Express.js Interview Questions and Answers
Freshers / Beginner level questions & answers
Ques 1. What is Express.js?
Express.js is a web application framework for Node.js designed for building web applications and APIs.
Ques 2. How to install Express.js?
You can install Express.js using npm (Node Package Manager) by running 'npm install express'.
Intermediate / 1 to 5 years experienced level questions & answers
Ques 3. What is middleware in Express.js?
Middleware functions are functions that have access to the request, response, and the next function in the application’s request-response cycle.
Ques 4. Explain routing in Express.js.
Routing refers to how an application’s endpoints (URIs) respond to client requests. In Express, you can define routes to handle different HTTP methods and URL patterns.
Ques 5. What is the purpose of app.use() in Express.js?
app.use() is used to mount middleware functions in the application. It is executed every time a request is received.
Ques 6. Explain the difference between res.send() and res.json() in Express.js.
res.send() sends a response of various types, while res.json() specifically sends a JSON response.
Ques 7. What is Express Router?
Express Router is a middleware that can be used to define modular routes for your application.
Ques 8. How to handle errors in Express.js?
You can handle errors in Express.js using middleware with four parameters (err, req, res, next) or using try-catch blocks.
Ques 9. What is the purpose of body-parser middleware?
body-parser is used to parse the incoming request bodies in a middleware before the handlers, making it easier to handle form data and JSON payloads.
Ques 10. What is the purpose of the Express.js 'app.listen()' method?
The 'app.listen()' method is used to bind and listen for connections on the specified host and port. It starts the Express.js application.
Ques 11. Explain the difference between PUT and PATCH HTTP methods in the context of Express.js.
PUT is used to update or create a resource entirely, while PATCH is used to apply partial modifications to a resource.
Ques 12. How can you set up a route parameter in Express.js?
You can define a route parameter by using a colon (:) followed by the parameter name in the route definition. For example, '/users/:id'.
Ques 13. What is middleware chaining in Express.js?
Middleware chaining is the process of calling multiple middleware functions in sequence for a specific route or globally using 'app.use()'.
Ques 14. Explain the role of the 'static' middleware in Express.js.
The 'static' middleware is used to serve static files, such as images, CSS, and JavaScript files, from a specified directory.
Ques 15. How can you handle CORS in Express.js?
You can handle CORS by using the 'cors' middleware or by manually setting the appropriate headers in the response.
Ques 16. What is the purpose of the 'express.Router' class?
The 'express.Router' class is used to create modular, mountable route handlers. It can be thought of as a mini Express application.
Ques 17. Explain the concept of 'cookie-parser' middleware in Express.js.
'cookie-parser' is a middleware used to parse cookie headers and populate 'req.cookies' with an object keyed by the cookie names.
Ques 18. What is the purpose of the 'helmet' middleware in Express.js?
'helmet' is a middleware that helps secure Express.js applications by setting various HTTP headers to prevent common web vulnerabilities.
Ques 19. What is the role of the 'express.json()' middleware?
'express.json()' is a middleware that parses incoming JSON requests, populating the 'req.body' property with the parsed data.
Ques 20. Explain the purpose of the 'express.static()' middleware.
'express.static()' is a middleware that serves static files, such as images, CSS, and JavaScript files, from a specified directory.
Ques 21. How can you handle query parameters in Express.js?
You can handle query parameters in Express.js using the 'req.query' object, which contains key-value pairs of the query parameters.
Ques 22. Explain the purpose of the 'express.urlencoded()' middleware.
'express.urlencoded()' is a middleware that parses incoming requests with URL-encoded payloads and is based on the 'body-parser' library.
Experienced / Expert level questions & answers
Ques 23. Explain the concept of view engines in Express.js.
View engines are used to render dynamic content. Popular view engines for Express include EJS, Pug, and Handlebars.
Ques 24. How can you implement session management in Express.js?
Session management in Express.js can be implemented using middleware like 'express-session' along with a session store.
Ques 25. Explain the purpose of the 'multer' middleware in Express.js.
'multer' is a middleware used for handling 'multipart/form-data,' primarily used for file uploads. It adds a 'body' object and a 'file' or 'files' object to the 'request' object.
Ques 26. How can you implement authentication in Express.js?
Authentication in Express.js can be implemented using middleware like 'passport' along with relevant strategies (e.g., local strategy, OAuth).
Ques 27. Explain the purpose of the 'express-validator' library in Express.js.
'express-validator' is a set of Express.js middlewares that wraps validator.js validator and sanitizer functions, providing validation and sanitation of user input.
Ques 28. How can you implement rate limiting in Express.js?
Rate limiting in Express.js can be implemented using middleware such as 'express-rate-limit' to restrict the number of requests from a client in a given time frame.
Ques 29. What is the purpose of the 'compression' middleware in Express.js?
'compression' is a middleware that compresses the response body for requests that traverse through it, reducing the size of the response and improving performance.
Ques 30. Explain the concept of 'Promise' in the context of asynchronous operations in Express.js.
Promises in Express.js are used to handle asynchronous operations, providing a cleaner and more readable way to structure asynchronous code.
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