Node.js

Node.js provides server-side functionality in familiar Javascript syntax and an asynchronous model. To be clear, Node.js is not a programming language. It adds features to Javascript that allow developers to write server-side code in familiar front-end language.

If you've already learned some Node in the past, GitHub's Learning Lab Intermediate NodeJS Course might be worth a try.

How It Works

As an interpreted language (like Python), Javascript requires a runtime engine to be run. How Node js functions as a server side language is its runtime engine allows it to interact with a server. In the browser, a Javascript engine provides access to Web Platform APIs and the window HTML (DOM). Node uses Google's V8 Javascript engine instead to access files, send data, and provide other server capabilities. The V8 engine powers Chrome, but there are other engines out there:

1. Firefox uses SpiderMonkey
2. Safari uses JavaScriptCore
3. IE uses who on 🌍 cares

The Chrome V8 Javascript engine provides C++ bindings for Javascript. The V8 Engine then passes events and data between the server and client.

Let's take a small example. Javascript has no concept of accessing system files, but in Node.js, we gain this functionality through modules. We'll get to modules later, but here's a small example of what you can do with Node.js using the built in filesystem module fs.

• Create helloworld.js

  // ./helloworld.js
  const fs = require('fs')
  const story = 'It was the best of JS, it was the worst of JS'
  fs.writeFileSync(
    './story.txt',
    story
  )
  

  ./story.txt
  It was the best of JS, it was the worst of JS
  

Alternatively, you can run this in the node console. In your terminal, typing node creates a shell utilizing a Read Evaluate Print Loop (REPL)

What this means is that as expressions are evaluated, their return value is printed in the console. You can invoke the REPL by just typing node in your terminal. Then, you can test what expressions are doing. Which is useful for debugging expressions and playing around with Javascript.

/* Enter the number 4, then press enter */
4 // <Enter>
++_ // Increment the previous expression by 1

You'll notice that the return value is printed below each statement executed:

v = "Vim"
// => 'Vim'
e = "Emacs"
// => 'Emacs'
console.log("I'm a pro since I use ", v)
// => I'm a pro since I use Vim

• Using the dictionary and list data structures:

  listOfEditors = [v, e]
  // => [ 'Vim', 'Emacs' ]
  editors = {}
  // => {}
  editors[v] = "superior"
  // => 'superior'
  editors[e] = "inferior"
  // => 'inferior'
  editors
  // => { Vim: 'superior', Emacs: 'inferior }
  

You can exit the REPL by pressing ^D or ^C^C.

The process variable, only accessible to you in Node, not in the browser. process is a list of current tasks that Node is executing.

• Typing process in the REPL will print out all environment variables

• The process.exit() method is another way of exiting from the REPL, or from a program you've written.

Module System

Modules in Nodejs are essentially Javascript libraries. There are quite a few built-in libraries available by default in Nodejs, including (thanks to W3 Schools for the rundown):

Package management with NPM

The Node Package Manager (NPM) is the default package manager for Node.js. It has a very frequently used command.

--save yourself the effort

If you see a line like the one below in package documentation, ignore the --save argument. It does nothing. It used to do something, but it doesn't anymore.

npm install PACKAGE --save

Before NPM v5.0.0, it was necessary to add --save after package name because it will save the installed package to package.json file in the dependency section. If you are using a recent version of NPM, save yourself some effort, and just type

npm install PACKAGE

By default, npm will now add the installed package to the dependency list in the package.json file.

To learn more, visit the official npm install documentation.

Killing the fund

As of npm version 6.13.0, npm now displays this message after installing a package usingnpm install:

13 npm install packages are looking for funding run `npm fund` for details

If you support the JavaScript community in other ways (such as by making a wiki), you may prefer not to see this message every single time you install a package using npm. To remove it, you could include a --no-fund flag every time you run npm install, but it's easier to add it to your config file.

• Disabling funding requests for NPM packages

  npm config set fund false
  

This will add fund=false to your ~/.npmrc(https://docs.npmjs.com/files/npmrc) file, preventing you from seeing the message in future package installations.

Importing Modules

• To import and use code you wrote in another file, use the require() function, passing in the parameter of the filepath

• Importing code from the file hello.js

  const hello = require('./hello')
  

The best documentation to learn more about module imports is the Typescript article Modules

The Core

Nodejs requires a few critical components that allow it to provide its functionality. These objects are part of what separate Nodejs applications from vanilla Javascript code:

1. Global Objects
2. Timer Methods
3. Sockets and Streams
4. Utilities
5. Events

Global Objects

There are three primary global objects available in Nodejs. They are:

1. global
2. process
3. Buffer

Each of these objects are in a Node application's namespace, meaning a require statement is not required (bad pun intended).

Variables and required objects are accessible within the application's namespace. This access is restricted to the variables within the application, meaning you are unable to change the value of variables in other other modules. This prevents accidental collisions when code from another module runs.

The process object provides access to the Node installation, the three methods of standard I/O (stdin, stderr, stdout), and application memory usage.

