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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.

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').

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 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.

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.

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.

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.

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); } });`.

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.

`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.

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.

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.

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' } }.

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.

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.

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.

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.

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.

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; } }});.

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(); });.

Database migration involves transferring data between different database systems or versions. This can be achieved using tools like `mysqldump` for exporting and importing data or third-party migration tools. For instance, exporting a database with `mysqldump` and importing it to a new server using `mysql` command facilitates migration.

User-defined functions (UDFs) allow users to create custom functions to encapsulate reusable logic in SQL. UDFs can take parameters and return values. For example, a UDF to calculate tax could be defined as `CREATE FUNCTION CalculateTax(amount DECIMAL) RETURNS DECIMAL BEGIN RETURN amount * 0.1; END;`.