Express.js Interview Questions and Answers for freshers

1. What is Express.js?

   • Answer: Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for building web and mobile applications. It's known for its simplicity, speed, and middleware support.

2. How does Express.js handle HTTP requests?

   • Answer: Express.js uses middleware functions to handle incoming HTTP requests. These functions have access to the request and response objects, allowing you to perform actions like parsing data, authenticating users, and sending responses.

3. Explain the concept of middleware in Express.js.

   • Answer: Middleware functions 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. They can perform various tasks such as logging, authentication, and data parsing before the request reaches the final route handler.

4. What are the different HTTP methods supported by Express.js?

   • Answer: Express.js supports all standard HTTP methods, including GET, POST, PUT, DELETE, PATCH, HEAD, OPTIONS, etc. These methods define the type of operation being performed on a resource.

5. How do you define routes in Express.js?

   • Answer: Routes are defined using the `app.METHOD(path, handler)` method, where METHOD is the HTTP method (e.g., GET, POST), path is the URL path, and handler is a function that processes the request.

6. Explain the use of `req` and `res` objects in Express.js.

   • Answer: `req` (request) object contains information about the incoming HTTP request, such as headers, body, parameters, etc. `res` (response) object is used to send the response back to the client, including status codes, headers, and body.

7. What is the purpose of `app.use()` in Express.js?

   • Answer: `app.use()` is used to register middleware functions. It can be used to apply middleware to all requests or to specific routes.

8. How do you handle errors in Express.js?

   • Answer: Errors can be handled using error-handling middleware. This middleware is placed after all other middleware and route handlers. It takes four arguments (err, req, res, next) and should handle the error appropriately.

9. What are Express.js templates? Name some popular template engines.

   • Answer: Express.js templates are used to dynamically generate HTML responses. Popular template engines include Pug (formerly Jade), EJS, Handlebars, and Mustache.

10. How do you install Express.js?

    • Answer: Use npm (Node Package Manager): `npm install express`

11. Explain the difference between `app.get()` and `app.post()`.

    • Answer: `app.get()` handles GET requests, typically used for retrieving data. `app.post()` handles POST requests, typically used for submitting data to the server.

12. How do you send a JSON response in Express.js?

    • Answer: Use `res.json(data)` where `data` is the JavaScript object you want to send as a JSON response.

13. What is a route parameter in Express.js, and how is it accessed?

    • Answer: A route parameter is a dynamic part of a URL path. It's accessed using `req.params.`.

14. How do you handle query parameters in Express.js?

    • Answer: Query parameters are accessed using `req.query.`.

15. Explain the concept of request body parsing in Express.js.

    • Answer: Request body parsing is the process of extracting data from the request body (e.g., data sent in a POST request). Middleware like `body-parser` is typically used for this purpose.

16. What is the purpose of the `next()` function in middleware?

    • Answer: `next()` function passes control to the next middleware function in the stack. If it's not called, the request processing stops at that middleware.

17. How do you set headers in an Express.js response?

    • Answer: Use `res.set('Header-Name', 'Header-Value')` or `res.setHeader('Header-Name', 'Header-Value')`.

18. What is the difference between `res.send()` and `res.json()`?

    • Answer: `res.send()` sends a plain text or HTML response. `res.json()` sends a JSON response.

19. How do you handle static files (like images, CSS, JS) in Express.js?

    • Answer: Use `express.static()` middleware to serve static files from a specified directory.

20. Explain how to create a simple REST API using Express.js.

    • Answer: Define routes for different HTTP methods (GET, POST, PUT, DELETE) to handle CRUD operations on a resource. Use appropriate middleware for request body parsing and error handling.

21. What are some common security considerations when building an Express.js application?

    • Answer: Input validation, output encoding, proper authentication and authorization, using HTTPS, protecting against common vulnerabilities like SQL injection and cross-site scripting (XSS).

22. How can you use environment variables in your Express.js application?

    • Answer: Use the `process.env` object to access environment variables.

23. What are some popular testing frameworks for Express.js applications?

    • Answer: Supertest, Mocha, Jest.

24. What is the purpose of the `app.listen()` method?

    • Answer: `app.listen()` starts the Express.js server and listens for incoming connections on a specified port.

25. How do you handle 404 (Not Found) errors in Express.js?

    • Answer: Create a middleware function that handles requests that don't match any defined routes. This middleware should typically send a 404 response.

26. What is the difference between a GET and a POST request?

    • Answer: GET requests retrieve data from the server, while POST requests send data to the server to create or update a resource. GET requests are typically idempotent (performing them multiple times has the same effect), while POST requests are not.

27. What is the role of a router in Express.js?

    • Answer: A router is an instance of `express.Router()` that allows you to organize routes into modules and mount them to the main application.

28. How do you use a router in Express.js?

    • Answer: Create a router using `const router = express.Router()`, define routes on the router, and then mount it to the app using `app.use('/path', router)`.

29. What is a middleware stack in Express.js?

    • Answer: A middleware stack is the sequence of middleware functions that are executed for each request. The order in which they are registered is crucial.

30. How do you handle asynchronous operations within Express.js middleware?

    • Answer: Use promises or async/await to handle asynchronous operations within middleware. Make sure to call `next()` after the asynchronous operation is complete.

