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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.

Redux Thunk is a middleware that allows action creators to return a function instead of an action object. This enables handling asynchronous operations. For example, you can fetch data from an API and dispatch an action once the data is received.

Actions in Redux are plain JavaScript objects that represent an intention to change the state. They must have a 'type' property. For example, an action to add a user might look like this: { type: 'ADD_USER', payload: { id: 1, name: 'John' } }.

The Redux store holds the application's state. It provides methods to access the state, dispatch actions, and register listeners. For example, when an action is dispatched to add a new item, the store updates its state and notifies subscribers.

Redux Toolkit is the official, recommended way to write Redux logic. It simplifies store setup and reduces boilerplate. With features like 'createSlice', it allows for easy state management and action creation, making development faster and more intuitive.

To create a slice in Redux Toolkit, use 'createSlice'. It takes an object with a name, initial state, and reducers. For example: const userSlice = createSlice({ name: 'user', initialState: {}, reducers: { addUser: (state, action) => { state[action.payload.id] = action.payload; } }});.

Redux is a predictable state container for JavaScript apps. It helps manage the state of an application in a centralized way, allowing for easier debugging and testing. For example, in a React app, Redux can store user authentication status, which can be accessed by any component.

createAsyncThunk is a utility in Redux Toolkit for handling asynchronous actions. It simplifies the process of creating thunks. For example: const fetchUser = createAsyncThunk('user/fetch', async (userId) => { const response = await fetch(`/api/users/${userId}`); return response.json(); });.

Add custom properties to `req` in middleware. For example: `app.use((req, res, next) => { req.customProperty = 'value'; next(); });` allows access to `req.customProperty` in subsequent middleware and routes.

Implement authentication using middleware like `passport` or `jsonwebtoken`. For example: `passport.authenticate('local')` or verify JWT tokens in middleware to control access based on user credentials.

Use the `dotenv` package to manage environment variables. Install it with `npm install dotenv`. Create a `.env` file with variables like `PORT=3000`, and access them with `process.env.PORT` in your code.

Set response headers using `res.set()`. For example: `res.set('Content-Type', 'application/json');` sets the `Content-Type` header. You can also use `res.header()` for similar functionality.

Synchronous errors are caught using try-catch blocks, while asynchronous errors should be handled with `.catch()` or async error-handling middleware. For example: `app.use(async (req, res, next) => { try { await asyncFunction(); } catch (err) { next(err); } });`.

A reducer is a pure function that takes the current state and an action as arguments, returning a new state. For instance, a user reducer might handle actions like 'ADD_USER' or 'REMOVE_USER' and return the updated user list.

Middleware in Redux provides a way to extend Redux's capabilities, allowing for side effects like API calls. For instance, 'redux-thunk' enables action creators to return functions instead of actions, facilitating asynchronous logic within your app.

Middleware functions process requests before they reach route handlers. They can modify `req` or `res`, end the request-response cycle, or pass control. For example: `app.use((req, res, next) => { console.log('Request received'); next(); });` logs every request.

`app.use()` registers middleware to process requests. For example: `app.use(express.json());` applies JSON parsing middleware globally. You can also use it for routing, e.g., `app.use('/api', apiRoutes);` to mount routers.

Selectors are functions that extract specific pieces of state from the Redux store. They enhance performance and readability. For instance, a selector like 'getUserById' can retrieve a user by their ID, allowing components to access only the necessary data.

MyISAM is a non-transactional storage engine, ideal for read-heavy applications, while InnoDB supports transactions, foreign keys, and row-level locking, making it suitable for high-concurrency environments. For instance, InnoDB is preferable for e-commerce sites where data integrity is critical.

Data validation in MongoDB can be implemented using schema validation rules defined in collections. By specifying validation criteria using JSON Schema, you can enforce data integrity. For example, a `users` collection can have a validation rule that ensures the 'age' field is an integer between 0 and 120, preventing invalid data entries.