The Buffer object handles binary data. You can read any type of data through a buffer, including a file, input stream, or network connection. A Buffer object take up to two parameters:

Timers

Timers in Nodejs allow programmers to delay and schedule execution of functions. The timer module includes the functions:

• setTimeout
• clearTimeout
• setInterval
• clearInterval

Sockets and Streams

There are four major types of streamed connections in Nodejs:

1. TCP - net module
2. HTTP - http module
3. UDP/Datagram Socket - dgram module
4. Readline/Child_Process - readline, child_process modules

Using a TCP connection, we can recieve messages from client's standard input on the server program. The process is used directly here to access standard I/O streams.

• Example of a TCP Server:

  const net = require('net')
  
  let listener = (connection) => {
    /* Print 'Connected' */
    console.log('Client has created a connection')
    /* Print when data is received from this connection */
    connection.on('data', (data) => {
      /* Print the data and its source */
      console.log(`data() called, received data from ${connection.remoteAddress}:${connection.remotePort}\n${data}`)
      /* Repeat the data sent back to the client */
      connection.write(`'server': ${data}`)
    })
    /* Print when the connection is closed by the client */
    connection.on('close', () => {
      console.log(`Client has closed the connection`)
    })
  }
  
  /* Create a server object, with a callback function for a connection */
  let server = new net.Server(listener)
  
  server.on('error', (error) => {
    if (error.code === 'EADDRINUSE') {
      console.error(`Error: Port ${error.port} already in use.`)
    }
    else {
      console.error(error)
    }
  })
  
  /* Set up a port number to be used by this server */
  const PORT = 1234
  
  /* Spin up the server, listening on port 1234 */
  server.listen(PORT)
  console.log(`Listening on port ${PORT}`)
  

• Example of a TCP Client:

  const net = require('net')
  
  /* Create a client object with the corresponding listener */
  let client = new net.Socket()
  
  const HOST = '127.0.0.1'
  const PORT = 1234
  
  // Connect to the server
  client.connect(PORT, HOST, () => {
    console.log('Connected to server')
    // Send a message to the server
    client.write('This is a message')
  })
  
  // prepare for input from terminal
  process.stdin.resume()
  
  // When user inputs data on `stdin`, send data to server
  process.stdin.on('data', (data) => {
    client.write(data)
  })
  
  // When receive data back, print to console
  client.on('data', (data) => {
    console.log(`data() called, received data: ${data}`)
  })
  
  client.on('close', () => {
    console.log('close() called, connection to server has been closed')
  })
  
  /* Print when the connection is closed by the client */
  client.on('end', () => {
    console.log('end() called, server no longer receiving new connections')
  })
  

Working with I/O

The .pipe(<stream>) is used to send the ouput of one stream to another.

• Sending the output of one process to another:

  process.stdin.pipe(process.stdout)
  

Reading and Writing

Node's documentation has a great article about reading files

The readline module allows the reading of standard I/O streams line by line during program execution. When the programmer is done reading a stream, they must be closed. The REPL is implemented by piping your terminal input stdin, executing Javascript commands and piping stdout back to your terminal.

Using system streams with child_process

Using the child_process module, a Node application can make system calls and recieve input from standard I/O. We do this in Node by defining a child process and redirecting its input and output by defining functions for whenever specific input is recieved.

For example, let's take the command ls:

• Defining a function call

  // format: `spawn("<name>" , [ "flag1", "flag2", ...])`
  const { spawn } = require('child_process')
  const ls = spawn('ls', ['-la'])
  

You can specify how the function interacts with I/O by using the .on() method

Specifying how ls() interacts with stdin, stdout, and stderr:

ls.stdout.on('data', (data) => {
  console.log(`The output of ls is ${data}`)
})

ls.stderr.on('data', (data) => {
  console.log(`ls exited with error ${data}`)
})

ls.on('close', (code) => {
  console.log(`child process exited with code ${code}`)
}

The program below illustrates an elaborate way to execute the command: ls | grep "fork"

• Simulating the execution of the shell command ls | grep "fork"

  const {spawn} = require('child_process')
  const ls = spawn('ls')
  const grep = spawn('grep', ['fork'])
  
  ls.stdout.on('data', (data) => {
    grep.stdin.write(data)
  })
  
  ls.stderr.on('data', (data) => {
    grep.stderr.write(`Error: ${data}`)
  })
  
  ls.on('close', (code) => {
    grep.stdin.end()
  })
  
  grep.stdout.on('data', (data) => {
    grep.stdout.write(data.toString())
  })
  

Utilities

The utilities object enables inheritance in Node. An object can be inherited by another object using the inherits function:

util.inherits(<source-object>, <target-object>)

Events & EventEmitter

As mentioned previously, event based programming is fundamental to the Node philosophy. The events module allows programmers to define actions for events much like try/catch blocks in Java and C++ or try/except blocks in Python. This method of programming puts the programmer in an event driven mindset to focus on all the events that a program must handle. Node modules are written to minimize the amount of boilerplate code written and shift focus to the actions.

