Express.js Middleware - Example - Tutorial Kart
Express.js Middleware ? Example
Express.js Middleware
What is Middleware?
Middleware is a function that can access request and response objects and can also use next function in the application's request-response cycle. In this tutorial, we will learn how to define a middleware function in Node.js Express application and how to make a call to the middleware function.
Middleware Terminology
request ? is the HTTP request that reaches the Express application when a client makes HTTP request like PUT, GET, etc. It contains properties like query string, url parameters, headers, etc. response ? object represents the HTTP response that an Express application sends when it gets an HTTP request. next ? next is used to continue with the next middleware in the middleware stack. request-response cycle ? The cycle of operations that get executed starting with a request hitting the Express application till a response leaves the application for the request. middleware stack ? stack of middleware functions that get executed for a request-response cycle.
Define Middleware Function
As we have already mentioned in the definition of middleware function, it has access to request, response objects and next function. The syntax is same as that of a JavaScript Function. It accepts request, response objects and next function as arguments.
function logger(req, res, next) { } function logger(req, res, next) {
}
here, logger is the function name, req is the HTTP request object, res is the Node Response Object and next is the next function in request-response cycle. You can access all the properties and methods of request object req .
Similarly, you can access all the properties and methods of response object res .
Calling next() function inside the middleware function is optional. If you use next() statement, the execution continues with the next middleware function in request-response cycle. If you do not call next() function, the execution for the given request stops here.
function logger(req, res, next) {
// your code next() // calls the nfeuxntctfiuonncltoiogngeinr(rtehqe, res, next) { middleware stack } // your code next() // calls the next function in the middleware stack }
Call Middleware
In an Express application, you call middleware using use function on application object.
var express = require('express') var app = express() fvuanrc teixopnrelosgsg=err(ereqqu,ire('express') res, next) { va/r/ aypopur=ceoxdperess()
next() } function logger(req, res, next) { app/./uysoeu(lrogc goedre)
next() }
app.use(logger)
Express.js Middleware Example
In this example, we will define a middleware called logger which logs the current time and query string to the console.
app.js
var express = require('express') var app = express() // define middleware func t ion function logger(req, res, next) {
c onsole.log(new Date(), req.url)
next() }
var express = require('express') var app = express() // define middleware function function logger(req, res, next) {
console.log(new Date(), req.url) next() } // calls logger:middleware for each request-response cycle app.use(logger) // route that gets executed for the path '/' app.get('/', function (req, res) { res.send('This is a basic Example for Express.js by TUTORIALKART') }) // start the server var server = app.listen(8000, function(){ console.log('Listening on port 8000...') })
Start this application and hit the following urls in your browser.
The output would be
For each request made to the application listening on 8000, we attached a middleware function. For the url , the url is / and hence the output of logger is current time and `/'. Similarly for url '/hello-page/' .
Node.js
Node.js Tutorial
Get Started W ith Node.js
Install Node.js Ubuntu Linux Install Node.js Windows Node.js - Basic Example Node.js - Command Line Arguments Node.js - Modules Node.js - Create a module Node.js - Add new functions to Module Node.js - Override functions of Module Node.js - Callback Function Node.js - forEach
E xp re s s .j s
Express.js Tutorial What is Express.js? Express.js Application Example Install Express.js Express.js Routes Express.js Middleware Express.js Router
Node.js Buffers
Node.js Buffer - Create, Write, Read Node.js Buffer - Length Node.js - Convert JSON to Buffer Node.js - Array to Buffer
Node.js HTTP
Node.js - Create HTTP Web Server Node.js - Redirect URL
Node.js MySQL
Node.js MySQL Node.js MySQL - Connect to MySQL Database Node.js MySQL - SELECT FROM Node.js MySQL - SELECT WHERE Node.js MySQL - ORDER BY Node.js MySQL - INSERT INTO
Node.js MySQL - UPDATE Node.js MySQL - DELETE Node.js MySQL - Result Object
Node.js MongoDB
Node.js MongoDB Node.js - Connect to MongoDB Node.js - Create Database in MongoDB Node.js - Drop Database in MongoDB Node.js - Create Collection in MongoDB Node.js - Delete Collection in MongoDB Node.js - Insert Documents to MongoDB Collection MongoError: failed to connect to server
Node.js Mongoose
Node.js Mongoose Tutorial Node.js Mongoose - Installation Node.js Mongoose - Connect to MongoDB Node.js Mongoose - Define a Model Node.js Mongoose - Insert Single Document to MongoDB Node.js Mongoose - Insert Multiple Documents to MongoDB
Node.js URL
Node.js - Parse URL parameters
Node.js FS (File System)
Node FS Node FS - Read a File Node FS - Create a File Node FS - Write to a File Node FS - Append to a File Node FS - Rename a File Node FS - Delete a File Node FS Extra - Copy a Folder
Node.js JSON
Node.js Parse JSON Node.js Write JSON Object to File
Node.js Error Handling
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