Express app.use() Function

The app.use() function is used to mount the specified middleware function(s) at the path that is being specified. It is mostly used to set up middleware for your application. 

Syntax:

app.use(path, callback)

Parameters:

• path: It is the path for which the middleware function is being called. It can be a string representing a path or path pattern or a regular expression pattern to match the paths.
• callback: It is a middleware function or a series/array of middleware functions.

Steps to create the application:

Step 1: You can install this package by using this command.

npm install express

Step 2: After installing the express module, you can check your express version in the command prompt using the command.

npm version express

Project Structure:

The updated dependencies in package.json file will look like:

"dependencies": {
    "express": "^4.18.2",
} 

Example: Below is the code example of middleware.

Javascript

const express = require('express');
const app = express();
const PORT = 3000;

// This middleware will not allow the
// request to go beyond it
app.use(function (req, res, next) {
    console.log("Middleware called")
    next();
});

// Requests will never reach this route
app.get('/user', function (req, res) {
    console.log("/user request called");
    res.send('Welcome to GeeksforGeeks');
});

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

Steps to run the program:

node index.js

Console Output:

Output

Browser Output: Now open your browser and go to http://localhost:3000/user and you can see

Output

We have a complete list of Express Application module methods, properties and events, to check those please go through this Express.js Application Complete Reference article.