Express.js | router.use() Function

The router.use() function uses the specified middleware function or functions. It basically mounts middleware for the routes which are being served by the specific router. 

Syntax:

router.use( path, function )

Parameters:

• Path: It is the path to this middleware, if we can have /user, now this middleware is called for all API’s having /user of this router.
• function: This function is passed as a callback, it is called when the specified path is called in this router.

Installation of the express module:

You can visit the link to Install the express module. You can install this package by using this command.

npm install express

After installing the express module, you can check your express version in the command prompt using the command.

npm version express

After that, you can just create a folder and add a file, for example, index.js. To run this file you need to run the following command.

node index.js

Project Structure:

Filename: index.js

javascript

const express = require('express');
const app = express();
const router = express.Router();
const PORT = 3000;

// All requests to this router will
// first hit this middleware
router.use(function (req, res, next) {
    console.log("Middleware Called");
    next();
})

// Always invoked
router.use(function (req, res, next) {
    res.send("Greetings from GeeksforGeeks");
})

app.use('/user', router);

app.listen(PORT, function (err) {
    if (err) console.log(err);
    console.log("Server listening on PORT", PORT);
});

Steps to run the program:

The project structure will look like this: 

Make sure you have installed the express module using the following command:

npm install express

Run the index.js file using the below command:

node index.js

Output:

Server listening on PORT 3000

Now open your browser and go to http://localhost:3000/user, you can see the following output on your screen:

Server listening on PORT 3000
Middleware Called

And you will see the following output on your browser:

Greetings from GeeksforGeeks