Express.js Interview Questions and Answers

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 extensibility.

2. How does Express.js handle routing?

   • Answer: Express.js uses middleware functions and HTTP methods (GET, POST, PUT, DELETE, etc.) to define routes. Routes map specific HTTP requests to specific functions that handle those requests. It uses `app.METHOD(path, handler)` where METHOD is the HTTP verb and path specifies the URL.

3. Explain 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 tasks like logging, authentication, authorization, and data parsing before the request reaches the final route handler.

4. What is the difference between `app.get()` and `app.post()`?

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

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

   • Answer: Error handling in Express.js is typically done using error-handling middleware. This middleware is placed after other middleware functions and is designed to catch errors thrown by previous middleware or route handlers. It usually takes four arguments: (err, req, res, next).

6. Explain the use of `req` and `res` objects.

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

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

   • Answer: The `next()` function is a callback function passed to middleware. Invoking `next()` passes control to the next middleware function in the stack. If `next()` is not called, the request is halted at that middleware.

8. How to serve static files in Express.js?

   • Answer: Use the `express.static()` middleware. It takes the path to the directory containing static files as an argument. For example: `app.use(express.static('public'))` will serve files from the 'public' directory.

9. What are template engines and how are they used with Express.js?

   • Answer: Template engines like EJS, Pug, or Handlebars allow you to generate dynamic HTML content using templates. Express.js integrates with them using `app.set('view engine', 'engineName')` and `res.render('templateName', data)` to render templates with data.

10. How do you handle POST request body data in Express.js?

    • Answer: Use a body-parsing middleware like `body-parser` (deprecated, use express.json() and express.urlencoded()) to parse the request body into a JavaScript object. `express.json()` for JSON data and `express.urlencoded()` for url-encoded data.

11. What is a RESTful API? How can you build one with Express.js?

    • Answer: A RESTful API follows REST architectural constraints (like using HTTP methods for CRUD operations). You build one in Express.js by defining routes that map HTTP methods (GET, POST, PUT, DELETE) to specific resources, using appropriate HTTP status codes and adhering to REST principles.

12. Explain the use of environment variables in Express.js.

    • Answer: Environment variables provide a way to configure your application without modifying the code. You access them using `process.env.VARIABLE_NAME`. This is crucial for managing sensitive information like database credentials.

13. How can you implement authentication in Express.js?

    • Answer: Use middleware like Passport.js or build custom authentication mechanisms using sessions, JWTs (JSON Web Tokens), or other authentication strategies. These middleware verify user identities and grant access to protected routes.

14. How to use sessions in Express.js?

    • Answer: Use session middleware (like `express-session`) to manage user sessions. Sessions store data on the server associated with a user’s browser, allowing you to track user login status and other session-specific information.

15. What are the benefits of using a reverse proxy with Express.js?

    • Answer: A reverse proxy (like Nginx or Apache) sits in front of your Express.js app, handling tasks like load balancing, SSL termination, caching, and security. It improves performance, security, and scalability.

16. How to test Express.js applications?

    • Answer: Use testing frameworks like Jest, Mocha, or Supertest to write unit and integration tests for your routes and middleware. Supertest is particularly useful for testing HTTP requests and responses.

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

    • Answer: `app.use()` registers middleware functions that are executed for all requests (or for a specific path if a path is provided). `app.get()` registers a route handler specifically for GET requests to a particular path.

18. How to handle different HTTP methods (PUT, DELETE, PATCH) in Express.js?

    • Answer: Use `app.put()`, `app.delete()`, and `app.patch()` respectively. These are similar to `app.get()` and `app.post()` but handle the specific HTTP methods.

19. Explain the concept of routing parameters in Express.js.

    • Answer: Routing parameters allow you to create dynamic routes that handle different URLs. They are defined using colons (:) in the route path (e.g., `/users/:id`). The value of the parameter is accessible in the request object (req.params).

20. How to deploy an Express.js application?

    • Answer: Several options are available: using platforms like Heroku, Netlify, AWS, Google Cloud, or setting up your own server using PM2 or similar process managers.

21. What is the role of a process manager in deploying Express.js applications?

    • Answer: A process manager like PM2 handles starting, stopping, restarting, and monitoring your Node.js application. It ensures the application runs reliably and handles crashes gracefully.

22. How to secure an Express.js application?

    • Answer: Employ various security measures: input validation, output encoding, using HTTPS, implementing proper authentication and authorization, regularly updating dependencies, using a web application firewall (WAF), and following secure coding practices.

23. What are some common security vulnerabilities in Express.js applications and how to prevent them?

    • Answer: Common vulnerabilities include SQL injection (prevent using parameterized queries or prepared statements), cross-site scripting (XSS) (prevent using output encoding and input sanitization), and cross-site request forgery (CSRF) (prevent using CSRF tokens).

24. Explain how to handle file uploads in Express.js.

    • Answer: Use middleware like `multer` to handle file uploads. It allows you to easily parse multipart/form-data requests and save uploaded files to the server.

25. How to implement rate limiting in Express.js?

    • Answer: Use rate-limiting middleware like `rate-limit` to prevent abuse of your API by limiting the number of requests from a single IP address or user within a specific time frame.

26. How to use Express.js with a database (e.g., MongoDB)?

    • Answer: Use a database driver (like `mongoose` for MongoDB) to interact with your database. Your Express.js routes will use the driver to perform database operations (CRUD).

27. What are some popular middleware packages for Express.js?

    • Answer: `morgan` (logging), `helmet` (security), `cors` (Cross-Origin Resource Sharing), `cookie-parser`, `compression`, and many more.

