ExpressJs Interview Questions & Answers

1. What is Express.js?

   • Answer: Express.js is a web application framework for Node.js, designed to build web applications and APIs quickly and with minimal code.

2. Explain middleware in Express.js.

   • Answer: Middleware in Express.js are functions that have access to the request, response, and the next middleware function in the application's request-response cycle.

3. How does routing work in Express.js?

   • Answer: Routing in Express.js involves defining routes for specific HTTP methods and URL patterns, and associating them with functions (handlers) that execute when the route is matched.

4. What is the purpose of the app.use() function in Express.js?

   • Answer: app.use() is used to mount middleware functions in the Express application, specifying the middleware to be executed for every request.

5. How can you handle GET and POST requests in Express.js?

   • Answer: GET and POST requests can be handled using the app.get() and app.post() methods, respectively, defining routes and providing handler functions.

6. What is the purpose of the express.Router class?

   • Answer: express.Router is a class in Express.js that allows you to create modular route handlers, providing a way to organize routes and middleware in a separate file.

7. Explain the difference between app.get() and app.all() in Express.js.

   • Answer: app.get() is used to handle HTTP GET requests, while app.all() is a method that matches any HTTP method (GET, POST, etc.) and can be used for any route.

8. What is the role of the req and res objects in Express.js?

   • Answer: The req object represents the HTTP request and contains information about the request, while the res object represents the HTTP response and is used to send the response to the client.

9. Explain the concept of middleware chaining in Express.js.

   • Answer: Middleware functions can be chained together using app.use() or method-specific middleware functions (app.get(), app.post()). They execute in the order they are defined.

10. How can you handle static files in Express.js?

    • Answer: Static files can be served using the built-in express.static middleware. For example, app.use(express.static('public')) will serve files from the public directory.

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

    • Answer: body-parser is used to parse incoming request bodies in middleware before handlers, making data available under the req.body property.

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

    • Answer: Errors in Express.js can be handled using middleware functions with four parameters (err, req, res, next). These middleware functions are defined after the regular routes.

13. What is the significance of the app.listen() method in Express.js?

    • Answer: app.listen() is used to bind and listen for connections on a specified port. It starts the Express.js application and makes it available to handle requests.

14. Explain the concept of view engines in Express.js.

    • Answer: View engines in Express.js are responsible for rendering dynamic content on the server. Examples include EJS, Handlebars, and Pug.

15. How can you set up route parameters in Express.js?

    • Answer: Route parameters are defined using a colon : followed by the parameter name in the route pattern. For example, app.get('/users/:id', handler).

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

    • Answer: The next() function is used to pass control to the next middleware function in the stack. It is called within a middleware function to proceed to the next middleware or route handler.

17. How can you handle cookies in Express.js?

    • Answer: Cookies can be handled in Express.js using the cookie-parser middleware, which parses the Cookie header and populates req.cookies.

18. What is the role of the app.route() method in Express.js?

    • Answer: app.route() is used to create a chainable route handler for a single route. It is a cleaner way to define multiple route handlers for a single route.

19. How do you perform routing in Express.js using the MVC (Model-View-Controller) pattern?

    • Answer: Routes can be organized into controllers, where each controller handles a specific set of routes. This follows the MVC pattern for better code organization.

20. What is the purpose of the app.locals object in Express.js?

    • Answer: app.locals is an object in Express.js that provides a way to pass variables to views. Variables set using app.locals are available in all views rendered by the application.

21. Explain the concept of middleware error handling in Express.js.

    • Answer: Middleware error handling in Express.js involves defining a middleware function with four parameters (err, req, res, next) and using it after regular routes to catch errors and respond appropriately.

22. How can you implement session management in Express.js?

    • Answer: Session management in Express.js can be implemented using middleware like express-session to store and manage user sessions.

23. What is the role of the express.json() middleware in Express.js?

    • Answer: express.json() is middleware that parses incoming JSON requests, making the parsed data available under req.body.

24. How can you set up route middleware in Express.js?

    • Answer: Route middleware can be set up using the app.use() method. For example, app.use('/admin', adminMiddleware) applies adminMiddleware only to routes under the /admin path.

25. What is the purpose of the app.route() method in Express.js?

    • Answer: app.route() is used to create a chainable route handler for a single route. It is a cleaner way to define multiple route handlers for a single route.

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

    • Answer: File uploads in Express.js can be handled using the multer middleware, which processes multipart/form-data forms and stores uploaded files.

