Middleware functions are functions that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle. The next function is a function in the Express router which, when invoked, executes the middleware succeeding the current middleware.

Middleware functions can perform the following tasks:

• Execute any code.
• Make changes to the request and the response objects.
• End the request-response cycle.
• Call the next middleware in the stack.

Here is an example of a simple “Hello World” Express application.

var express = require('express')
var app = express()

app.get('/', function (req, res) {
  res.send('Hello World!')
})

app.listen(3000)

Middleware function logger

Here is a simple example of a middleware function called “logger”. This function just prints “logged” when a request to the app passes through it.

var logger = function (req, res, next) {
  console.log('logged')
  next()
}

To load the middleware function, call app.use(), specifying the middleware function. The following code loads the logger middleware function before the route to the root path (/). Here is whole code.

var express = require('express')
var app = express()

var logger = function (req, res, next) {
  console.log('logged')
  next()
}

app.use(logger)

app.get('/', function (req, res) {
  res.send('Hello World!')
})

app.listen(3000)