Express.js Interview Questions and Answers
Intermediate / 1 to 5 years experienced level questions & answers
Ques 1. 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 2. 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 3. 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 4. 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 5. What is Express Router?
Express Router is a middleware that can be used to define modular routes for your application.
Ques 6. 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 7. 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 8. 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 9. 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 10. 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 11. 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 12. 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 13. 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 14. 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 15. 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 16. 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 17. 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 18. 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 19. 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 20. 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.
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