---
###### tags: `Note` `js` `javascript` `node` `node.js` `web`
---
# Node.js

Refence: https://www.youtube.com/watch?v=TlB_eWDSMt4

## node module system

[Node.js v14.7.0 Documentation](https://nodejs.org/api/index.html)

example:
* os
* fs
* http
* events
* else(create own module)

### global module

console.log("hello world!!"); //global module

setTimeout();
clearTimeout();

setInterval();
clearInterval();

//the above methods is belong to window module and we call it global

### make own module

//show the module
console.log(module);

//add something like function or varible into exports
function func() {
    console.log("Hello World!!");
}

module.exports.func = func;

and now, we can required this file into other file.

var temp = require('./filename.js');

temp.func();
//output: Hello World!!

In fact, when the file is required, it will run a function like this:

(function (exports,require,module,__filename,__dirname) {
    //show the module
	console.log(module);
	
	//add something like function or varible into exports
	function func() {
	    console.log("Hello World!!");
	}
	
	module.exports.func = func;
})

And it is called <mark>module wrapper function</mark>.

### Path module

var path = require('path');

var pathObj = path.parse(__filename);

/* output:

{ root: '/',
  dir: '/mnt/d/code/node.js',
  base: 'path.js',
  ext: '.js',
  name: 'path' }
  */

### OS module

const os = require('os');

var totalMemory = os.totalmem();
var freeMemory = os.freemem();

console.log(`Total Memory: ${totalMemory}`);
//Total Memory: 8461193216

console.log(`Free Memory: ${freeMemory}`);
//Free Memory: 1103503360

### fs module

const fs = require('fs');

var files = fs.readdirSync('./');
console.log(files);
//[ 'app.js', 'fs.js', 'log.js', 'os.js', 'path.js' ]

fs.readdir('*',function(err,files) {
    if (err) console.log('Error',err);
    else console.log(files);
});
//Error { [Error: ENOENT: no such file or directory, scandir '*'] errno: -2, code: 'ENOENT', syscall: 'scandir', path: '*' }

### event module

const EventEmitter = require('events');
const emitter = new EventEmitter();

//register a listener
emitter.on('messageLogged',function(args) {
    console.log('the event is happened');
});

//raise an evetn. that means make a event happened
emitter.emit('messageLogged');

//if you want to send some messages.
emitter.emit('messageLogged',{message:'hello world'});

the above example, EventEmitter is a class. certainly, we can use extends.

const EventEmitter = require('events');

class myClass extends EventEmitter {

}

### HTTP module

const http = require('http');

const server = http.createServer();

server.o('connection', function(socket) {
    console.log("New Connection...");
})

//make the sever listen onthe port 3000
server.listen(3000);
console.log("the server is listening on port 3000...");

and we can set the various routes in server.

const http = require('http');

const server = http.createServer(function(req,res) {
    if (req.url == '/') {
        res.write("Hello world!!");
        res.end();
    }
    if (req.url == '/url') {
        //statements
    }
});

//make server listen on port 3000
server.listen(3000);