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Express.js | app.listen() Function

  • Last Updated : 18 Jun, 2020

The app.listen() function is used to bind and listen the connections on the specified host and port. This method is identical to Node’s http.Server.listen() method.

If the port number is omitted or is 0, the operating system will assign an arbitrary unused port, which is useful for cases like automated tasks (tests, etc.).

The app returned by express() is in fact a JavaScript function, designed to be passed to Node’s HTTP servers as a callback to handle requests. This makes it easy to provide both HTTP and HTTPS versions of your app with the same code base, as the app does not inherit from these (it is simply a callback).




var express = require('express')
var https = require('https')
var http = require('http')
var app = express()
  
http.createServer(app).listen(80)
https.createServer(options, app).listen(443)

Syntax:

app.listen([port[, host[, backlog]]][, callback])

Installation of express module:



  1. You can visit the link to Install express module. You can install this package by using this command.
    npm install express
  2. After installing express module, you can check your express version in command prompt using the command.
    npm version express
  3. After that, you can just create a folder and add a file for example, index.js. To run this file you need to run the following command.
    node index.js

Filename: index.js




var express = require('express');
var app = express();
var PORT = 3000;
  
app.listen(PORT, function(err){
    if (err) console.log("Error in server setup")
    console.log("Server listening on Port", PORT);
})

Steps to run the program:

  1. The project structure will look like this:
  2. Make sure you have installed express module using the following command:
    npm install express
  3. Run index.js file using below command:
    node index.js

    Output:

    Server listening on Port 3000
    

So this is how you can use the express app.listen() function which Binds and listens for connections on the specified host and port.

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