nodeJS-get-started

nodeJS - get started

What is Node.js

Node App are:

• Build twice as fast with fewer people
• 33% fewer lines of code
• 40% fewer files
• 2x request/sec
• 35% faster response time

Why it’s popular?

• Great for prototyping and agile development
• Superfast and highly scalable
• JavaScript everywhere
• Cleaner and more consistent codebase
• Large ecosystem of open-source libs

What it is? - Node.js is a JavaScript runtime built on Chrome’s V8 JavaScript engine, Node is not a programming language. Node is not a framework.

NOTE: NodeJs is suitable for handling requests asynchronously, not suitable for compute-heavy applications.

Setup Environment

Install Node.js

Open command prompt, create a new folder:

mkdir first-app
cd first-app

Use npm to initialized a new Node.js app, this will create a package.json file containing metadata that describes the app.

npm init -y

Open this folder in Visual Studio Code:

code .

Add a new file app.js, and in the js file, write a regular javascript code, for example, a function:

// define a function
function sayHello(name){
    console.log("Hello, "+ name);
}

// Call the function
sayHello('Qijie');

Run this code in command line with command:

node app.js

Node Core

console.log();

setTimeout();
clearTimeout();

setInterval();
clearInterval();

Module

In Node.js, every js file is called a Module, the variables and functions that are defined inside a module are private by default, they are not accessible from outside the module. In order to make a variable or function to be public, use the keyword exports.

How to create a module

In VS Code, create a new js file called logger.js, then define a variable and a function as following:

var url = 'http://mylogger.io/log';

var log = function(message){
    console.log(message);
}

module.exports.log = log;
module.exports.endPoint = url;

How to use a module

In the main module app.js, we import logger.js and call the log function:

const mylog = require('./logger'); // mylog is an instance of the module, use mylog.xx to use the functions and variables
mylog.log('hello from another module');

NOTE: In each js file, the code will be wrapped inside a function like following:

(function (exports, require, module, __filename, __dirname){
    var url = 'http://mylogger.io/log';

    var log = function(message){
        console.log(message);
    }

    module.exports.log = log;
    module.exports.endPoint = url;
})

you could see exports, require, module are parameters of this function, therefore we could use it in our js file.

Use path module

const path = require('path');
var pathObj = path.parse(__filename);
console.log(pathObj);

Use os module

const os = require('os');

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

console.log(`Total Memory: ${totalMemory}`);
console.log(`Free Memory: ${freeMemory}`);

Use fs module

const fs = require('fs');

// Synchronous
const files = fs.readdirSync('./');
console.log(files);

//Asyncrhonous
fs.readdir('./',function(err, files){
    if(error){
        console.log("Error: "+error)
    }
    if(files){
        console.log("Results: "+files);
    }
})

Example of fs, output the files under a folder path:

const fs = require('fs');
fs.readdir('d:\\',function(error,files){
   	if(error){
    	console.log(error);
	}        

    files.forEach(function(file){
        console.write(file+"\r\n");
    })
           
});

Use events module

const EventEmitter = require('events'); // Upper case E for EventEmitter, means this is a Class

const emitter = new EventEmitter();
emitter.on("MessageLogged",function(arg){
    console.log("listener called", arg)
});

emitter.emit('MessageLogged',{id:1,url:"http://"});

Use http module

Http module is an EventEmitter, you could use the .on events.

const http = require('http');
var server = http.createServer();

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

server.listen(3000,function(){
    console.log('Listening on port 3000...');
});

Example of http, print Multiplication Table to browser:

const http = require('http');
var server = http.createServer((req, res) =>{
    if(req.url ==="/"){
        for(let i=1; i<=9; i++)
        {
            for(let j=1; j<=i; j++)
            {
                res.write(j+'*'+i+"="+j*i+" ");
            }
            res.write('\r\n');
        }

        res.end();
    }
});

server.listen(3000,function(){
    console.log('Listening on port 3000...');
});

output result: