Node.js

What is Node.js?
Node.js is an open source server framework
Node.js is free
Node.js runs on various platforms (Windows, Linux, Unix, Mac OS X, etc.)
Node.js uses JavaScript on the server

Why Node.js?
Node.js uses asynchronous programming!
A common task for a web server can be to open a file on the server and return the content to the client.
Here is how PHP or ASP handles a file request:
1. Sends the task to the computer's file system.
2. Waits while the file system opens and reads the file.
3. Returns the content to the client.
4. Ready to handle the next request.

Here is how Node.js handles a file request:
1. Sends the task to the computer's file system.
2. Ready to handle the next request.
3. When the file system has opened and read the file, the server returns the content to the client.

Node.js eliminates the waiting, and simply continues with the next request.
Node.js runs single-threaded, non-blocking, asynchronously programming, which is very memory efficient.

What Can Node.js Do?

Node.js can generate dynamic page content
Node.js can create, open, read, write, delete, and close files on the server
Node.js can collect form data
Node.js can add, delete, modify data in your database

What is a Node.js File?
Node.js files contain tasks that will be executed on certain events
A typical event is someone trying to access a port on the server
Node.js files must be initiated on the server before having any effect
Node.js files have extension ".js"

why use Node.js? 

Features of Node.js

Following are some of the important features that make Node.js the first choice of software architects.
Asynchronous and Event Driven − All APIs of Node.js library are asynchronous, that is, non-blocking. It essentially means a Node.js based server never waits for an API to return data. The server moves to the next API after calling it and a notification mechanism of Events of Node.js helps the server to get a response from the previous API call.
Very Fast − Being built on Google Chrome's V8 JavaScript Engine, Node.js library is very fast in code execution.
Single Threaded but Highly Scalable − Node.js uses a single threaded model with event looping. Event mechanism helps the server to respond in a non-blocking way and makes the server highly scalable as opposed to traditional servers which create limited threads to handle requests. Node.js uses a single threaded program and the same program can provide service to a much larger number of requests than traditional servers like Apache HTTP Server.
No Buffering − Node.js applications never buffer any data. These applications simply output the data in chunks.
License − Node.js is released under the MIT license.

Who Uses Node.js?

Following is the link on github wiki containing an exhaustive list of projects, application and companies which are using Node.js. This list includes eBay, General Electric, GoDaddy, Microsoft, PayPal, Uber, Wikipins, Yahoo!, and Yammer to name a few.

• Projects, Applications, and Companies Using Node

Concepts

Where to Use Node.js?

Following are the areas where Node.js is proving itself as a perfect technology partner.
1. I/O bound Applications
2. Data Streaming Applications
3. Data Intensive Real-time Applications (DIRT)
4. JSON APIs based Applications
5. Single Page Applications

Events in Node.js

Every action on a computer is an event. Like when a connection is made or a file is opened.
Objects in Node.js can fire events, like the readStream object fires events when opening and closing a file:

Example
var fs = require('fs');
var rs = fs.createReadStream('./demofile.txt');
rs.on('open', function () {
console.log('The file is open');
});

Events Module

Node.js has a built-in module, called "Events", where you can create-, fire-, and listen for- your own events.
To include the built-in Events module use the require() method. In addition, all event properties and methods are an instance of an EventEmitter object. To be able to access these properties and methods, create an EventEmitter object:

var events = require('events');
var eventEmitter = new events.EventEmitter();

The EventEmitter

ObjectYou can assign event handlers to your own events with the EventEmitter object.
In the example below we have created a function that will be executed when a "scream" event is fired.
To fire an event, use the emit() method.

Example

var events = require('events');
var eventEmitter = new events.EventEmitter();
//Create an event handler:var myEventHandler = function () {
console.log('I hear a scream!');
}
//Assign the event handler to an event:eventEmitter.on('scream', myEventHandler);
//Fire the 'scream' event:eventEmitter.emit('scream');