27. Explain the concept of RESTful routing in Express.js.

    • Answer: RESTful routing in Express.js follows the principles of Representational State Transfer (REST), using standard HTTP methods (GET, POST, PUT, DELETE) to perform CRUD operations on resources.

28. What is the significance of the express.Router class in Express.js?

    • Answer: express.Router allows you to create modular route handlers in separate files or modules, making it easier to organize and maintain a large application.

29. How can you handle CORS (Cross-Origin Resource Sharing) in Express.js?

    • Answer: CORS can be handled in Express.js using the cors middleware

, which sets the appropriate headers to allow or restrict cross-origin requests.

30. Explain the purpose of the app.param() method in Express.js.

    • Answer: app.param() is used to define middleware functions for specific route parameters, allowing you to perform actions or validation based on parameter values.

31. How can you implement HTTPS in an Express.js application?

    • Answer: HTTPS in Express.js can be implemented by creating an HTTPS server using the https module and passing it the Express application.

32. What is the role of the express.urlencoded() middleware in Express.js?

    • Answer: express.urlencoded() is middleware that parses incoming URL-encoded data (form submissions) and makes the parsed data available under req.body.

33. Explain the concept of clustering in Express.js.

    • Answer: Clustering in Express.js involves creating a cluster of worker processes to handle incoming requests, improving application performance and scalability.

34. How can you secure an Express.js application?

    • Answer: Express.js applications can be secured by using HTTPS, implementing proper session management, validating user input, handling CORS, and using security middleware.

35. What is the purpose of the app.locals object in Express.js?

    • Answer: app.locals is an object in Express.js that provides a way to pass variables to views. Variables set using app.locals are available in all views rendered by the application.

36. How do you handle validation in Express.js?

    • Answer: Validation in Express.js can be handled using middleware functions that check and validate request data, ensuring it meets the required criteria.

37. Explain the concept of Express.js middleware error handling.

    • Answer: Middleware error handling in Express.js involves defining a middleware function with four parameters (err, req, res, next) and using it after regular routes to catch errors and respond appropriately.

38. What is the purpose of the express.static middleware in Express.js?

    • Answer: express.static is middleware in Express.js that serves static files, such as images, CSS, and JavaScript files, from a specified directory.

39. How can you implement WebSocket communication in Express.js?

    • Answer: WebSocket communication in Express.js can be implemented using libraries like socket.io. It allows bidirectional communication between the server and the client.

40. Explain the concept of cookie-parser in Express.js.

    • Answer: cookie-parser is middleware in Express.js that parses the Cookie header and populates req.cookies with an object containing the parsed cookies.

41. How can you set up a reverse proxy in Express.js?

    • Answer: A reverse proxy in Express.js can be set up using the http-proxy-middleware middleware to forward requests to another server or endpoint.

42. What is the significance of the morgan middleware in Express.js?

    • Answer: morgan is middleware in Express.js used for logging HTTP requests. It provides information about each incoming request, including the request method, status code, and response time.

43. How do you implement rate limiting in Express.js?

    • Answer: Rate limiting in Express.js can be implemented using middleware like express-rate-limit to control the number of requests a client can make within a specified time frame.

44. Explain the concept of templating engines in Express.js.

    • Answer: Templating engines in Express.js, such as EJS or Pug, are used to dynamically generate HTML content on the server, allowing for dynamic and data-driven web pages.

45. How can you handle sessions in Express.js?

    • Answer: Sessions in Express.js can be handled using middleware like express-session. It provides a way to store and retrieve user session data.

46. What is the role of the express.json() middleware in Express.js?

    • Answer: express.json() is middleware that parses incoming JSON requests, making the parsed data available under req.body.

47. How can you implement Cross-Site Scripting (XSS) protection in Express.js?

    • Answer: XSS protection in Express.js can be implemented using middleware like helmet, which sets the appropriate headers to prevent XSS attacks.

48. Explain the concept of Express.js subapplications.

    • Answer: Subapplications in Express.js involve creating smaller Express instances and mounting them at specific paths, allowing for modularization and code organization.

49. How do you handle authentication in Express.js?

    • Answer: Authentication in Express.js can be handled using middleware, Passport.js, or other authentication libraries. User credentials are validated before allowing access to protected routes.

50. What is the purpose of the express.urlencoded() middleware in Express.js?

    • Answer: express.urlencoded() is middleware that parses incoming URL-encoded data (form submissions) and makes the parsed data available under req.body.