Express.js & Web APIs

Question 1

What is Express.js?

Answer 1

Express.js is a fast, unopinionated, minimalist web framework for Node.js used to build web and API applications.

Question 2

What are routes in Express.js?

Answer 2

Routes in Express.js define how the application responds to a client request for a particular endpoint and HTTP method.

Question 3

How do you define a GET route in Express.js?

Answer 3

app.get(‘/route’, (req, res) => { res.send(‘response’); });

Question 4

What is middleware in Express.js?

Answer 4

Middleware is a function that has access to the request, response, and next function in the request-response cycle and can modify them.

Question 5

What are common types of middleware in Express?

Answer 5

Application-level, Router-level, Built-in, and Error-handling middleware.

Question 6

What is the purpose of REST APIs?

Answer 6

REST APIs expose resources using standard HTTP methods, enabling stateless communication between client and server.

Question 7

What is the lifecycle of a request in Express?

Answer 7

Request → Middleware stack → Route handler → Response → Optional error handling middleware.

Question 8

How does Express handle errors?

Answer 8

By passing an error to next(err), Express will skip remaining middleware and move to the error-handling middleware.

Question 9

What is authentication in Express.js?

Answer 9

Authentication verifies a user’s identity using credentials like username/password or tokens.

Question 10

What is authorization in Express.js?

Answer 10

Authorization determines what an authenticated user is allowed to do (e.g., access resources or endpoints).

Question 11

What’s the difference between authentication and authorization?

Answer 11

Authentication verifies who the user is; authorization verifies what the user has access to.

Question 12

How can you implement JWT-based authentication in Express?

Answer 12

By signing a token during login and verifying it in a middleware using a library like jsonwebtoken.

Question 13

What are the advantages of Express.js?

Answer 13

Simple to use, large ecosystem, flexible, supports middleware, and excellent community support.

Question 14

What are the disadvantages of Express.js?

Answer 14

Less structured, requires manual error handling, lacks built-in advanced features, and can get messy with large codebases.

Question 15

What is a best practice for structuring Express apps?

Answer 15

Use MVC or modular folder structure, separate routes/controllers/middleware, and apply error handling globally.

Question 16

What’s a common use case for Express.js?

Answer 16

Building REST APIs, backend for web or mobile apps, or proxy servers.

Question 17

How does Express.js impact system design?

Answer 17

It promotes modular and scalable design, supports middleware chaining, and allows easy integration with databases and services.

Question 18

Give an example of middleware usage.

Answer 18

app.use(express.json()); parses incoming JSON requests.

Question 19

Give an example of using async/await in route handlers.

Answer 19

app.get(‘/data’, async (req, res) => { const result = await db.query(); res.send(result); });

Question 20

What are the architectural implications of using middleware?

Answer 20

Middleware encourages a pipeline architecture where requests pass through layered processing logic.

Question 21

How can you make Express apps fault-tolerant?

Answer 21

Use global error handling, process monitoring (e.g., PM2), retries, and validation.

Question 22

How do you monitor Express applications?

Answer 22

Using tools like morgan, winston, Prometheus, Grafana, or integrating APMs like New Relic or Datadog.

Question 23

How can you debug an Express.js app?

Answer 23

Use logging (e.g., console.log, winston), the Node.js debugger, or VS Code debugging tools.

Question 24

What are tradeoffs of using Express over a full-fledged framework?

Answer 24

Express offers flexibility but lacks conventions and features, requiring more setup for larger applications.

Question 25

What is a common gotcha with asynchronous error handling in Express?

Answer 25

For async route handlers, unhandled errors won't reach error middleware unless wrapped or using async middleware libraries.

Question 26

Why is input validation important in Express apps?

Answer 26

To prevent security issues like injection attacks and ensure the integrity of incoming data.

Question 27

Which library is commonly used for validation in Express?

Answer 27

Joi, express-validator, or Zod are commonly used libraries.

Question 28

What is a common mistake when using middleware?

Answer 28

Not calling `next()`, which causes requests to hang indefinitely.

Question 29

What’s the difference between `app.use()` and `app.get()`?

Answer 29

`app.use()` applies middleware to all HTTP methods, `app.get()` handles only GET requests at the specified path.

Question 30

Why use Helmet in Express?

Answer 30

Helmet helps secure Express apps by setting various HTTP headers.

Question 31

What’s the benefit of CORS middleware in Express?

Answer 31

It allows or restricts cross-origin requests to your Express API.

Question 32

What is an interview question about Express routing?

Answer 32

How does Express match route handlers and what happens if no route matches?

Question 33

What is a potential security risk with Express?

Answer 33

Not sanitizing user input or exposing sensitive error messages.

Question 34

How can Express.js scale horizontally?

Answer 34

By running multiple instances behind a load balancer using clustering or PM2.

Question 35

What is an example of route-level middleware?

Answer 35

`router.get('/user', authMiddleware, getUserHandler);`

Question 36

What is a 404 handler in Express?

Answer 36

A middleware that handles requests with no matching route: `app.use((req, res) => res.status(404).send('Not Found'))`

Question 37

What’s the role of `next()` in middleware?

Answer 37

It passes control to the next middleware in the stack.

Question 38

What does `express.json()` do?

Answer 38

It parses incoming JSON payloads and makes them available on `req.body`.

Question 39

What is the role of body-parser in Express?

Answer 39

`body-parser` parses incoming request bodies; it's now included in Express as `express.json()` and `express.urlencoded()`.

Question 40

What is the impact of poor error handling in Express?

Answer 40

It can cause app crashes, unhandled promise rejections, or expose internal logic to clients.

Question 41

What are common interview questions about Express?

Answer 41

Explain middleware, how Express handles errors, or how you structure an Express app.

Question 42

What should you log in a production Express app?

Answer 42

Request info, errors, timestamps, user IDs, and trace IDs for correlation.

Question 43

How does Express compare to Fastify or Koa?

Answer 43

Express is more popular and flexible, while Fastify offers better performance and Koa focuses on modern async patterns.