React Js Interview Questions and Answers

​React Questions:

1. What is React?

   • Answer: React is a JavaScript library used for building user interfaces, especially single-page applications. It allows developers to create reusable UI components and efficiently update the view when data changes.

2. What are the main features of React?

   • Answer: The main features of React are:

     • Virtual DOM: React uses a virtual DOM to improve performance by minimizing direct interactions with the real DOM.

     • Components: UI elements are built using reusable components.

     • JSX: React uses JSX, a syntax extension for JavaScript, to describe what the UI should look like.

     • State and Props: State stores data within a component, while props are used to pass data between components.

     • One-way Data Flow: React uses unidirectional data flow to make the application more predictable.

3. What is JSX in React?

   • Answer: JSX stands for JavaScript XML. It's a syntax extension that allows writing HTML-like code in JavaScript. It gets transpiled to JavaScript using tools like Babel.

4. Explain the concept of Components in React.

   • Answer: Components are the building blocks of a React application. They are JavaScript functions or classes that return React elements (UI). Components can be either Functional Components (written as functions) or Class Components (written as classes).

5. What is the Virtual DOM?

   • Answer: The Virtual DOM is an in-memory representation of the actual DOM elements. React updates the virtual DOM first and then compares it with the real DOM using a diffing algorithm. Only the changed elements are updated in the real DOM, improving performance.

6. What are State and Props in React?

   • Answer:

     • State: A state is an object that holds data that may change over time. It is used within a component.

     • Props: Props (short for properties) are read-only objects passed to components from their parent. They allow data flow from parent to child components.

7. What is the difference between a Class component and a Functional component in React?

   • Answer:

     • Class components are ES6 classes that extend React.Component and have access to lifecycle methods like componentDidMount().

     • Functional components are simple JavaScript functions. They were stateless initially but now can use hooks like useState and useEffect to manage state and side effects.

8. What are React Hooks?

   • Answer: React Hooks are functions that allow functional components to use state and other React features without writing a class component. Some commonly used hooks are useState, useEffect, useContext, useReducer, and useRef.

9. What is the purpose of useState and useEffect hooks?

   • Answer:

     • useState: A hook that allows a functional component to maintain local state.

     • useEffect: A hook that runs side effects (like data fetching, subscriptions, or manual DOM manipulations) in a functional component after render.

10. What is Context API in React?

    • Answer: The Context API is used to manage global state in React applications. It provides a way to share values (like theme or authentication status) between components without passing props down manually at every level.

11. What is Redux and how is it used with React?

    • Answer: Redux is a state management library that helps manage application-level state in a predictable way. It works with React by providing a centralized store where the state is stored and allows components to subscribe to state changes and dispatch actions to modify state.

12. What is the significance of the key prop in React lists?

    • Answer: The key prop helps React identify which items in the list are changed, added, or removed, optimizing re-renders. Each key must be unique within the list.

13. What is the difference between componentDidMount and useEffect?

    • Answer:

      • componentDidMount is a lifecycle method used in class components to run code once after the component is mounted.

      • useEffect serves the same purpose in functional components and can also be used to run code on every render or only when specific dependencies change.

14. What is a higher-order component (HOC) in React?

    • Answer: A Higher-Order Component is a function that takes a component and returns a new component with additional props or functionality. It is used for code reuse and component logic sharing.

15. What are controlled and uncontrolled components?

    • Answer:

      • Controlled components: Components that rely on React state for their values. Example: A form input whose value is set by the state.

      • Uncontrolled components: Components that manage their own state internally using the DOM, without relying on React state.

Backend/Full-Stack Questions:

1. What is Node.js and how does it work?

   • Answer: Node.js is a runtime environment that allows running JavaScript code outside the browser. It is built on Chrome's V8 engine and is used for building scalable server-side applications.

2. What are Express.js and how does it relate to Node.js?

   • Answer: Express.js is a lightweight, minimal web application framework for Node.js that simplifies routing, handling HTTP requests, and managing middleware.

3. What is RESTful API?

   • Answer: RESTful APIs follow the principles of Representational State Transfer (REST) and use HTTP methods (GET, POST, PUT, DELETE) to interact with resources represented in URLs.

4. What are HTTP methods and their differences?

   • Answer:

     • GET: Retrieves data from the server.

     • POST: Submits data to be processed to the server.

     • PUT: Updates existing data on the server.

     • DELETE: Removes data from the server.

5. Explain CORS (Cross-Origin Resource Sharing).

   • Answer: CORS is a security feature implemented by browsers to prevent web pages from making requests to a domain different from the one that served the web page. CORS headers must be configured on the server to allow cross-origin requests.

6. What is JWT (JSON Web Token)?

   • Answer: JWT is an open standard for securely transmitting information between parties as a JSON object. It is commonly used for authentication and authorization in web applications.

