Latest Top interview questions and answers for a Express JS developer for experienced and freshers, get ready to crack any interview.

Basics of Express.js interview question:

1. What is Express.js?
   • Answer: Express.js is a web application framework for Node.js. It simplifies the process of building robust and scalable web applications by providing a set of features for routing, middleware, and HTTP utilities.
2. Explain the concept of middleware in Express.js.
   • Answer: Middleware in Express.js are functions that have access to the request, response, and the next middleware function. They can modify request or response objects, end the request-response cycle, or call the next middleware.
3. How do you install Express.js in a Node.js project?
   • Answer: You can install Express.js using npm (Node Package Manager) with the following command:

npm install express

1. What is routing in Express.js, and how is it implemented?
   • Answer: Routing in Express.js involves defining routes for different HTTP methods and URLs. It is implemented using the app.get(), app.post(), and similar methods to handle specific routes.
2. Explain the role of the app.use method in Express.js.
   • Answer: The app.use method in Express.js is used to mount middleware functions. It is often used for setting up middleware that should be executed for every request, such as logging or error handling.
3. What is the purpose of the express.static middleware?
   • Answer: The express.static middleware in Express.js is used to serve static files, such as images, CSS, and JavaScript, from a specified directory. It simplifies the process of serving static assets.
4. How do you handle query parameters in Express.js?
   • Answer: Query parameters in Express.js can be accessed using the req.query object. For example:

app.get('/api/user', (req, res) => {
  const userId = req.query.id;
  // Handle userId
});

1. What is the purpose of the body-parser middleware in Express.js?
   • Answer: The body-parser middleware in Express.js is used to parse the request body in different formats (e.g., JSON, URL-encoded). It allows easy access to the request payload.
2. Explain the difference between req.params and req.query in Express.js.
   • Answer: req.params in Express.js is used to access route parameters, while req.query is used to access query parameters. Route parameters are part of the URL, while query parameters are appended to the URL.
3. How can you handle form submissions in Express.js?
   • Answer: Form submissions in Express.js can be handled using the body-parser middleware to parse the form data. For example:

const bodyParser = require('body-parser');
app.use(bodyParser.urlencoded({ extended: false }));

app.post('/submit', (req, res) => {
  const formData = req.body;
  // Handle form data
});

Advanced Express.js Concepts interview question:

11. Explain the concept of routing middleware in Express.js.
    • Answer: Routing middleware in Express.js allows the organization of routes into separate files or modules. It improves code organization by breaking down the application into smaller, manageable pieces.
12. What is JWT authentication, and how is it implemented in Express.js?
    • Answer: JWT (JSON Web Token) authentication involves issuing a token to a client upon successful authentication. In Express.js, libraries like jsonwebtoken can be used to generate and verify JWTs for user authentication.
13. How do you handle errors in Express.js?
    • Answer: Errors in Express.js can be handled using middleware with four parameters (err, req, res, next). Additionally, the next(err) function can be used to pass errors to the next middleware.
14. Explain the purpose of the cors middleware in Express.js.
    • Answer: The cors middleware in Express.js is used to handle Cross-Origin Resource Sharing. It allows or restricts cross-origin requests based on server configurations.
15. What is session management in Express.js, and how is it implemented?
    • Answer: Session management in Express.js involves maintaining user sessions. It can be implemented using middleware like express-session to store session data on the server or using JWTs for stateless session management.
16. How does Express.js handle file uploads?
    • Answer: Express.js can handle file uploads using the multer middleware. It allows the parsing of multipart/form-data and facilitates handling file uploads.
17. Explain the concept of template engines in Express.js.
    • Answer: Template engines in Express.js allow the dynamic generation of HTML. Popular template engines include EJS, Pug, and Handlebars. They allow embedding dynamic content within HTML markup.
18. What is the purpose of the cookie-parser middleware in Express.js?
    • Answer: The cookie-parser middleware in Express.js is used to parse and handle cookies in HTTP requests. It facilitates working with cookies in a web application.
19. How can you implement server-side rendering (SSR) in Express.js?
    • Answer: Server-side rendering in Express.js involves rendering React components on the server before sending the HTML to the client. Libraries like react-dom/server and frameworks like Next.js can be used for SSR.
20. What is clustering in the context of Express.js, and why is it useful?
    • Answer: Clustering in Express.js involves creating multiple instances (workers) of the application to handle concurrent connections. It is useful for utilizing multiple CPU cores and improving performance.

Testing in Express.js interview question:

21. How can you perform unit testing in Express.js?
    • Answer: Unit testing in Express.js can be performed using testing frameworks like Mocha or Jest. Assertions can be done using libraries like Chai or built-in assert module.
22. What is the purpose of the supertest library in Express.js testing?
    • Answer: The supertest library is used for testing HTTP assertions in Express.js applications. It allows making requests to the application and asserting the response.
23. How do you mock API calls in Express.js testing?
    • Answer: API calls in Express.js testing can be mocked using tools like sinon or by creating manual mocks. Mocks help isolate the code under test and control the behavior of dependencies.
24. What is end-to-end testing, and how is it performed in Express.js applications?
    • Answer: End-to-end testing in Express.js involves testing the complete application workflow. Tools like Supertest, Cypress, or Puppeteer can be used for end-to-end testing.
25. Explain the concept of code coverage in Express.js testing.
    • Answer: Code coverage measures the percentage of code that is executed during testing. Tools like Istanbul or Jest provide code coverage reports to identify untested code.