31. What is the purpose of the `req.body` object?

    • Answer: `req.body` contains the parsed data from the request body (e.g., data submitted in a POST request). You need body-parsing middleware (like `body-parser`) to populate this object.

32. How do you redirect a user to a different URL in Express.js?

    • Answer: Use `res.redirect(url)`.

33. What are some best practices for writing clean and maintainable Express.js code?

    • Answer: Use consistent naming conventions, organize code into modules, use routers, write unit tests, handle errors gracefully, follow DRY (Don't Repeat Yourself) principles.

34. How can you improve the performance of an Express.js application?

    • Answer: Use caching, optimize database queries, use a load balancer, use efficient middleware, profile your application to identify bottlenecks.

35. What is CORS and how do you handle it in Express.js?

    • Answer: CORS (Cross-Origin Resource Sharing) is a security mechanism that restricts requests from different origins. You can handle it in Express.js using the `cors` middleware to allow specific origins.

36. How do you implement authentication in Express.js?

    • Answer: Use middleware to verify user credentials (e.g., using Passport.js or a custom authentication system) and protect routes based on user roles or permissions.

37. What are some common HTTP status codes and their meanings?

    • Answer: 200 (OK), 201 (Created), 400 (Bad Request), 401 (Unauthorized), 404 (Not Found), 500 (Internal Server Error).

38. How do you debug an Express.js application?

    • Answer: Use console logging, debugging tools in your IDE, and logging libraries (like Winston or Bunyan).

39. What is the difference between `app.get()` and `app.param()`?

    • Answer: `app.get()` defines a route handler for GET requests. `app.param()` defines middleware that runs before route handlers, allowing you to perform actions on route parameters before they're used.

40. What is the role of the `next()` function in error-handling middleware?

    • Answer: In error-handling middleware, `next()` is not typically used because it would pass the error to subsequent middleware, which is usually not desired. Instead, the error should be handled within the error-handling middleware itself.

41. How can you handle multiple middleware functions for a single route?

    • Answer: By passing multiple functions as arguments to `app.METHOD()`.

42. Explain the importance of input validation in Express.js.

    • Answer: Input validation helps prevent security vulnerabilities (like SQL injection or XSS) and improves the overall robustness of your application by ensuring data integrity.

43. How would you structure a large Express.js application?

    • Answer: By dividing the application into modules (based on functionality), using routers to organize routes, and adhering to a well-defined folder structure. Consider using a design pattern like MVC (Model-View-Controller).

44. What are some ways to improve the scalability of an Express.js application?

    • Answer: Use a load balancer, employ techniques like horizontal scaling (adding more servers), utilize caching mechanisms, optimize database queries, and use asynchronous operations where appropriate.

45. How do you implement logging in an Express.js application?

    • Answer: Use logging middleware like `morgan` or dedicated logging libraries like `winston` or `bunyan` to record requests, errors, and other relevant events.

46. What is the difference between a regular middleware and an error-handling middleware?

    • Answer: Regular middleware takes three arguments (`req`, `res`, `next`). Error-handling middleware takes four arguments (`err`, `req`, `res`, `next`), and it's specifically designed to handle errors that occur in other middleware or route handlers.

47. How do you deploy an Express.js application?

    • Answer: Various methods exist, including deploying to platforms like Heroku, AWS, Google Cloud, or using PM2 (a process manager) for local or server deployment.

48. What are some tools you can use for performance monitoring and profiling of your Express.js application?

    • Answer: Various tools exist, depending on your deployment environment. Examples include tools provided by your cloud provider (like AWS X-Ray or Google Cloud Profiler) and application performance monitoring (APM) services.

49. How can you secure your API keys and sensitive information in an Express.js application?

    • Answer: Store them in environment variables, utilize secrets management services provided by your cloud provider, and avoid hardcoding them directly into the code.

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

    • Answer: Use middleware like `multer` to handle file uploads and store the uploaded files in a designated directory.

51. Describe your experience with testing in the context of Express.js.

    • Answer: [Describe your experience, even if limited, focusing on any testing frameworks used and the types of tests written. If you have no experience, explain your understanding of the importance of testing and the testing process.]

52. What are some of the limitations of Express.js?

    • Answer: It's a minimalist framework, so it may require additional libraries for more advanced features. For very large and complex applications, a more full-featured framework might be considered.

53. What are some alternatives to Express.js?

    • Answer: NestJS, Koa.js, Fastify, Hapi.js

54. Explain your understanding of RESTful API design principles.

    • Answer: [Explain your understanding of constraints like using appropriate HTTP methods (GET, POST, PUT, DELETE), resource-based URLs, statelessness, client-server architecture, caching, etc.]

55. How would you handle rate limiting in your Express.js application?

    • Answer: Use middleware designed for rate limiting, such as `rate-limit` or implement a custom solution using a cache (like Redis) to track requests.

56. How would you approach designing an API for a specific scenario? (Example: An e-commerce API for products).

    • Answer: [Provide a detailed outline of the API endpoints, HTTP methods, data structures, and considerations for security and scalability for the e-commerce product example or a similar scenario.]

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