What does middleware and app.use actually mean in Expressjs?

Question

Almost every Express app I see has an app.use statement for middleware but I haven\'t found a clear, concise explanation of what middleware actually is and what

Answer 1

expressjs guide has pretty neat answer to your question, I highly recommend you to read that, I am posting a short snippet of the guide, the guide is quite good.

Writing middleware for use in Express apps

Overview

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.

If the current middleware function does not end the request-response cycle, it must call next() to pass control to the next middleware function. Otherwise, the request will be left hanging.

Example

Here is an example of a simple “Hello World” Express application. The remainder of this article will define and add two middleware functions to the application: one called myLogger that prints a simple log message and another called requestTime¹ that displays the timestamp of the HTTP request.

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

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

app.listen(3000)   

Middleware function myLogger

Here is a simple example of a middleware function called “myLogger”. This function just prints “LOGGED” when a request to the app passes through it. The middleware function is assigned to a variable named myLogger.

var myLogger = function (req, res, next) {
  console.log('LOGGED')
  next()
}

Notice the call above to next(). Calling this function invokes the next middleware function in the app. The next() function is not a part of the Node.js or Express API, but is the third argument that is passed to the middleware function. The next() function could be named anything, but by convention it is always named “next”. To avoid confusion, always use this convention.

To load the middleware function, call app.use(), specifying the middleware function. For example, the following code loads the myLogger middleware function before the route to the root path (/).

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

var myLogger = function (req, res, next) {
  console.log('LOGGED')
  next()
}

app.use(myLogger)

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

app.listen(3000)

Every time the app receives a request, it prints the message “LOGGED” to the terminal.

The order of middleware loading is important: middleware functions that are loaded first are also executed first.

If myLogger is loaded after the route to the root path, the request never reaches it and the app doesn’t print “LOGGED”, because the route handler of the root path terminates the request-response cycle.

The middleware function myLogger simply prints a message, then passes on the request to the next middleware function in the stack by calling the next() function.

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1. This post will only contain myLogger middleware, for further post you could go to the original expressjs guide here.