Express.js Interview Questions and Answers for internship
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What is Express.js?
- Answer: Express.js is a minimal and flexible Node.js web application framework that provides a robust set of features for building web and mobile applications. It's known for its simplicity, speed, and middleware support.
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Explain the role of middleware in Express.js.
- 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 perform tasks like logging, authentication, authorization, and data parsing before the request reaches the final route handler.
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How do you define routes in Express.js? Give an example.
- Answer: Routes are defined using the `app.METHOD(path, handler)` methods, where METHOD is GET, POST, PUT, DELETE, etc., path is the URL path, and handler is a function that processes the request. Example: `app.get('/users', (req, res) => { res.send('List of users'); });`
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What are request and response objects in Express.js?
- Answer: The `req` (request) object contains information about the incoming HTTP request, such as headers, URL parameters, body data, and more. The `res` (response) object is used to send the HTTP response back to the client, including status codes, headers, and data.
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Explain the use of `app.use()` in Express.js.
- Answer: `app.use()` is used to mount middleware functions. It can be used to apply middleware to all routes or specific routes based on the order in which it is called. For example, to apply a logging middleware to all requests, you would use `app.use(loggerMiddleware);` before defining any routes.
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How do you handle POST requests in Express.js?
- Answer: POST requests are handled using `app.post(path, handler)`. You'll typically need middleware (like `body-parser`) to parse the request body (often JSON) before accessing the data within the handler function. Example: `app.post('/users', (req, res) => { res.send(req.body); });`
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What is the purpose of `res.send()` and `res.json()`?
- Answer: `res.send()` sends a response to the client. It can handle various data types including strings, objects, and buffers. `res.json()` is a specialized version that automatically converts a JavaScript object into a JSON string and sets the Content-Type header appropriately.
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How do you handle errors in Express.js?
- Answer: Error handling is typically done using error-handling middleware. This middleware is placed after your route handlers. It takes four arguments (err, req, res, next), allowing you to catch and handle errors gracefully and send appropriate responses to the client.
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Explain the use of template engines with Express.js. Give an example with EJS.
- Answer: Template engines like EJS, Pug, or Handlebars are used to generate dynamic HTML. They allow you to separate presentation logic from application logic. With EJS, you can create `.ejs` files containing HTML with embedded JavaScript code. Example: `res.render('index', {user: 'John'});` would render `index.ejs` passing the `user` variable.
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What are some common HTTP status codes and when would you use them?
- Answer: 200 OK (successful request), 400 Bad Request (client-side error), 401 Unauthorized (authentication required), 404 Not Found, 500 Internal Server Error (server-side error). The choice depends on the outcome of the request.
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How do you use environment variables in Express.js?
- Answer: Environment variables are accessed using `process.env.
`. This is crucial for separating configuration details from code, keeping sensitive information out of version control.
- Answer: Environment variables are accessed using `process.env.
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What are some popular middleware packages for Express.js?
- Answer: `body-parser` (for parsing request bodies), `morgan` (for logging), `cookie-parser` (for handling cookies), `helmet` (for security headers), `cors` (for enabling CORS).
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Explain the concept of RESTful APIs.
- Answer: RESTful APIs follow architectural constraints to create a simple, scalable, and maintainable web service. They use standard HTTP methods (GET, POST, PUT, DELETE) to interact with resources. Resources are identified by URLs, and responses are typically in JSON or XML.
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How do you test an Express.js application?
- Answer: Testing is crucial! You can use tools like Supertest to make HTTP requests to your application and assert the responses. Unit tests focus on individual components, while integration tests verify the interaction between different parts of the application.
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What is a server-side rendering (SSR) and why is it used?
- Answer: Server-side rendering involves generating the HTML on the server instead of solely relying on the client-side JavaScript. This improves SEO, initial load time, and can enhance performance in some cases.
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How would you implement authentication in an Express.js application?
- Answer: Authentication can be implemented using various strategies like Passport.js, which supports different authentication mechanisms (like OAuth, JWT). It involves verifying the user's identity based on credentials or tokens.
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What is the difference between `app.get()` and `app.route()`?
- Answer: `app.get()` directly defines a route handler for a specific HTTP GET request. `app.route()` creates a route object that allows chaining multiple handlers for different HTTP methods (GET, POST, PUT, DELETE) on the same path, improving code organization.
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How do you handle file uploads in Express.js?
- Answer: Middleware like `multer` is commonly used to handle file uploads. It parses multipart/form-data requests and provides access to the uploaded files.
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Describe your experience with databases and how you've integrated them with Express.js.
- Answer: [Answer should reflect the candidate's actual experience. Example: "I've worked with MongoDB and PostgreSQL, using Mongoose and pg, respectively, as ORMs to interact with the database from within my Express.js applications. I'm familiar with creating database models, querying data, and handling transactions."]
