Node.js

How node.js works?

Node.js works on a v8 environment, it is a virtual machine that utilizes JavaScript as its scripting language and achieves high output via non-blocking I/O and single threaded event loop.

Thread

OS starting to execute the program

What are the pros and cons of Node.js?

Pros: a) If your application does not have any CPU intensive computation, you can build it in Javascript top to bottom, even down to the database level if you use JSON storage object DB like MongoDB. b) Crawlers receive a full-rendered HTML response, which is far more SEO friendly rather than a single page application or a websockets app run on top of Node.js. Cons: a) Any intensive CPU computation will block node.js responsiveness, so a threaded platform is a better approach. b) Using relational database with Node.js is considered less favourable

True globals of Node.js:

*global - The global namespace. Setting a property to this namespace makes it globally visible within the running process. *process - Node's built-in process module, which provides interaction with the current Node process. *console - Node's built-in console module, which wraps various STDIO functionality in a browser-like way. *setTimeout(), clearTimeout(), setInterval(), clearInterval() - The built-in timer functions are globals. Read More

What are the Challenges with Node.js ?

Emphasizing on the technical side, it's a bit of challenge in Node.js to have one process with one thread to scale up on multi core server.

Why is non-blocking good?

Ex. multiple functions can be read at the same time

Pseudo-globals of Node.js:

* module, module.exports, exports - These objects all pertain to Node's module system. * filename - The `filename` keyword contains the path of the currently executing file. Note that this is not defined while running the Node REPL. * dirname - Like `filename, the __dirname` keyword contains the path to the root directory of the currently executing script. Also not present in the Node REPL. * require() - The require() function is a built-in function, exposed per-module, that allows other valid modules to be included. Read More

Mention the framework most commonly used in node.js?

"Express" is the most common framework used in node.js

Callbacks are placed on the

event queue

What is control flow function?

A generic piece of code which runs in between several asynchronous function calls is known as control flow function.

Mention the steps by which you can async in Node.js?

By following steps you can async Node.js a) First class functions b) Function composition c) Callback Counters d) Event loops

What is 'Callback' in node.js?

Callback function is used in node.js to deal with multiple requests made to the server. Like if you have a large file which is going to take a long time for a server to read and if you don't want a server to get engage in reading that large file while dealing with other requests, call back function is used. Call back function allows the server to deal with pending request first and call a function when it is finished.

Node Globals

Class: Buffer __dirname __filename clearImmediate(immediateObject) clearInterval(intervalObject) clearTimeout(timeoutObject) console exports global module process require() require.cache require.extensions require.resolve() setImmediate(callback[, ...args]) setInterval(callback, delay[, ...args]) setTimeout(callback, delay[, ...args])

What is the command that is used in node.js to import external libraries?

Command "require" is used for importing external libraries, for example, "var http=require ("http")". This will load the http library and the single exported object through the http variable.

Why Node.js is single threaded?

For async processing, Node.js was created explicitly as an experiment. It is believed that more performance and scalability can be achieved by doing async processing on a single thread under typical web loads than the typical thread based implementation.

What do you mean by the term I/O ?

I/O is the shorthand for input and output, and it will access anything outside of your application. It will be loaded into the machine memory to run the program, once the application is started.

What does event-driven programming mean?

In computer programming, event driven programming is a programming paradigm in which the flow of the program is determined by events like messages from other programs or threads. It is an application architecture technique divided into two sections 1) Event Selection 2) Event Handling

What does it mean "non-blocking" in node.js?

In node.js "non-blocking" means that its IO is non-blocking. Node uses "libuv" to handle its IO in a platform-agnostic way. On windows, it uses completion ports for unix it uses epoll or kqueue etc. So, it makes a non-blocking request and upon a request, it queues it within the event loop which call the JavaScript 'callback' on the main JavaScript thread.

What does setImmediate do?

It puts function at start of next event loop, less hacky way of doing setTimeout(function(){},0) to move function to event loop

Can you access DOM in node?

No, you cannot access DOM in node.

How Node.js overcomes the problem of blocking of I/O operations?

Node.js solves this problem by putting the event based model at its core, using an event loop instead of threads.

Where can we use node.js?

Node.js can be used for the following purposes a) Web applications ( especially real-time web apps ) b) Network applications c) Distributed systems d) General purpose applications

What is node.js?

Node.js is a Server side scripting which is used to build scalable programs. Its multiple advantages over other server side languages, the prominent being non-blocking I/O.

Why node.js is quickly gaining attention from JAVA programmers?

Node.js is quickly gaining attention as it is a loop based server for JavaScript. Node.js gives user the ability to write the JavaScript on the server, which has access to things like HTTP stack, file I/O, TCP and databases.

What is the difference between Node.js vs Ajax?

The difference between Node.js and Ajax is that, Ajax (short for Asynchronous Javascript and XML) is a client side technology, often used for updating the contents of the page without refreshing it. While,Node.js is Server Side Javascript, used for developing server software. Node.js does not execute in the browser but by the server.

What are the two arguments that async.queue takes?

The two arguments that async.queue takes a) Task function b) Concurrency value

What are the two types of API functions in Node.js ?

The two types of API functions in Node.js are a) Asynchronous, non-blocking functions b) Synchronous, blocking functions

What is an event loop in Node.js ?

To process and handle external events and to convert them into callback invocations an event loop is used. So, at I/O calls, node.js can switch from one request to another .

Node.js is

V8 JS engine embedded inside OS level process, allowing us to access the file system and open up network connections

Does node run on windows?

Yes - it does. Download the MSI installer from http://nodejs.org/download/

Explain the steps how "Control Flow" controls the functions calls?

a) Control the order of execution b) Collect data c) Limit concurrency d) Call the next step in program

Using the event loop what are the tasks that should be done asynchronously?

a) I/O operations b) Heavy computation c) Anything requiring blocking

What is the advantage of using node.js?

a) It provides an easy way to build scalable network programs b) Generally fast c) Great concurrency d) Asynchronous everything e) Almost never blocks

What are most other backend languages vs. Node.js?

blocking, multithreaded vs. non-blocking, single-threaded, uses callbacks

I/O

communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals or data received by the system and outputs are the signals or data sent from it.

"I/O" refers primarily to

interaction with the system's disk and network supported by libuv.

libuv

libuv is a multi-platform support library with a focus on asynchronous I/O.

Express

npm package/ webserver framework built on top of http

Process

program and state of all the threads executing in the program

REPL

read-eval-print loop

V8

runtime environment for JS, parses JS and runs it

The event queue is

single-threaded

Sending info from a file to client via streams

var server = http.createServer(function(req,res){ res.writeHead(200,{'Content-Type':'text/plain'}); var myReadStream = fs.createReadStream(__dirname+'/readMe.txt','utf8'); myReadStream.pipe(res); }

How to use clearTimeout example

var timer = setTimeout(fn, 2); if(secs<1){ clearTimeout(timer); }