7. What is MongoDB and how does it work with Node.js?

   • Answer: MongoDB is a NoSQL database that stores data in JSON-like documents. It works with Node.js through the MongoDB Node.js driver or an Object Data Modeling (ODM) library like Mongoose, allowing easy data manipulation and storage.

8. What is the difference between SQL and NoSQL databases?

   • Answer:

     • SQL databases are relational and use structured query language for managing data. They are best suited for structured data with predefined schemas.

     • NoSQL databases are non-relational and flexible, allowing the storage of unstructured or semi-structured data. They are best for scalability and handling large volumes of diverse data types.

9. What is the role of middleware in Express.js?

   • Answer: Middleware functions are functions that have access to the request, response, and the next function in the request-response cycle. They can modify the request or response, end the request-response cycle, or pass control to the next middleware function.

10. What is the difference between synchronous and asynchronous code in JavaScript?

    • Answer:

      • Synchronous: Code that runs sequentially, blocking further execution until the current task is completed.

      • Asynchronous: Code that allows tasks to run concurrently without blocking the main thread, using callbacks, promises, or async/await.

11. Q: Explain the difference between ref and out parameters in C#.

• ref: Requires the variable to be initialized before it is passed to the method.

• out: Does not require initialization; must be assigned within the method.

void RefExample(ref int x) { x += 10; } void OutExample(out int y) { y = 10; }

12. Q: Differentiate between value types and reference types in .NET.

• Value Types: Stored in stack, contain actual data. (e.g., int, struct)

• Reference Types: Stored in heap, contain reference to the data. (e.g., class, array)

13. Q: What is ASP.NET MVC?

• ASP.NET MVC is a design pattern for web applications.

  • Model: Business/data logic

  • View: UI elements

  • Controller: Handles input and updates Model/View

14. Q: What is Entity Framework (EF)?

• EF is an ORM that enables .NET developers to work with databases using .NET objects, eliminating the need for most raw SQL queries.

using (var context = new AppDbContext()) { var users = context.Users.ToList(); }

15. Q: Difference between IEnumerable<T> and IQueryable<T>?

• IEnumerable<T>: Executes queries in memory (LINQ to Objects).

• IQueryable<T>: Builds query expression and executes on the database server (LINQ to Entities).

IQueryable<User> query = context.Users.Where(u => u.Age > 20);

16. Q: What is a Partial View in ASP.NET MVC?

• A reusable part of a view used for rendering common UI elements (e.g., header, form).

@Html.Partial("_LoginPartial")

17. Q: What is the using statement used for in C#?

• Automatically disposes of objects that implement IDisposable.

using (SqlConnection conn = new SqlConnection(connectionString)) { conn.Open(); }

18. Q: What is Middleware in ASP.NET Core?

• Middleware components form a pipeline to handle HTTP requests/responses.

• Examples: Authentication, Logging, CORS.

app.UseRouting(); app.UseAuthorization();

19. Q: Explain Dependency Injection (DI) in .NET Core.

• DI is a technique to inject service objects into classes, promoting loose coupling.

public class HomeController { private readonly IProductService _service; public HomeController(IProductService service) { _service = service; } }

20. Q: Difference between abstract class and interface in C#?

FeatureAbstract ClassInterface

ImplementationCan have methodsCannot (until C# 8)

InheritanceSingleMultiple

FieldsCan have fieldsCannot (before C# 8)

21. Q: What is Razor in ASP.NET MVC?

• Razor is a markup syntax to embed C# code in HTML views.

<p>Hello @Model.Name!</p>

22. Q: Difference between ViewData, ViewBag, and TempData?

• ViewData: Dictionary (string/object), requires casting

• ViewBag: Dynamic property, wrapper around ViewData

• TempData: Persists across one redirect (uses session)

ViewBag.Name = "John"; ViewData["Name"] = "John"; TempData["Name"] = "John";

23. Q: How do you connect to a SQL Server database in ADO.NET?

using (SqlConnection conn = new SqlConnection(connStr)) { conn.Open(); SqlCommand cmd = new SqlCommand("SELECT * FROM Users", conn); SqlDataReader reader = cmd.ExecuteReader(); }

24. Q: What are the access modifiers in C#?

• public: Accessible from anywhere

• private: Accessible only in the class

• protected: Accessible in class and derived classes

• internal: Accessible within the same assembly

• protected internal, private protected: Combinations for finer control

25. Q: Synchronous vs Asynchronous methods in .NET?

• Synchronous: Blocks the thread until task is complete

• Asynchronous: Uses async/await, doesn't block thread

public async Task<IActionResult> GetDataAsync() { var data = await _service.GetDataAsync(); return View(data); }

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