28. Explain the concept of middleware stacks in Express.js.

    • Answer: Middleware functions are executed in the order they are added to the application. This creates a stack, where each middleware can modify the request or response before passing it to the next middleware or the final route handler.

29. How to implement logging in Express.js?

    • Answer: Use a logging middleware like `morgan` to log HTTP requests and other relevant information to the console or a file. This helps with debugging and monitoring.

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

    • Answer: `app.listen()` starts the Express.js server and begins listening for incoming HTTP requests on the specified port and hostname (or IP address).

31. How to handle redirects in Express.js?

    • Answer: Use `res.redirect()` to redirect the client to a different URL. You can specify the status code (301, 302, etc.) to indicate the type of redirect.

32. What is the difference between a GET and a HEAD request?

    • Answer: A GET request retrieves the resource's content, while a HEAD request only retrieves the headers of the response. HEAD is often used for checking resource existence or metadata.

33. How to create a simple Express.js server?

    • Answer: `const express = require('express'); const app = express(); app.get('/', (req, res) => res.send('Hello World!')); app.listen(3000, () => console.log('Server listening on port 3000'));`

34. Explain the concept of request parameters versus query parameters.

    • Answer: Request parameters are part of the URL path (e.g., `/users/123`, where `123` is a request parameter). Query parameters are appended to the URL after a question mark (e.g., `/users?name=John&age=30`).

35. How to access query parameters in Express.js?

    • Answer: Access query parameters using `req.query`. For example, `req.query.name` would access the value of the `name` query parameter.

36. How to set headers in the response in Express.js?

    • Answer: Use `res.set()` or `res.setHeader()` to set headers in the response. For example: `res.set('Content-Type', 'application/json');`

37. How to send a JSON response in Express.js?

    • Answer: Use `res.json()` to send a JSON response. It automatically sets the `Content-Type` header to `application/json`.

38. How to send a file as a response in Express.js?

    • Answer: Use `res.sendFile()` to send a file as a response. Provide the path to the file.

39. How to handle 404 errors in Express.js?

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

40. What are the best practices for structuring an Express.js application?

    • Answer: Organize your code into separate modules for routes, middleware, models, etc. Use a clear directory structure and follow consistent coding conventions.

41. How to use Express.js with other frameworks or libraries (e.g., React)?

    • Answer: Express.js serves as the backend API server. The frontend (e.g., React) makes HTTP requests to the Express.js API to fetch and send data.

42. What are some common patterns used in Express.js applications?

    • Answer: Common patterns include middleware chains, route handlers, MVC (Model-View-Controller) architecture, and RESTful API design.

43. Explain the concept of request lifecycle in Express.js.

    • Answer: The request lifecycle starts when a request arrives at the server and ends when a response is sent back to the client. Middleware and route handlers participate in this cycle.

44. How to improve the performance of an Express.js application?

    • Answer: Optimize database queries, use caching, utilize compression, minimize unnecessary middleware, use efficient template engines, and optimize the server configuration.

45. What are some tools for debugging Express.js applications?

    • Answer: Use Node.js debuggers, logging statements, browser developer tools, and logging middleware like `morgan` to trace the flow of requests and identify problems.

46. How to handle asynchronous operations in Express.js?

    • Answer: Use Promises, async/await, or callbacks to handle asynchronous operations like database queries or API calls. Avoid blocking the main thread.

47. What are some alternatives to Express.js?

    • Answer: Other Node.js frameworks like NestJS, Koa.js, Fastify, and Meteor.

48. How to gracefully shut down an Express.js server?

    • Answer: Use event listeners (like `SIGTERM` and `SIGINT`) to handle termination signals and gracefully close connections before shutting down the server.

49. How to implement WebSocket functionality in an Express.js application?

    • Answer: Use libraries like `ws` or `socket.io` to add real-time bidirectional communication capabilities to your application.

50. Explain the difference between using `app.use()` with a path and without a path.

    • Answer: `app.use()` without a path applies the middleware to all routes. `app.use('/path')` applies the middleware only to routes starting with `/path`.

51. How to handle form submissions in Express.js?

    • Answer: Use `body-parser` (or `express.urlencoded()`) to parse form data. Handle the submitted data in your route handler.

52. What is the role of `app.engine()`?

    • Answer: `app.engine()` registers a template engine with Express.js, allowing you to use that engine for rendering views.

53. How to implement basic authentication in Express.js?

    • Answer: Basic authentication can be implemented using the `basic-auth` middleware or a custom implementation that compares username and password credentials.

54. How to use HTTPS with Express.js?

    • Answer: Use a reverse proxy like Nginx to handle SSL termination or configure Express.js with an SSL certificate (usually obtained from a trusted Certificate Authority).

55. What is the purpose of the `res.status()` method?

    • Answer: `res.status()` sets the HTTP status code of the response.

56. How to use Express.js with Docker?

    • Answer: Create a Dockerfile that installs Node.js and your application dependencies and runs your Express.js application. Then use Docker to build and run the container.

57. How to implement CORS in Express.js?

    • Answer: Use the `cors` middleware to set appropriate CORS headers in your response to allow requests from specific origins.

58. What is the difference between `app.set()` and `app.get()`?

    • Answer: `app.set()` sets application settings (e.g., view engine, port), while `app.get()` defines a route handler for GET requests.

59. How to programmatically close an Express.js server?

    • Answer: Call the `server.close()` method after gracefully handling any pending operations.

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