1. What is Express.js?

Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for web and mobile applications. It facilitates building APIs and web servers.

2. What are the key features of Express?

Middleware Support
Robust Routing
Template Engines
High Performance
Minimalist Design

3. What is the difference between Node.js and Express.js?

Node.js: A runtime environment for executing JavaScript on the server.
Express.js: A framework built on top of Node.js to simplify web development.

4. What is "Middleware" in Express?

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.

5. What are the different types of Middleware?

Application-level: Bound to an instance of express().
Router-level: Bound to an instance of express.Router().
Error-handling: Middleware for handling errors (takes err as first argument).
Built-in: Provided by Express (e.g., express.json()).
Third-party: Installed via npm (e.g., morgan, cors).

6. What is the signature of an Error-handling middleware?

An error-handling middleware function always takes four arguments: (err, req, res, next).

7. What is "Routing" in Express?

The process of defining how an application responds to a client request to a particular endpoint, which is a URI (or path) and a specific HTTP request method (GET, POST, PUT, DELETE).

8. What is `express.Router()`?

A mini-Express application that only performs routing and middleware functions. It's used to organize routes into separate modules.

9. What is the purpose of `next()`?

A function that, when invoked, executes the next middleware function in the current stack. If it's not called, the request will be left hanging.

10. How do you handle static files in Express?

By using the built-in express.static() middleware: app.use(express.static('public'));.

11. What is `res.send()` vs `res.json()`?

res.send(): Sends a variety of response types (string, object, buffer).
res.json(): Specifically sends a JSON response (automatically sets the Content-Type header).

12. What is `res.render()`?

Used to render a view (template) using a configured template engine (like EJS, Pug, or Handlebars).

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

app.get(): Responds only to GET requests for a specific path.
app.use(): Mounts middleware for all HTTP methods and paths that start with the specified path.

14. What are "Route Parameters"?

Dynamic parts of a URI that are used to capture the values specified at their position in the URL (e.g., /user/:id). Accessed via req.params.

15. What are "Query Parameters"?

Key-value pairs appended to the URL after the ? symbol (e.g., /search?q=node). Accessed via req.query.

16. What is the purpose of `req.body`?

It contains the data submitted in the request body (e.g., from a form or JSON payload). It's empty by default and requires a parser like express.json().

17. What is "Scaffolding" in Express?

The process of creating a basic structure for an application using tools like express-generator.

18. What is the "Template Engine"?

A tool that allows you to use static template files in your application (e.g., EJS, Pug). At runtime, the template engine replaces variables in a template file with actual values and transforms the template into an HTML file sent to the client.

19. How do you implement "CORS" in Express?

By using the cors middleware: app.use(require('cors')());.

20. How do you implement "HTTPS" in Express?

By using the https module from Node.js and providing your SSL/TLS certificates (key and cert) to the https.createServer() method.

21. What is "Security" best practice in Express?

Use helmet for secure headers.
Rate-limiting (e.g., express-rate-limit).
Input validation (e.g., express-validator).
Avoid common vulnerabilities (XSS, CSRF).

22. What is "Clustering" in Express?

Using the Node.js cluster module to run multiple instances of your Express app across multi-core systems for load balancing.

23. What is "Body-Parser"?

(Historically a separate package, now built-in as express.json() and express.urlencoded()). It parses the incoming request body.

24. What is the process of handling asynchronous errors in Express (Pre-v5)?

You must catch the error in the async handler and pass it to next(err) to trigger the error-handling middleware.

25. What is the new error handling in Express v5?

Express v5 automatically handles errors rejected from async route handlers and middleware without needing explicit try...catch and next(err).

26. What is `res.redirect()`?

Redirects the client to a different URL with a specific status code (default is 302).

27. What is `res.end()`?

Ends the response process without sending any data.

28. What is the purpose of `app.set()`?

Used to assign a setting name to a value (e.g., app.set('view engine', 'ejs')).

29. What is the purpose of `app.locals`?

An object that stores variables that are local to the application and available in all templates rendered with res.render().

30. What is `res.locals`?

An object that stores variables that are local to the current request-response cycle and available only to the views rendered during that cycle.

31. What is "Morgan" middleware?

A popular HTTP request logger middleware for Node.js.

32. What is "Cookie-Parser"?

Middleware used to parse cookies attached to the client request.

33. What is "Express-Session"?

A middleware used to create and manage sessions in an Express application.