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How would you deploy an Express.js application?
- Answer: Deployment options include platforms like Heroku, AWS, Google Cloud, or setting up a server yourself. The choice depends on project requirements and scalability needs. I'm familiar with [mention specific deployment methods used].
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Explain your understanding of asynchronous programming in Node.js and its relevance to Express.js.
- Answer: Node.js is event-driven and asynchronous. Express.js leverages this by using callbacks, promises, or async/await to handle requests concurrently without blocking the main thread. This is essential for performance and scalability.
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What are some security best practices for Express.js applications?
- Answer: Input validation (preventing injection attacks), using parameterized queries (against SQL injection), proper authentication and authorization, HTTPS, using a robust framework like Helmet, regularly updating dependencies.
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How do you handle different HTTP methods (GET, POST, PUT, DELETE) in Express.js?
- Answer: Each method has a corresponding Express.js method (e.g., `app.get`, `app.post`, `app.put`, `app.delete`) to define route handlers for the different request types.
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What is rate limiting and why is it important?
- Answer: Rate limiting controls the number of requests a client can make within a specific time period. It's essential to protect against denial-of-service (DoS) attacks and to manage server resources.
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How would you structure a large Express.js application?
- Answer: A modular approach is key, dividing the application into distinct modules (controllers, models, routes, middleware). This improves maintainability, testability, and collaboration.
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What is the difference between a library and a framework?
- Answer: A library is a collection of functions that can be called by your code. A framework dictates the structure and flow of your application, and your code is typically integrated within the framework's structure.
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Describe your experience working with version control systems like Git.
- Answer: [Answer should reflect the candidate's actual experience with Git, including branching, merging, pull requests, etc.]
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How familiar are you with the concept of RESTful API design principles?
- Answer: [Explain understanding of constraints like statelessness, client-server, cacheability, etc.]
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What are some common challenges you've faced working with Express.js, and how did you overcome them?
- Answer: [Describe specific challenges encountered and how they were addressed. Examples: Debugging asynchronous code, handling errors gracefully, optimizing performance.]
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What are your preferred methods for debugging Express.js applications?
- Answer: [Describe preferred debugging techniques like using console logs, debuggers, error handling middleware, logging libraries.]
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How would you handle a large volume of concurrent requests in an Express.js application?
- Answer: Discuss strategies like load balancing, using a message queue, scaling the application horizontally (multiple servers), optimizing database queries, and caching.
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Explain your understanding of different HTTP methods (GET, POST, PUT, DELETE, PATCH) and their appropriate usage.
- Answer: GET (retrieve data), POST (create data), PUT (update entire resource), DELETE (delete resource), PATCH (update part of a resource).
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How do you approach designing APIs for scalability and maintainability?
- Answer: Discuss principles like versioning, modular design, clear documentation, use of standard formats (JSON), and proper error handling.
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What are some tools you use for monitoring and logging in your Express.js projects?
- Answer: Mention tools like Winston, Bunyan, or centralized logging services. Also mention monitoring tools that provide insights into application performance.
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How would you implement role-based access control (RBAC) in your application?
- Answer: Describe strategies for implementing RBAC, including middleware that checks user roles against required permissions before granting access to resources.
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Describe your experience with different databases (e.g., relational vs. NoSQL).
- Answer: [Reflect candidate's database experience. Compare and contrast relational (SQL) and NoSQL databases and discuss appropriate use cases.]
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How familiar are you with Docker and containerization? How could it be used with Express.js?
- Answer: Describe understanding of Docker. Explain how Docker can be used to create and deploy consistent environments for Express.js applications.
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How do you handle authentication and authorization in a real-world Express.js application?
- Answer: Describe the use of Passport.js or similar libraries to handle authentication and authorization mechanisms.
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What is your experience with testing frameworks like Jest or Mocha?
- Answer: [Describe experience with testing frameworks and explain how they are used to write unit and integration tests for Express.js applications.]
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How would you improve the performance of a slow Express.js application?
- Answer: Discuss performance optimization techniques, such as caching, database query optimization, using asynchronous operations, and load balancing.
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What is your understanding of web sockets and how could they be used with Express.js?
- Answer: Explain understanding of web sockets and how they enable real-time, bidirectional communication between client and server, often using libraries like Socket.IO.
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Describe a time you had to debug a complex issue in an Express.js application.
- Answer: [Describe a specific situation, steps taken to diagnose the problem, and the solution that was implemented.]
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What are your thoughts on using TypeScript with Express.js?
- Answer: [Discuss the benefits and challenges of using TypeScript to improve code maintainability and reduce runtime errors.]
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What are your salary expectations for this internship?
- Answer: [Provide a realistic salary range based on research and experience.]
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