What is Node.js? [closed]

Answer 1

Q: The programming model is event driven, especially the way it handles I/O.

Correct. It uses call-backs, so any request to access the file system would cause a request to be sent to the file system and then Node.js would start processing its next request. It would only worry about the I/O request once it gets a response back from the file system, at which time it will run the callback code. However, it is possible to make synchronous I/O requests (that is, blocking requests). It is up to the developer to choose between asynchronous (callbacks) or synchronous (waiting).

Q: It uses JavaScript and the parser is V8.

Yes

Q: It can be easily used to create concurrent server applications.

Yes, although you'd need to hand-code quite a lot of JavaScript. It might be better to look at a framework, such as http://www.easynodejs.com/ - which comes with full online documentation and a sample application.

Answer 2

There is a very good fast food place analogy that best explains the event driven model of Node.js, see the full article, Node.js, Doctor’s Offices and Fast Food Restaurants – Understanding Event-driven Programming

Here is a summary:

If the fast food joint followed a traditional thread-based model, you'd order your food and wait in line until you received it. The person behind you wouldn't be able to order until your order was done. In an event-driven model, you order your food and then get out of line to wait. Everyone else is then free to order.

Node.js is event-driven, but most web servers are thread-based.York explains how Node.js works:

• You use your web browser to make a request for "/about.html" on a Node.js web server.

• The Node.js server accepts your request and calls a function to retrieve that file from disk.

• While the Node.js server is waiting for the file to be retrieved, it services the next web request.

• When the file is retrieved, there is a callback function that is inserted in the Node.js servers queue.

• The Node.js server executes that function which in this case would render the "/about.html" page and send it back to your web browser."

Answer 3

Well, I understand that

• Node's goal is to provide an easy way to build scalable network programs.
• Node is similar in design to and influenced by systems like Ruby's Event Machine or Python's Twisted.
• Evented I/O for V8 javascript.

For me that means that you were correct in all three assumptions. The library sure looks promising!

Answer 4

The closures are a way to execute code in the context it was created in.

What this means for concurency is that you can define variables, then initiate a nonblocking I/O function, and send it an anonymous function for its callback.

When the task is complete, the callback function will execute in the context with the variables, this is the closure.

The reason closures are so good for writing applications with nonblocking I/O is that it's very easy to manage the context of functions executing asynchronously.

Answer 5

Node.js is an open source command line tool built for the server side JavaScript code. You can download a tarball, compile and install the source. It lets you run JavaScript programs.

The JavaScript is executed by the V8, a JavaScript engine developed by Google which is used in Chrome browser. It uses a JavaScript API to access the network and file system.

It is popular for its performance and the ability to perform parallel operations.

Understanding node.js is the best explanation of node.js I have found so far.

Following are some good articles on the topic.

• Learning Server-Side JavaScript with Node.js
• This Time, You’ll Learn Node.js

Answer 6

V8 is an implementation of JavaScript. It lets you run standalone JavaScript applications (among other things).

Node.js is simply a library written for V8 which does evented I/O. This concept is a bit trickier to explain, and I'm sure someone will answer with a better explanation than I... The gist is that rather than doing some input or output and waiting for it to happen, you just don't wait for it to finish. So for example, ask for the last edited time of a file:

// Pseudo code
stat( 'somefile' )

That might take a couple of milliseconds, or it might take seconds. With evented I/O you simply fire off the request and instead of waiting around you attach a callback that gets run when the request finishes:

// Pseudo code
stat( 'somefile', function( result ) {
  // Use the result here
} );
// ...more code here

This makes it a lot like JavaScript code in the browser (for example, with Ajax style functionality).

For more information, you should check out the article Node.js is genuinely exciting which was my introduction to the library/platform... I found it quite good.