Node.js

• built on Google Chrome's Javascript Engine. (Written in Javascript)
• Single thread application but run in concurrency support by callbacks and event.
• Runtime environment + Javascript Library = Node.js
• Open Source.
• Cross platform ( run on OS X, Windows, and Linux)
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Features

• Acronymous and Event Driven 
• All API's of Node.js Library is Acronymous with non-blocking. (Never wait for API return data)
• E.g. 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
• Single Threaded but Highly Scalable 
• Use 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  not like traditional servers (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.

Blocking Code

• The program blocks until it reads the file and then only it proceeds to end the program.
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• Read From File!!!!!
  Program Ended

Non-Blocking Code

• The program does not wait for file reading and proceeds to print "Program Ended" and at the same time, the program without blocking continues reading the file.
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• Program Ended
  Read From File!!!!

Callback 

• is an asynchronous equivalent for a function. 
• is called at the completion of a given task. 
• Node.js heavy use of callbacks. All the APIs of Node.js are written in such a way that they support callbacks.
• E.g. a function to read a file may start reading the file and return the control to the execution environment immediately so that the next instruction can be executed. Once file I/O is complete, it will call the callback function while passing the callback function, the content of the file as a parameter. So there is no blocking or wait for File I/O. This makes Node.js highly scalable, as it can process a high number of requests without waiting for any function to return results.

Event-Driven Programming

• Node.js starts its server, it simply initiates its variables, declares functions and then simply waits for the event to occur.
• In an event-driven application, there is a main loop that listens for events and then triggers a callback function when one of those events is detected.
• Node.js uses observer pattern. Node thread keeps an event loop and whenever a task gets completed, it fires the corresponding event which signals the event listener function to execute.
• Whenever an event gets fired, its listener function starts executing. 
• Node.js has multiple in-built events (events module and EventEmitter) which used to bind events and event-listeners.

Different Event and Callback

• Callback functions are called when an asynchronous function return its result.
• Whereas event handling work on observer pattern.