How do you use environment variables in a Node.js application?

In Node.js, error handling is crucial for building robust applications. For synchronous code, use try-catch blocks. For asynchronous code, handle errors in callbacks or use promise-based methods with `.catch()`. Middleware functions in Express can centralize error handling. Always log errors and provide meaningful messages for debugging and user feedback.

File uploads in Node.js can be handled using middleware libraries like `multer`. Install it with `npm install multer`. Set up `multer` as middleware in your Express routes to handle multipart form data and save uploaded files. Configure storage options and file filters to manage file types and sizes. Handle uploaded files in your route handlers and save them to disk or a cloud service.

In Node.js, the Event Emitter class is used to handle events and listeners. The `events` module provides the `EventEmitter` class, which allows you to create instances that can emit events and register listeners for those events. Use `emitter.on('event', listener)` to add a listener and `emitter.emit('event', args)` to trigger the event. This pattern is useful for asynchronous programming and decoupled component interactions.

Middleware functions in Express are functions that have access to the request, response, and the next middleware function in the application’s request-response cycle. Implement middleware by defining a function with `(req, res, next)` parameters and using `app.use(middlewareFunction)` to apply it globally or `router.use(middlewareFunction)` for specific routes. Middleware can perform tasks such as logging, authentication, or request modification.

The `process.env` object in Node.js is used to access environment variables. It provides a way to store configuration settings, such as API keys or database connection strings, outside of the codebase. By using `process.env`, you can manage different configurations for development, testing, and production environments without hardcoding values into your application.

Manage environment variables in Node.js using a `.env` file and the `dotenv` package. Install it with `npm install dotenv` and require it at the beginning of your application with `require('dotenv').config()`. Define variables in `.env` like `PORT=3000` and access them using `process.env.PORT`. This approach helps keep sensitive information and configuration separate from code.

Node.js can interact with databases such as MongoDB, MySQL, or PostgreSQL using libraries like Mongoose or Sequelize. Example: Mongoose is an ORM for MongoDB, and Sequelize is used for SQL-based databases. The interaction is typically asynchronous using Promises or async/await.

readFile reads the entire file into memory, which can be inefficient for large files, whereas createReadStream reads the file in chunks, making it more memory efficient. Example: Use fs.createReadStream() when reading large files to prevent memory overload.

The 'require' function is used to import modules in Node.js, following the CommonJS module system. Example: To import the 'fs' module, you use 'const fs = require('fs');' to access file system operations like reading or writing files.

Express.js is a fast, minimal web framework for Node.js that simplifies server creation and request handling. Example: Express allows you to define routes and middleware in a structured way. A simple Express app might handle GET requests at '/home' with app.get('/home').

Clustering allows Node.js to create child processes (workers) that share the same server port to handle multiple requests in parallel. This improves application performance by leveraging multi-core systems. Example: Using the cluster module to spawn worker processes for increased throughput.

ES modules allow you to use JavaScript's import/export syntax in Node.js. Example: In an ES module, you can import another module using 'import { func } from './module.js';' instead of using 'require'. Node.js supports ES modules natively from version 12 onwards.

The crypto module in Node.js provides cryptographic functionalities like hashing, encryption, and decryption. Example: To hash a password before storing it, you can use crypto.createHash('sha256').update('password').digest('hex'). It ensures data security by generating unique and irreversible hashes.

npm (Node Package Manager) is the default package manager for Node.js, used to manage packages and dependencies in Node.js projects. Example: npm install installs dependencies, and npm init initializes a new project with a package.json file to manage those dependencies.

Environment variables in Node.js can be accessed via process.env. These variables allow you to configure application settings such as API keys or database URLs without hardcoding sensitive information. Example: Accessing an environment variable using process.env.DB_URL in the code.

Synchronous methods block the event loop until the operation is complete, while asynchronous methods allow the program to continue running while the operation completes in the background. Example: fs.readFileSync is synchronous, while fs.readFile is asynchronous, not blocking the event loop.