34. What is "Passport.js"?

A popular authentication middleware for Node.js that supports various strategies (local, OAuth, OpenID).

35. What is "Nodemon"?

A tool that automatically restarts a Node.js application when file changes are detected in the directory.

36. What is "Environment Variables" in Express?

Using a .env file and a library like dotenv to store sensitive information like port numbers and database URIs.

37. What is the `req.headers` object?

Contains the HTTP headers sent by the client.

38. What is the difference between `res.sendStatus()` and `res.status().send()`?

res.sendStatus(): Sets the status code and sends a string representation of the code (e.g., "OK" for 200).
res.status().send(): Sets the status code and then sends your custom message or data.

39. What is the purpose of "Gzip Compression" in Express?

To reduce the size of the response body, decreasing latency and improving performance (using compression middleware).

40. What is "Parameter Validation"?

Using libraries like joi or express-validator to ensure that incoming data meets specific requirements before processing.

41. What is "Route Chaining"?

A technique where you chain multiple handlers for a single route (e.g., app.route('/path').get(h1).post(h2)).

42. What is the "Event-driven" nature of Express?

Express is fundamentally based on Node's EventEmitter, allowing it to handle requests as events in an asynchronous loop.

43. What is the difference between "Mounting" and "Routing"?

Routing: Mapping a path to a handler.
Mounting: Attaching an entire router or middleware to a specific base path.

44. What is the "Full-Stack" context of Express?

Express is the "E" in the MEAN (MongoDB, Express, Angular, Node) and MERN (MongoDB, Express, React, Node) stacks.

45. What is the "Micro-framework" terminology?

Express is often called a micro-framework because it provides only the bare essentials, leaving other features (like database ORMs) to third-party libraries.

46. What is "Versioning" in APIs?

A strategy (like /v1/users vs /v2/users) to manage breaking changes in an API over time.

47. What is "Graceful Shutdown" in Express?

The process of closing the server and stopping the application only after all existing requests have been processed.

48. What is the `req.xhr` property?

A boolean that is true if the request's X-Requested-With header is "XMLHttpRequest".

49. What is the `app.on('mount')` event?

An event that is fired on a sub-app when it is mounted onto a parent app.

50. What is the "Express Ecosystem"?

The vast collection of community-built middleware and libraries that extend Express's functionality to handle almost any server-side task.

Express.js Interview Questions

Table of Contents

What is Express.js?

What are the key features of Express?

What is the difference between Node.js and Express.js?

What is "Middleware" in Express?

What are the different types of Middleware?

What is the signature of an Error-handling middleware?

What is "Routing" in Express?

What is <code>express.Router()</code>?

What is the purpose of <code>next()</code>?

How do you handle static files in Express?

What is <code>res.send()</code> vs <code>res.json()</code>?

What is <code>res.render()</code>?

What is the difference between <code>app.get()</code> and <code>app.use()</code>?

What are "Route Parameters"?

What are "Query Parameters"?

What is the purpose of <code>req.body</code>?

What is "Scaffolding" in Express?

What is the "Template Engine"?

How do you implement "CORS" in Express?

How do you implement "HTTPS" in Express?

What is "Security" best practice in Express?

What is "Clustering" in Express?

What is "Body-Parser"?

What is the process of handling asynchronous errors in Express (Pre-v5)?

What is the new error handling in Express v5?

What is <code>res.redirect()</code>?

What is <code>res.end()</code>?

What is the purpose of <code>app.set()</code>?

What is the purpose of <code>app.locals</code>?

What is <code>res.locals</code>?

What is "Morgan" middleware?

What is "Cookie-Parser"?

What is "Express-Session"?

What is "Passport.js"?

What is "Nodemon"?

What is "Environment Variables" in Express?

What is the <code>req.headers</code> object?

What is the difference between <code>res.sendStatus()</code> and <code>res.status().send()</code>?

What is the purpose of "Gzip Compression" in Express?

What is "Parameter Validation"?

What is "Route Chaining"?

What is the "Event-driven" nature of Express?

What is the difference between "Mounting" and "Routing"?

What is the "Full-Stack" context of Express?

What is the "Micro-framework" terminology?

What is "Versioning" in APIs?

What is "Graceful Shutdown" in Express?

What is the <code>req.xhr</code> property?

What is the <code>app.on('mount')</code> event?

What is the "Express Ecosystem"?