How do middleware functions work in Express?

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What are Middleware Functions in Express?

In Express.js, middleware functions are functions that sit in between the incoming request (req) from the client and the final response (res) sent by the server. They act like a pipeline, processing requests step by step before reaching the destination.

How They Work

1. Execution Order – Middleware runs in the order they are defined. Each function has access to the request (req), response (res), and a special next() function.

2. The next() Function – After doing its job (like logging, authentication, or data parsing), middleware calls next() to pass control to the next middleware in the stack. If next() is not called, the request will be stuck.

3. Types of Middleware:

   • Application-level middleware → Applied across the app (e.g., logging all requests).

   • Router-level middleware → Applied only to specific routes.

   • Built-in middleware → Provided by Express (like express.json() for parsing JSON).

   • Error-handling middleware → Captures and handles errors in the pipeline.

Common Uses of Middleware

• Logging: Keeping track of requests.

• Authentication/Authorization: Checking if a user is logged in or has access rights.

• Data Parsing: Converting request body into JSON or URL-encoded data.

• Error Handling: Catching and sending error responses gracefully.

• Serving Static Files: Delivering images, CSS, or JS directly.

✅ In short:
Middleware in Express works like a chain of functions. Each one processes the request and either sends a response or passes it along using next(). This modular design makes applications flexible, organized, and easier to maintain.

Read More :

How do you optimize performance in a React application?

What is Express.js, and why is it used in Node.js applications?

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