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1. What is the difference between a front-end and back-end developer?

   • A front-end developer focuses on client-side development, creating the user interface and user experience of a website or application using HTML, CSS, and JavaScript.

   • A back-end developer focuses on server-side development, creating the server-side logic, database integration, and API connectivity using languages like Java, Python, or Ruby.

2. What is RESTful API?

   • REST (Representational State of Resource) is an architectural style for designing networked applications. It uses HTTP methods (GET, POST, PUT, DELETE) to interact with resources.

   • RESTful APIs are stateless, meaning that each request contains all the information necessary to complete the request.

3. What is the difference between null and undefined in JavaScript?

   • In JavaScript, null is an object that represents the intentional absence of any object value.

   • Undefined is a primitive value that represents an uninitialized or non-existent variable.

4. How do you handle errors in JavaScript?

   • There are several ways to handle errors in JavaScript, including try-catch blocks, error handling libraries like jQuery's $.ajaxError(), and error reporting services like Rollbar.

5. What is a data structure in programming?

   • A data structure is a way to organize and store data in a program. Examples of data structures include arrays, linked lists, stacks, queues, trees, and graphs.

Intermediate Level Questions (6-15)

6. What is the difference between var, let, and const in JavaScript?

   • Var declares a variable with block-level scoping.

   • Let declares a variable with block-level scoping and allows reassignment.

   • Const declares a constant with block-level scoping and does not allow reassignment.

7. How do you optimize the performance of a web application?

   • Optimizing performance involves techniques like minifying and compressing code, caching frequently accessed data, optimizing database queries, using lazy loading, and implementing caching mechanisms like Redis or Memcached.

8. What is the difference between async and sync code in JavaScript?

   • Async code uses callbacks or promises to handle asynchronous operations.

   • Sync code executes operations synchronously, blocking other code execution until the operation is complete.

9. What is the purpose of a router in a web application?

   • A router is responsible for mapping URLs to specific routes or controllers in a web application. It helps determine which controller or action to execute based on the URL.

10. How do you handle form validation in JavaScript?

    • Form validation involves using techniques like regular expressions, DOM manipulation, and JavaScript libraries like jQuery Validate or VanillaJS Validate.

11. What is JSON and how do you use it in JavaScript?

    • JSON (JavaScript Object Notation) is a lightweight data interchange format that represents data as key-value pairs.

    • In JavaScript, you can use JSON.parse() to parse JSON data from a string and JSON.stringify() to convert JSON data to a string.

12. What are some common design patterns in software development?

    • Some common design patterns include Singleton, Factory, Observer, MVC (Model-View-Controller), and Repository.

13. How do you implement authentication and authorization in a web application?

    • Authentication verifies the identity of a user or system.

    • Authorization controls access to resources based on user roles or permissions.

14. What are some common security vulnerabilities in web applications?

    • Common security vulnerabilities include SQL injection, cross-site scripting (XSS), cross-site request forgery (CSRF), insecure password storage, and weak authentication.

15. How do you debug a web application using console.log() or other debugging tools?

    • Console.log() helps you debug by printing messages to the console for inspection.

    • Other debugging tools include Chrome DevTools' Console panel, Node.js Inspector, or third-party debugging libraries like Debug.js.

Advanced Level Questions (16-30)

16. How do you implement caching in a web application using Redis or Memcached?

    • Caching involves storing frequently accessed data in memory (Redis) or disk-based storage (Memcached) to reduce database queries and improve performance.

17. What is service-oriented architecture (SOA) and how do you implement it in a web application?

    • SOA involves designing applications as loosely coupled services that communicate with each other using APIs or messaging protocols.

18. How do you implement internationalization (i18n) and localization (L10n) in a web application?

    • I18n involves adapting software to different languages and cultures.

    • L10n involves adapting software for specific regions or languages.

19. What is WebSockets and how do you use them in real-time communication?

    • WebSockets establish persistent bi-directional communication between the client and server over HTTP connections.

20. How do you implement load balancing and scaling in a web application?

    • Load balancing distributes incoming traffic across multiple servers to improve performance and availability.

    • Scaling involves increasing capacity by adding more servers or resources.

21. What is GraphQL and how do you use it for querying data?

    • GraphQL is a query language for APIs that allows clients to specify exactly what data they need from the server.

22. How do you implement microservices architecture in a web application?

    • Microservices architecture involves breaking down monolithic applications into smaller independent services that communicate with each other using APIs or messaging protocols.

23. What are some common testing frameworks for JavaScript testing?

    • Some common testing frameworks include Jest, Mocha, Chai, Enzyme, and Cypress.

24. How do you implement dependency injection in JavaScript?

    • Dependency injection involves injecting dependencies (objects or services) into components rather than instantiating them directly.

25. What is TypeScript and how do you use it for type safety?

    • TypeScript is a superset of JavaScript that adds optional static typing and other features for better code maintainability and readability.

26. How do you implement Observables in RxJS for reactive programming?

    • Observables are used to manage asynchronous data streams by observing changes to the stream without blocking execution.

27. What is Firebase Realtime Database or Firestore and how do you use it for real-time data synchronization?

    • Firebase Realtime Database or Firestore are NoSQL databases that provide real-time data synchronization across multiple clients.

28. How do you implement Service Workers for offline-first applications?

    • Service workers allow developers to cache resources for offline-first applications by intercepting network requests and responding with cached content when offline.

29. What are some common issues with scalability in web applications?

    • Common issues with scalability include database bottlenecks, slow query performance, inadequate caching, insufficient resources (CPU/memory), and inefficient algorithms.

30. How do you implement A/B testing in web applications using libraries like Redux or React Hooks?

    • A/B testing involves randomly dividing users into two groups (control group vs treatment group) to test different variations of an application feature or design change.