is to use a caching mechanism to store frequently accessed data in memory, reducing the number of database queries and improving response times. Another way is to use compression middleware to compress response bodies and reduce the amount of data sent over the network. Additionally, using asynchronous programming techniques, such as promises or async/await, can improve the application’s responsiveness and reduce the number of blocked threads.

26. How can you handle concurrency in an Express.js application?

Concurrency can be handled in Express.js using various techniques, such as thread pooling or event-driven programming. Thread pooling involves creating a pool of worker threads that can handle incoming requests concurrently. This approach can improve the application’s throughput, but it can also increase its memory usage and overhead. Event-driven programming, on the other hand, involves using a single event loop that processes incoming requests asynchronously.

27. How can you secure an Express.js application against common web vulnerabilities?

Express.js provides various security mechanisms, such as rate limiting, input validation, and session management, to protect against common web vulnerabilities, such as cross-site scripting (XSS), SQL injection, and CSRF attacks. Additionally, using HTTPS for secure communication and implementing proper access control mechanisms, such as role-based access control (RBAC), can further enhance the application’s security.

28. How can you handle real-time communication in an Express.js application?

Real-time communication can be handled in Express.js using libraries such as Socket.IO, which provides a real-time, bidirectional communication channel between the server and the client. Socket.IO uses WebSockets and long-polling techniques to establish a persistent connection and send data in real-time.

29. How can you deploy an Express.js application to production?

Deploying an Express.js application to production involves several steps, such as configuring the environment, setting up a production database, and configuring the server. One approach is to use a containerization platform, such as Docker, to package the application and its dependencies into a portable image. The image can then be deployed to a cloud provider, such as AWS or Google Cloud, using a container orchestration tool, such as Kubernetes or Docker Swarm.

30. Write a function in Express.js that calculates the sum of two numbers passed as query parameters.

Code

app.get('/sum', (req, res) => {
  const { num1, num2 } = req.query;
  const sum = parseInt(num1) + parseInt(num2);
  res.send(`The sum of ${num1} and ${num2} is ${sum}.`);
});

This function handles a GET request to the /sum endpoint and extracts the num1 and num2 query parameters from the request object using object destructuring. It then parses the parameters as integers using the parseInt function and calculates their sum.

Conclusion:

In conclusion, by understanding these 30 interview questions, you will have a better understanding of the framework and be well-prepared to handle any questions that might come your way in an interview. Whether you are a beginner or an experienced developer, these questions will help you deepen your knowledge of Express.js and become a better developer.