Express.js Middleware Patterns

# Express.js Middleware Patterns
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## Core Principles
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* **Separation of Concerns**: Each middleware function should have a single responsibility and not mix multiple, unrelated functionalities.
* **Reusability**: Middleware functions should be designed to be reusable across different routes and applications.
* **Readability**: Middleware functions should be easy to understand and follow a consistent naming convention.

## Code Style Guidelines
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### Naming Conventions

* **Middleware Function Names**: Use camelCase and include a descriptive suffix (e.g., `authenticateUser`, `validateRequestData`).
* **Variable Names**: Use meaningful and descriptive variable names (e.g., `req`, `res`, `next`).

### Typing

* **Type Definitions**: Use JSDoc-style comments to define the types of function parameters and return values.
* **Type Checking**: Use a type checker like TypeScript to ensure type safety.

## Best Practices
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### Middleware Function Structure

* **Keep it Simple**: Avoid complex logic and conditional statements within middleware functions.
* **Error Handling**: Handle errors and exceptions properly, and pass them to the next error-handling middleware function using `next(err)`.
* **Async/Await**: Use async/await syntax to handle asynchronous operations and avoid callback hell.

### Middleware Ordering

* **Order Matters**: The order of middleware functions in the stack matters, as each function will be executed in sequence.
* **Error-Handling Middleware**: Place error-handling middleware functions at the end of the stack to catch any errors that occur in previous middleware functions.

### Request and Response Objects

* **Request Object**: Use the `req` object to access request data, such as query parameters, body, and headers.
* **Response Object**: Use the `res` object to send responses, such as JSON data, redirects, and errors.

## Common Pitfalls to Avoid
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### Overusing Middleware Functions

* **Avoid Over-Engineering**: Don't create too many middleware functions, as this can lead to a complex and hard-to-maintain codebase.
* **Keep it Focused**: Ensure each middleware function has a clear and specific purpose.

### Not Handling Errors Properly

* **Error Handling**: Always handle errors and exceptions properly, and pass them to the next error-handling middleware function using `next(err)`.
* **Don't Swallow Errors**: Avoid swallowing errors, as this can lead to unexpected behavior and make debugging difficult.

### Not Following Best Practices

* **Follow Established Patterns**: Follow established middleware patterns and best practices to ensure consistency and maintainability.
* **Keep it Simple and Consistent**: Avoid complex and inconsistent code, as this can lead to maintenance issues and make the codebase harder to understand.