Web Application Interview Questions

What is a web application?

A web application is a software application that runs in a web browser. Unlike traditional desktop applications, web apps are accessed via the internet and use web technologies such as HTML, CSS, JavaScript on the front-end, and various back-end frameworks like Node.js, Django, or Laravel.

What’s the difference between a website and a web application?

While both are accessed via a browser, a website is mostly informational and static, whereas a web application is interactive, dynamic, and often performs specific business functions such as user authentication, file uploads, or data processing.

What is the client-server architecture?

Web applications follow the client-server model, where:

• The client (browser) sends requests to the server.
• The server processes the request and sends back a response, often in the form of HTML, JSON, or XML.

What are the essential components of a web application?

• Frontend (Client-side): Handles UI and UX, typically using HTML, CSS, and JavaScript (with frameworks like React, Angular, or Vue).
• Backend (Server-side): Manages application logic, database operations, and server responses.
• Database: Stores application data (e.g., MySQL, PostgreSQL, MongoDB).
• APIs: Enable communication between front-end and back-end or with external services.

How do HTTP methods work in web applications?

Common HTTP methods include:

• GET: Retrieve data from the server.
• POST: Submit data to the server.
• PUT: Update existing data.
• DELETE: Remove data.

Each method has a specific purpose in RESTful APIs.

What is REST API?

REST (Representational State Transfer) is an architectural style used in web services. REST APIs use standard HTTP methods and are stateless, scalable, and allow communication between different parts of an application or between apps.

GET /api/users/123

Explain the concept of Single Page Application (SPA).

A SPA loads a single HTML page and dynamically updates content as the user interacts with the app, without reloading the page. Technologies like React, Angular, and Vue are commonly used to build SPAs.

What is CORS, and why is it important?

CORS (Cross-Origin Resource Sharing) is a security feature implemented in browsers that restricts web pages from making requests to a different domain than the one that served the page. It’s vital for security, especially in API calls across different origins.

How do sessions and cookies work in web applications?

• Cookies store data in the browser and are sent with every request to the server.
• Sessions store data on the server, with the client holding only a session ID.

They’re used to track user authentication, preferences, and other states across multiple requests.

What is CSRF and how do you prevent it?

CSRF (Cross-Site Request Forgery) is an attack that tricks a user into submitting unwanted actions on a web app where they are authenticated.

Prevention techniques:

• Use anti-CSRF tokens.
• Validate the origin and referrer headers.
• Use SameSite cookie attributes.

What are some common security practices in web applications?

• Input validation to prevent SQL Injection.
• Use HTTPS for secure communication.
• Store passwords securely using hashing (e.g., bcrypt).
• Implement proper authentication and authorization checks.
• Sanitize user inputs to prevent XSS (Cross-Site Scripting).

What is the difference between SQL and NoSQL databases?

• SQL databases (e.g., MySQL, PostgreSQL): Use structured schemas and tables; ideal for complex queries and transactions.
• NoSQL databases (e.g., MongoDB, Redis): Use flexible data models like JSON, key-value pairs; suitable for unstructured data and high scalability.

What is WebSockets and how is it different from HTTP?

WebSockets provide full-duplex communication channels over a single, long-lived TCP connection. Unlike HTTP, which is request-response-based, WebSockets allow real-time data exchange, ideal for chat apps, live notifications, or multiplayer games.

Explain MVC architecture in web development.

MVC stands for:

• Model: Manages data and business logic.
• View: Handles UI representation.
• Controller: Interprets user inputs and calls Model/View accordingly.

It's a common architectural pattern in frameworks like Django, Laravel, and Ruby on Rails.

What is middleware in web development?

Middleware is software that acts as a bridge between an HTTP request and response. It can modify the request, check authentication, handle errors, or perform logging. Common in Express.js, Django, and Laravel.

How do you handle form validation?

There are two levels:

• Client-side validation: Done with JavaScript to provide quick feedback to users.
• Server-side validation: Ensures data integrity and security regardless of client behavior.

Always implement both for robust validation.

What are JWTs and how are they used?

JWT (JSON Web Token) is a compact, URL-safe token used for securely transmitting information between parties. It's often used for stateless authentication:

• The user logs in and receives a JWT.
• The JWT is sent with each request in the Authorization header.
• The server validates the token.

How would you optimize the performance of a web application?

• Minify CSS, JS, and HTML.
• Use lazy loading for images and components.
• Implement server-side caching (Redis, Memcached).
• Use CDNs for static assets.
• Optimize database queries and indexes.
• Enable gzip compression.

What is the role of DevOps in web application deployment?

DevOps practices automate and streamline application deployment using CI/CD pipelines, containers (Docker), orchestration (Kubernetes), and infrastructure as code. This ensures faster and more reliable delivery of web apps.

How do you debug a web application?

• Use browser dev tools (Console, Network tab).
• Check server logs for errors.
• Use debugging tools and breakpoints (Node.js debugger, Chrome DevTools).
• Log errors on the client and server side using tools like Sentry or LogRocket.