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What is NodeJS :

  • Node.js lets us use JavaScript language on the server… so it allows us to write JavaScript outside the browser!
  • Node.js has all the power of JavaScript and offers us a whole new way of developing dynamic websites.
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    • Asynchronous Event Driven Server Side JavaScript
    • Non-blocking I/O and File API
    • Runs on Google JS V8 Engine
    • Single Threaded
    • Highly Scalable
    • Node Apps are created using JavaScript
    • Faster in performance
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  • 1.Multiple requests are sent to the Node.js server.
  • 2.The event-driven nature of Node.js creates a loop of events for all requests and handles them under a single thread.
  • 3.These events are then delegated to the appropriate non-blocking event handler.
  • 4.The response received travels the reverse way to the requested end point.
  • 5.Since the process happens under a single thread, the response is sent asynchronously without causing page refresh.
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Explain NodeJS :

  • An open-source runtime environment that contains JavaScript code for execution in the server to aid in asynchronous request-response strategy.
  • A library with variety of modules needed for network and web application development.
  • Is developed by Ryan Dahl in 2009.
  • Is part of V8 JavaScript, a Google's implementation for the Chrome browser, with "Just in Time" compilation services to achieve high speeds for executions.
  • Uses a non-blocking, event-driven I/O environment to handle high-critical real-time applications running across distributed devices.
  • Prevents its applications from data buffers, regardless of its extended service processing tasks
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History of NodeJS :

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learn nodejs - nodejs tutorial - nodejs example programs

How webpage works :

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How webpage works

how nodejs works :

  • Node.js offers a server-side environment, which allows us to also use JavaScript language to generate web pages. Basically, it replaces server languages such as PHP, Java EE, etc.
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learn nodejs - nodejs tutorial - What is nodejs - nodejs example programs

Blocking model vs. non-blocking model :

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  • Considering a program, which upload a file and then display it. Below is the code written in a blocking model:
    • Upload the file
    • Display the file
    • Do something else
  • The actions are carried out in top to bottom order.
    • The program will upload a file to the Internet.
    • The program will display the file to the user.
    • Then, the program can do other things (i.e. carry out other actions).
  • Lets see the same code in a non-blocking model:
    • Upload a file
    • As soon as it’s finished, display the file
    • Do something else
  • The program wont carry out the tasks in the order that they are written. It does this:
    • The program launches the upload of the file to the internet.
    • The program does other things (the program follows its course).
    • As soon as the upload is finished, the program carries out the actions that we asked it to do: it displays the file.

On the whole, the running of the program can therefore be represented as in the next figure.

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learn nodejs - nodejs tutorial - Non blocking mode of nodejs - nodejs examples - nodejs programs

This is exactly how Node.js works. As soon as "File uploaded" appears, a function known as a callback function is called and carries out actions (here, the callback function displays the file).

  // // Identical result to the previous code

var callback = function (error, response, body) {
console.log ("Hey Wikitechy File uploaded!");
});

request (‘http://www.site.com/file.zip',callback);

console.log ("I do other things while I’m waiting… "); 
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nodejs sample code :

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  • Line 1
    • Loading http module
    • It is required to work with HTTP request and response
    • It is required to create HTTP Server
  • Line 2
    • Create server using createServer() function
    • It takes a callback as parameter . It takes HTTP request and response as paramtere
    • Callback got two input parameter .
    • Both input parameters are readable and writeable stream
  • Line 4
    • Writing response header
    • It takes a JSON object as optional second parameter
    • second parameter contains informations like
    • Content Length
    • Content Type
    • Connection
    • Accept Type
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  • Line 5-6
    • Writing data in response
    • Data can be written in form of string or buffer
    • ServerResponse.end indicates the communication has been finished
  • Line 9
    • createServer() method is finished with chained http.Server.listen() method
    • It specifies port number on which request will be listen Optional host name
  • Line 10
    • Printing a message
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nodejs workflow :

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  • It works on single thread
  • It handles all request asynchronously on same thread
  • It does not create new thread for each request which is very unlikely like other web servers
  • It does not wait to complete a request

node js - Asynchronous Vs Event-Driven :

  • Asynchronous code execution -
    • Not all code is executed in the same order it is written
    • Node (and you) divide the code into small pieces and fire them in parallel
    • Typically the code announces its state of execution (or completion) via events
  • Event-Driven code execution -
    • A code block gets into the queue of execution when an event happens
    • ------------- It gets executed on its turn
    • Anyone can raise (or listen to) an event
    • ------------- System
      ------------- Some library (module)
      ------------- Your code
    • You have the choice (and ways) to attach a code block to an event
    • ------------- Also known as event callbacks
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