Express.js Interview Questions and Answers for 10 years experience

1. What is Express.js and why is it popular for building web applications?

   • Answer: Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for creating single-page, multi-page, and hybrid web applications. Its popularity stems from its ease of use, minimal overhead, large community support, and extensive middleware ecosystem, allowing developers to quickly build scalable and efficient applications.

2. Explain the concept of middleware in Express.js. Give examples.

   • Answer: 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. They can execute any code, make changes to the request and response objects, end the request-response cycle, or call the next middleware function in the stack. Examples include logging middleware (logging request details), authentication middleware (verifying user credentials), and body-parsing middleware (parsing JSON or form data from the request body).

3. How do you handle routing in Express.js? Explain different HTTP methods.

   • Answer: Routing in Express.js is handled using the `app.METHOD(path, callback)` functions, where METHOD can be GET, POST, PUT, DELETE, etc., representing different HTTP methods. The path specifies the URL route, and the callback function handles the request. GET is used for retrieving data, POST for creating data, PUT for updating data, and DELETE for deleting data. Other methods like PATCH (partial updates) and HEAD (retrieving headers only) are also supported.

4. Describe how to create a RESTful API with Express.js.

   • Answer: Creating a RESTful API involves defining routes for each CRUD (Create, Read, Update, Delete) operation using appropriate HTTP methods (POST for create, GET for read, PUT or PATCH for update, DELETE for delete). Each route should return appropriate HTTP status codes (e.g., 200 OK, 201 Created, 400 Bad Request, 404 Not Found, 500 Internal Server Error) and data in a consistent format (e.g., JSON). Proper error handling and input validation are crucial.

5. How do you handle errors in Express.js? Explain different error handling strategies.

   • Answer: Express.js provides mechanisms for handling both synchronous and asynchronous errors. For synchronous errors, a `try...catch` block can be used within route handlers. For asynchronous errors (e.g., database errors), error handling middleware can be added after route definitions. This middleware takes four arguments (err, req, res, next), allowing you to catch errors, log them, and send appropriate responses to the client. Centralized error handling is recommended for better maintainability.

6. Explain the use of request parameters and query parameters in Express.js.

   • Answer: Request parameters are part of the URL path, accessed using `req.params`. They are typically used to identify specific resources (e.g., `/users/:id`). Query parameters are appended to the URL after a question mark, accessed using `req.query`. They are used for filtering or modifying requests (e.g., `/users?name=John`).

7. How to use body-parser middleware in Express.js?

   • Answer: `body-parser` is a middleware that parses incoming request bodies in a variety of formats, enabling access to data sent from forms or APIs. It's no longer included in Express.js core, so you must install it separately (`npm install body-parser`). Then you use `app.use(bodyParser.json())` for JSON data and `app.use(bodyParser.urlencoded({ extended: false }))` for URL-encoded data.

8. What are some common security best practices when developing with Express.js?

   • Answer: Security best practices include input validation (sanitizing user inputs to prevent injection attacks), output encoding (preventing cross-site scripting attacks), using HTTPS to encrypt communication, proper authentication and authorization (using JWTs or OAuth), using parameterized queries to prevent SQL injection, and regularly updating dependencies to patch vulnerabilities.

9. How do you implement authentication and authorization in an Express.js application?

   • Answer: Authentication verifies the identity of the user, while authorization determines what actions the user is permitted to perform. Common approaches include using JWT (JSON Web Tokens) for authentication and role-based access control (RBAC) for authorization. Middleware can be used to intercept requests and check for valid tokens and permissions before proceeding.

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