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2016-02-17

Important concepts about Javascript for Node JS Developer

Special Values
Null,undefined,{},NaN,Infinity

Special Datatype
object,function,string,number

Typecase
parseInt(),parseFloat(),Boolean()

check datatype :
typeof()-return as string like 'number','object'
instanceof-[] instanceof Array=>true
isNaN(),isFinite()

String Functions:
length,indexOf(),substr(),slice(),split,trim(),search(REGULAR EXPR)

Create Object:
var obj1={first_name='manab',last_name='basu'}

delete properties from object:
delete user.first_name

to add properties in object :
user["first_name"]="Manab"

lenght Of Object
Object.keys(user).length

Array:
Create Array:var arr=[] or var arr=['a','b','c']
typeof arr=Object

Check : Array.isArray(arr)

Add Item: arr.push('a') or arr[4]='a' or arr.unshift('c')- add element at beginning

Delete Item-delete arr[3] or arr.pop()-pop item from the end,arr.shift()-Remove item from begening

Extract Element from array-arr.splice(START_INDEX,NO OF ELEMENT) & it reduce the array size based on NO OF ELEMENT

String to array:"A,b,c".split(',');

Array to string:[1,2,3].join(":");=>1:2:3

Sort:arr.sort()

Sort String of array(names):
names.sort(function(a, b){
var a1=a.toLowerCase(),b1=b.toLowerCase();
    if(a1 < b1) return -1;
    if(a1 > b1) return 1;
    return 0;
})

Functions:
check arguments:write- console.log(arguments);- within function

Constraint
Ternary Operator: var x=today?"X":"Y"

prototype:The prototype property is initially an empty object, and can have members added to it - as you would any other object.
var myObject = function(name){
    this.name = name;
    return this;
};
console.log(typeof myObject.prototype); // object
myObject.prototype.getName = function(){//adding a new member getName() to the class myObject
    return this.name;
};
or
function Shape(){
};
Shape.prototype.X=0;
Shape.prototype.Y=0;
Shape.prototype.move=function(x,y){
 this.X=x;
 this.Y=y;
}
Shape.prototype.distance=function(){
 return Math.sqrt(this.X*this.X+this.Y*this.Y);
}
//call
var s=new Shape();
s.move(10,10);
console.log(s.distance());

Inheritance:
function Square(){}
Square.prototype=new Shape();
Square.prototype.__proto__=Shape().prototype;
Square.prototype.width=0;
Square.prototype.area=function(){
 return this.width*this.width;
}
var sq=new Square();
sq.move(5,5);
sq.width=15;
console.log(sq.distance());
console.log(sq.area());
Error Handling:
function test(){
 throw new Error("Bad Error");
}
//Calling Function
try{
 test();
}
catch(e){
 console.log(e.message);
}
Global: global object
global.name="Manab";
global["name"];

Node Js Cluster

A cluster is a pool of similar workers running under a parent Node process. Workers are spawned using the fork() method of the child_processes module. This means workers can share server handles and use IPC (Inter-process communication) to communicate with the parent Node process.

The master process is in charge of initiating workers and controlling them. You can create an arbitrary number of workers in your master process. Moreover, remember that by default incoming connections are distributed in a round-robin approach among workers (except in Windows).

A cluster module contains several events. Two common events related to the moments of start and termination of workers are the online and the exit events. online is emitted when the worker is forked and sends the online message. exit is emitted when a worker process dies.

Example:

package.json
{
  "name": "democluster",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies":{
    "express":"*"
  }
}


index.js

var cluster = require('cluster');
if(cluster.isMaster) {
    var numWorkers = require('os').cpus().length;
    console.log('Master cluster setting up ' + numWorkers + ' workers...');
    for(var i = 0; i < numWorkers; i++) {
        cluster.fork();
    }
    cluster.on('fork', function(worker) {
        console.log('worker:' + worker.id + " is forked");
    });
    cluster.on('online', function(worker) {
        console.log('worker:' + worker.id + " is online");
    });
    cluster.on('listening', function(worker) {
        console.log('worker:' + worker.id + " is listening");
    });
    cluster.on('disconnect', function(worker) {
        console.log('worker:' + worker.id + " is disconnected");
    });
    cluster.on('exit', function(worker) {
        console.log('worker:' + worker.id + " is dead");
        console.log('Starting a new worker');
        cluster.fork();
    });

