Top Full Stack Developer Interview Questions & Answers (August 2025)

1. Explain the difference between the virtual DOM and the real DOM. Why is the virtual DOM faster?

Answer: "The Real DOM is the standard tree-like structure of HTML elements that the browser creates. Manipulating it directly is slow because every change causes the browser to repaint and reflow the entire page. The Virtual DOM, used by frameworks like React, is a lightweight JavaScript copy of the Real DOM. When state changes, React creates a new virtual DOM, compares it to the previous one (a process called 'diffing'), and then calculates the most efficient way to update the Real DOM with only the necessary changes. This batching of updates minimizes direct manipulation of the slow Real DOM, making the application much faster and more performant."

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

Answer: "`var` is function-scoped and hoisted, meaning it's treated as if it were declared at the top of the function, which can lead to unexpected behavior. `let` and `const` were introduced in ES6 and are block-scoped, which is more intuitive. `let` allows you to reassign the variable's value, while `const` declares a constant whose value cannot be reassigned after it's initialized. In modern JavaScript, we should always prefer `const` by default and use `let` only when we know the variable needs to be reassigned."

3. What is middleware in the context of a back-end framework like Express.js or ASP.NET Core?

Answer: "Middleware is a function that has access to the request object, the response object, and the next middleware function in the application’s request-response cycle. It's a chain of functions that a request passes through before it hits the actual route handler. Middleware is used for tasks like logging, authentication, authorization, error handling, and parsing request bodies. It's a powerful way to modularize and manage cross-cutting concerns in an application."

4. Explain the difference between SQL and NoSQL databases. When would you choose one over the other?

Answer: "SQL databases (like PostgreSQL, MySQL) are relational, with a predefined schema and structured data stored in tables. They are excellent for applications requiring strong transactional consistency (ACID properties), like a banking system. NoSQL databases (like MongoDB, Cassandra) are non-relational, with dynamic schemas for unstructured data. I would choose a NoSQL database for applications with massive scalability needs, flexible data models, and where high write throughput is critical, such as an IoT application or a social media feed."

5. What is a database index and why is it important for performance?

Answer: "An index is a data structure, like a B-tree, that improves the speed of data retrieval operations on a database table at the cost of slower writes and increased storage space. Without an index, the database engine has to scan the entire table (a 'full table scan') to find the rows that match a query's condition. With an index on a specific column, the database can quickly look up the location of the data, similar to looking up a word in a book's index instead of reading the entire book. It's crucial for the performance of `SELECT` queries on large tables."

6. What are the key principles of a RESTful API?

Answer: "The key principles are:
1. Client-Server Architecture: The client and server are separate concerns.
2. Statelessness: Each request from a client to the server must contain all the information needed to understand and complete the request. The server does not store any client context between requests.
3. Cacheability: Responses must define themselves as cacheable or not to improve performance.
4. Uniform Interface: Using standard conventions, like using HTTP verbs (`GET`, `POST`, `PUT`, `DELETE`) to perform actions on resources identified by URIs."