React Js, Node fullstack interview questions

What is React and why use it?

Answer: React is a JavaScript library for building user interfaces, primarily for single-page applications. It's used for handling the view layer and allows developers to create reusable UI components. React's key features include its virtual DOM that optimizes rendering and improves app performance, and its declarative nature that makes the code more predictable and easier to debug.

How does React's virtual DOM work?

Answer: The virtual DOM is a programming concept implemented by React that provides a virtual representation of the UI. It's a lightweight copy of the actual DOM. When a component's state changes, React first updates the virtual DOM. Then, it compares the updated virtual DOM with a pre-update version and determines the minimal changes needed to be applied to the real DOM. This process, known as "diffing," minimizes direct manipulations of the DOM, leading to better performance and a smoother user experience.

Explain the lifecycle methods of a React component.

Answer: React components go through a lifecycle that allows you to run custom behavior during different stages of its rendering process. Key lifecycle methods include:

• componentDidMount(): Invoked after the component is mounted to the DOM. It's used for initialization that requires DOM nodes, such as network requests.

• `shouldComponentUpdate(nextProps, nextState): Determines if the component should update in response to prop or state changes. Returning false` prevents the re-rendering.

• componentDidUpdate(prevProps, prevState): Invoked after updating occurs. It's useful for DOM manipulations and further state updates.

• componentWillUnmount(): Called before the component is removed from the DOM. It's used for cleanup, like invalidating timers and cleaning up subscriptions.

React 16.3 introduced new lifecycle methods (getDerivedStateFromProps and getSnapshotBeforeUpdate) and deprecated some older ones (componentWillMount, componentWillReceiveProps, and componentWillUpdate).

Node.js Interview Questions

What is Node.js and where can it be used?

Answer: Node.js is an open-source, cross-platform JavaScript runtime environment that executes JavaScript code outside a web browser. It's designed to build scalable network applications. Node.js can be used for various applications like command-line tools, web servers, real-time chat applications, RESTful APIs, and more, thanks to its non-blocking, event-driven architecture.

Explain the event loop in Node.js.

Answer: The event loop is a fundamental aspect of Node.js, enabling it to handle asynchronous operations. The event loop allows Node.js to perform non-blocking I/O operations despite JavaScript being single-threaded. It processes operations by cycling through phases like timers, I/O callbacks, idle, prepare, and close callbacks. Operations are queued and executed sequentially, allowing for efficient execution of operations like network requests or file I/O without blocking the main thread.

What are Streams in Node.js?

Answer: Streams are collections of data, like arrays or strings, that might not be available all at once and don't have to fit in memory. This makes streams ideal for reading or writing large amounts of data, like files or web requests. Node.js provides four types of streams: Readable, Writable, Duplex (both readable and writable), and Transform (a type of duplex stream that can modify or transform the data as it is written and read).

PostgreSQL Interview Questions

What is PostgreSQL and what are its features?

Answer: PostgreSQL is a powerful, open-source object-relational database system. It uses and extends the SQL language combined with many features that safely store and scale the most complicated data workloads. Key features include advanced data types, full support for different character sets, robust transaction and concurrency support, and extensibility. PostgreSQL is highly customizable, running stored procedures in more than a dozen programming languages, and includes powerful add-ons like PostGIS for geographic data.

How does PostgreSQL handle transactions?

Answer: PostgreSQL handles transactions within the scope of the ACID properties (Atomicity, Consistency, Isolation, Durability), ensuring data integrity and reliability. A transaction in PostgreSQL is a sequence of operations performed as a single logical unit of work. If a transaction is committed, all of the operations are successfully applied to the database. If it is rolled back, none of the changes are applied. PostgreSQL uses Multi-Version Concurrency Control (MVCC) to maintain consistency without locking the database for read operations, improving performance for multi-user environments.

Explain JOIN operations in PostgreSQL.

Answer: JOIN operations in SQL are used to combine rows from two or more tables based

on a related column between them. PostgreSQL supports several types of JOINs:

• **INNER JOIN**: Returns rows when there is at least one match in both tables.

• **LEFT JOIN** (or LEFT OUTER JOIN): Returns all rows from the left table, and the matched rows from the right table. If there's no match, the result is NULL on the right side.

• **RIGHT JOIN** (or RIGHT OUTER JOIN): Opposite of LEFT JOIN, returning all rows from the right table and matched rows from the left table.

• **FULL JOIN** (or FULL OUTER JOIN): Combines LEFT JOIN and RIGHT JOIN, returning rows when there is a match in one of the tables.

Each JOIN type serves different needs and choosing the right one depends on the relationship between the tables and the result you want to achieve.