  
} else {
    var app = require('express')();
    app.all('/*', function(req, res) {res.send('process ' + process.pid + ' says hello!').end();})
    var server = app.listen(8000, function() {
        console.log('Process ' + process.pid + ' is listening to all incoming requests');
    });
}


OUTPUT IN CONSOLE:

Master cluster setting up 4 workers...
worker:1 is forked
worker:2 is forked
worker:3 is forked
worker:4 is forked
worker:1 is online
worker:4 is online
worker:2 is online
worker:3 is online
Process 8892 is listening to all incoming requests
worker:1 is listening
Process 16192 is listening to all incoming requests
worker:4 is listening
Process 9412 is listening to all incoming requests
Process 15596 is listening to all incoming requests
worker:3 is listening
worker:2 is listening

Reference: http://www.sitepoint.com/how-to-create-a-node-js-cluster-for-speeding-up-your-apps/

2016-02-09

Node js Child Process -exec, spawn and fork

There are two methods for running child processes under Node.js - exec and spawn.

child_process.exec(command, [options], callback)=>exectest.js
child_process.spawn(command, [args], [options])=>spawntest.js
There is an alternative of exec called spawn. Unlike exec it launches a new process and should be used if we expect long-time communication with the running command.

Exec() and spawn() both create processes, but they differ in what they return, spawn returns streams (stdout & stderr), while exec returns a buffer with a max size. Spawn() should be used when the process returns large amount of data.

 Another difference is that spawn() starts receiving the response as soon as the process starts executing, and exec() otherwise waits for the process to end and tries to return all the buffered data at once

 The fork method is a special case of the spawn() functionality, it only creates Node processes. Its signature is the following:

 child_process.fork(modulePath, [args], [options])=>forktest.js

Difference Between Spawn, fork and exec :

 require('child_process').spawn() starts sending back data from the child process in a stream as soon as the child process starts executing. When you run this command, it send a system command that will run on its own process rather than executing code within your node process. In this no new V8 instance will be created and only one copy of the node module will be active on the processor. It is used when you want the child process to return large amount of data to Node.

require('child_process').fork() is a special instance of spawn thats runs a new instance of the V8 engine. Which actually means you are creating multiple workers running on the same Node code base for different task.

require('child_process').exec() returns a buffer from the child process. The default buffer size is 200k. It is asynchronous, but it waits for the child process to end and tries to return all the buffered data at once. If your return data from the child process is greater than 200k then you will get maxBuffer exceeded.

ref:http://www.codingdefined.com/2014/08/difference-between-fork-spawn-and-exec.html

Example :

package.json:

{
  "name": "demochildprocess",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

exectest.js

const
fs = require('fs'),
process = require('child_process');

for(var i=0; i<3; i++) {
 var ls = process.exec('node worker.js '+i, function (error, stdout, stderr) {
    if (error) {
      console.log(error.stack);
      console.log('Error code: '+error.code);
      console.log('Signal received: '+error.signal);
    }
    console.log('stdout: ' + stdout);
    console.log('stderr: ' + stderr);

  });

  ls.on('exit', function (code) {
    console.log('Child process exited with exit code '+code);
  });

}

/*Output:
Child process exited with exit code 0
stdout: Process 0 at work
stderr:

Child process exited with exit code 0
stdout: Process 1 at work
stderr:

Child process exited with exit code 0
stdout: Process 2 at work
stderr:*/

spawntest.js

const
 fs = require('fs'),
 process = require('child_process');

/*Create 10 worker process*/
for(var i=0; i<3; i++) {
 var ls = process.spawn('node', ['worker.js', i]);

 ls.stdout.on('data', function (data) {
   console.log('stdout: ' + data);
 });

 ls.stderr.on('data', function (data) {
   console.log('stderr: ' + data);
 });

 ls.on('close', function (code) {
   console.log('child process exited with code ' + code);
 });
}

/*
stdout: Process 0 at work

stdout: Process 1 at work

child process exited with code 0
child process exited with code 0
stdout: Process 2 at work

child process exited with code 0
*/

forktest.js

const
 fs = require('fs'),
 process = require('child_process');
   

for(var i=0; i <3; i++) {
 var ls = process.fork("worker.js", [i]);

 ls.on('close', function (code) {
   console.log('child process exited with code ' + code);
 });
}

/*
Process 0 at work
child process exited with code 0
Process 1 at work
Process 2 at work
child process exited with code 0
child process exited with code 0

*/


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I am Manab from kolkata. Engineering graduate and presently working in an IT company. Love to share my knowledge with all and to make lots of friends.
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