Node.js

About node.js

• JavaScript is a programming language, and Node.js is a runtime environment that allows you to execute JavaScript code outside of a web browser.
• https://nodejs.org/en/about/ As an asynchronous event-driven JavaScript runtime, Node.js is designed to build scalable network applications. In the example, many connections can be handled concurrently. Upon each connection, the callback is fired, but if there is no work to be done, Node.js will sleep.
• https://www.javatpoint.com/nodejs-tutorial Node.js is a cross-platform environment and library for running JavaScript applications which is used to create networking and server-side applications. Node.js is mostly used to run real-time server applications.

Installation

See Javascript

Tutorials

• https://www.w3schools.com/nodejs/default.asp
• https://www.tutorialspoint.com/nodejs/index.htm
• https://www.javatpoint.com/nodejs-tutorial
• Node.js Tutorial for Beginners: Learn Node in 1 Hour | Mosh

node.js prompt

How to exit

brb@brb-P45T-A:~/Downloads$ node
> Math.random()
0.14142140373587608
> .exit

HTTP server

http://www.youtube.com/watch?v=jo_B4LTHi3I

Note HTTP is just one application of TCP.

First create a js file

var http = require('http');

var s = http.createServer(function (req, res) {
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.write('Hello \n');
  setTimeout(function() {
    res.end("world\n");
  }, 2000);
});

s.listen(8000);

Then using node program to run the js script. We can test the program by using a web browser or curl program.

# Terminal 1
node web-server.js

# Terminal 2
curl http://localhost:8000
OR use "-i" to include HTTP-header in the output
curl -i http://localhost:8000

The output I get is

$ curl -i http://localhost:8000
HTTP/1.1 200 OK
Content-Type: text/plain
Date: Sat, 19 Oct 2013 20:55:17 GMT
Connection: keep-alive
Transfer-Encoding: chunked

Hello 
world

We can compare this with the Example from Hypertext Transfer Protocol in Wikipedia.

Test the http server

Apache benchmark testing

ab -n 1000 -c 100 http://localhost:8000

Setup Nginx as Frontend Server for Node.js

• How to Install Latest Node.js and NPM on Ubuntu with PPA
• How to Setup Nginx as Frontend Server for Node.js
• How to Install Node.js on Debian 10 and configure Nginx as a Frontend Proxy Server (include PM2 installation)

PM2

How to Install and Set Up PM2 on Linux Servers

TCP server

Simple example

Create <tcp-server.js> file

var net= require('net')

var server = net.createServer(function(socket) {
  socket.write('hello\n');
  socket.end('world\n');
});

server.listen(8000);

After running this program, we can test it by using telnet or nc.

telnet localhost 8000
OR
nc localhost 8000

Echo server

This is modified from Simple server.

var net= require('net')

var server = net.createServer(function(socket) {
  socket.write('hello\n');
  socket.write('world\n');

  socket.on('data', function(data) {
    socket.write(data);
  });
});

server.listen(8000);

We can use netstat -an | grep tcp to lists all the TCP ports on which your Linux server is listening including all the active network connections to and from your server. This can be very helpful in determining whether slowness is due to high traffic volumes:

telnet needs to use Ctrl+] to break out the connection but nc can disconnect by using Ctrl+C.

$ telnet localhost 8000
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
hello
world
klllkafda
klllkafda
^]

telnet> quit
Connection closed.
$
$ nc localhost 8000
hello
world


adaf
adaf
^C
$

Chat server

net = require('net');

var sockets = [];

var s = net.Server(function(socket) {
  sockets.push(socket);

  socket.on('data', function(d) {
    for(var i=0; i<sockets.length; i++) {
      if (sockets[i] == socket) continue;
      sockets[i].write(d);
    }
  });

  socket.on('end', function() {
    var i = socksets.indexOf(socket);
    sockets.splice(i, 1);
  });
});

s.listen(8000);

Test it by

Terminal 1: telnet localhost 8000

Terminal 2: telnet localhost 8000

http get

Node.js - RESTful API from tutorialspoint.com

$ cat hello-world-server.js 
setInterval(function() {
  console.log("world");
 }, 2000)

console.log("hello");

var http = require('http')
setInterval(function() {
  console.log("fetching google.com");
  http.get({ host: 'google.com' }, function(res) {
    console.log(res.headers);
  });
}, 2000)

console.log("hello");

Modules, npm and package.json

https://www.javatpoint.com/nodejs-package-manager

• npm version # the packages are not complete?

  $ npm version
  { npm: '6.9.0',
    ares: '1.15.0',
    brotli: '1.0.7',
    cldr: '35.1',
    http_parser: '2.8.0',
    icu: '64.2',
    modules: '64',
    napi: '4',
    nghttp2: '1.39.2',
    node: '10.16.3',
    openssl: '1.1.1c',
    tz: '2019a',
    unicode: '12.1',
    uv: '1.28.0',
    v8: '6.8.275.32-node.54',
    zlib: '1.2.11' }
  

• npm install <Module Name>

  $ npm install tz
  npm WARN saveError ENOENT: no such file or directory, open '/Users/XXX/Downloads/package.json'
  npm notice created a lockfile as package-lock.json. You should commit this file.
  npm WARN enoent ENOENT: no such file or directory, open '/Users/XXX/Downloads/package.json'
  npm WARN Downloads No description
  npm WARN Downloads No repository field.
  npm WARN Downloads No README data
  npm WARN Downloads No license field.
  
  + [email protected]
  added 2 packages from 1 contributor and audited 2 packages in 0.904s
  found 0 vulnerabilities
  $ ls -lt | head
  # <package-lock.json> file & directory "node_modules" are created
  # there is no need to worry about the package-lock.json file 
  

• npm ls # List packages under ./node_modules
• npm install -g <Module Name> # install packages in global environment
• npm uninstall <Module Name>
• npm search <Module Name>

require()

• https://nodejs.org/api/modules.html
• os module. https://www.javatpoint.com/nodejs-os
• dns module. https://www.javatpoint.com/nodejs-dns
• net module for socket programming. https://www.javatpoint.com/nodejs-net
• fs module for file system. https://www.javatpoint.com/nodejs-streams, https://www.javatpoint.com/nodejs-file-system
• path module for file path. https://www.javatpoint.com/nodejs-path
• assert module for testing. https://www.javatpoint.com/nodejs-assertion-testing
• http module for creating a server. https://www.javatpoint.com/nodejs-web-modules
• mysql module.

Global vs Local installation

npm 1.0: Global vs Local installation. The rule of thumb is:

• If you’re installing something that you want to use in your program, using require('whatever'), then install it locally, at the root of your project.
• If you’re installing something that you want to use in your shell, on the command line or something, install it globally, so that its binaries end up in your PATH environment variable.

npm ls: check all the modules installed

https://nodejs.org/en/blog/npm/npm-1-0-the-new-ls/

$ npm ls # locally

$ npm ls -g # globally

package.json

• https://docs.npmjs.com/files/package.json
• package.json is present in the root directory of any Node application/module and is used to define the properties of a package.

Docker

Official Docker Image for Node.js