Express app.use() Function

The app.use() function in Express.js adds middleware to the application's request-processing pipeline. It applies the specified middleware to all incoming requests or to specific routes, allowing you to modify request/response objects, perform operations, or handle errors throughout the 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": "^5.1.0",
} 

Example: Below is the code example of middleware.

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

// This middleware will log the request and 
// allow it to proceed to the next handler
app.use(function (req, res, next) {
    console.log("Middleware called");
    next();
});

// Requests will reach this route after 
// passing through the middleware
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:

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

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.