Middleware in express js

What is Middleware in Express.js?

Middleware in Express.js is a function that gets executed before the final route handler.

It can be used to check requests, log activities, authenticate users, validate data, and more.


How Middleware Works

function middleware(req, res, next) {
// Do something with req or res
next(); // Pass control to the next middleware or route
}
  1. req is the request object
  2. res is the response object
  3. next() moves to the next middleware or route handler


How the Client, Middleware, and Server Work Together

Client → Sends HTTP request to server (e.g., GET, POST)

Middleware → Intercepts and processes the request

Server → Executes the final route handler and sends a response


Middleware Example (code)

import express from "express";

const app = express();

function checkRoute(req, resp, next) {
console.log("user is acessing " + req.url + " Page");
next();
}

app.use(checkRoute);

app.get("/", (req, resp) => {
resp.send("Home Page");
});

app.get("/users", (req, resp) => {
resp.send("users Page");
});

app.get("/products", (req, resp) => {
resp.send("Products Page");
});

app.listen(3200);

This middleware (checkRoute) runs for every route and logs the page being accessed before the actual route is handled.

You can also write it like this using an arrow function:

app.use((req, resp, next) => {
console.log("user is acessing " + req.url + " Page");
next();
});


Types of Middleware

Application-level Middleware

  1. Registered using: app.use() or app.get()
  2. Purpose: Used to apply middleware globally or to specific routes in the main app.
  3. Example Use: Logging, request parsing, etc.

Router-level Middleware

  1. Registered using: router.use()
  2. Purpose: Works like application-level middleware, but applies to route instances (e.g., for grouping /admin or /user routes).
  3. Example Use: Authorization checks in a specific route group.

Built-in Middleware

  1. Comes with Express.js.
  2. Common example: express.static() to serve static files (like CSS, images).
  3. Purpose: Provides ready-to-use functionality without extra installation.

Error-handling Middleware

  1. Defined with four arguments: (err, req, res, next)
  2. Purpose: Catches and handles errors across the application.
  3. Example Use: Sending custom error messages, logging errors.

Third-party Middleware

  1. Installed via npm, and used with require() or import.
  2. Purpose: Add advanced functionalities.
  3. Common examples:
  4. morgan (logging)
  5. cors (Cross-Origin Resource Sharing)
  6. body-parser (parsing form data)