Here are the steps to define an event:

1. require: require('events')
2. instantiate
3. define the callback
4. define the event

• Defining and triggering an event:

  // import specific module function to use
  var eventEmitter = require('events').EventEmitter
  var counter = 0
  
  // create an event emitter object
  var em = new eventEmitter()
  
  // define a callback when the event 'timed' occurs
  em.on('timed', (data) => console.log('timed ' + data))
  
  // trigger the event for example
  setInterval( () => em.emit('timed', counter++), 3000)
  

JavaScript

Data Types

JavaScript has primitive data types, which are immutable

1. Number
2. String
3. Boolean
4. Null
5. Undefined
6. Symbol

JavaScript also has some built-in objects

1. Array
2. Date
3. RegExp
4. Map & WeakMap
5. Set & WeakSet

Specifying Integer Type

• Specifying integers using different formats:

  let val = 10
  let hex = 0x2f // (47)
  let exponential = 1.2e4 // (1.2 * 10^4)
  let max = Infinity
  let min = -Infinity
  

Number Properties

• Specifying values using the properties of Number:

  let maxValue = Number.MAX_VALUE
  let minValue = Number.MIN_VALUE
  
  let maxInt = Number.MAX_SAFE_INTEGER
  let minInt = Number.MIN_SAFE_INTEGER
  

Special Characters

String Substitutions

• Using the backtick syntax to substitue a variable's value inside of a string.

  const temp = 22.5
  console.log(`The value is ${temp}`)
  

null and undefined

• A variable is undefined when it is initialized without being assigned a value.
• A variable is only null when it is assigned as such

Objects

• Creating an object:

  const person = {
    name: "Austin",
    age: 21
  }
  

UTF-8

A variable can be defined using normal letters, as well as UTF-8 characters

• Creating a variable using UTF-8 characters

  const 你好 = "Hello"
  console.log(你好)
  // => "Hello"
  

Arrow Functions

Arrow functions allow us to give a function an identifier.

// Create a function 'squareIt' accepting an input 'x' and returning 'x' squared
const squareIt = (x) => x * x

// After 'squareIt' has been written, we can now call the identifier and give it input
squareIt(10) // '100'

Equality '==' vs. '==='

• === is known as the identity operator
• == is known as the equality operator

The identity operator === will compare both types and values between two variables. JavaScript objects are compared by reference, not by value. An object is equal to itself, but not equal to a different object with the same value.

It's really complicated so keep that link under your pillow, you'll need it, trust me.

• Examples of the two operators being different:

  true === '1' // false
  true == '1' // true
  
  true === 1 // false
  true == 1 // true
  
  null === undefined // false
  null == undefined // true
  

• Examples of the two operators being the same:

  [1, 2] === [1, 2] // false
  [1, 2] == [1, 2] // false
  
  0 === 0.0 // true
  0 == 0.0 // true
  

There are also some edge cases worth noting:

• The equality operator == will attempt to convert the two variables

• Not a number or NaN is not equal in comparison to anything, even with itself

  NaN === NaN // false
  NaN == NaN // false
  x = NaN
  x === NaN // false
  x !== x // true
  

JavaScript Arrays

The Mozilla Javascript Documentation for Arrays will explain it better than I can.

The "in" operator

The in operator evaluates to true if the left-side value is the name of a property of the right-side object. This results in some counterintuitive logic at times, an example is provided below:

// Arrays
let trees = ['redwood', 'bay', 'cedar', 'oak', 'maple']
0 in trees        // returns true (array index 0 exists)
3 in trees        // returns true (array index 3 exists)
6 in trees        // returns false (array index 6 does not exist)
'bay' in trees    // returns false (you must specify the index number, not the value at that index)
'length' in trees // returns true (length is an Array property)
Symbol.iterator in trees // returns true (arrays are iterable, works only in ES2015+)

// Predefined objects
'PI' in Math          // returns true

// Custom objects
let mycar = {make: 'Honda', model: 'Accord', year: 1998}
'make' in mycar  // returns true
'model' in mycar // returns true

const data = [7, 8, 9]
"0" in data // true: array has an element "0"
1 in data // true: numbers are converted into strings
7 in data // false: no 7th element in this array

GitHub Package Registry

Add the following two lines to your ~/.npmrc

//npm.pkg.github.com/:_authToken=${GITHUB_TOKEN}
registry = https://npm.pkg.github.com

Add the read:packages and write:packages permissions to your GITHUB_TOKEN environment variable in the GitHub tokens page

Install a Package

To add this Package registered on the GitHub NPM Registry, enter the following command

npm install @codertocat/hello-world-npm

Alternatively, you can add the package as a dependency to the project's package.json

{
  "name": "testjs",
  "version": "1.0.0",
  "main": "index.js",
  "license": "MIT",
  "dependencies": {
    "@codertocat/hello-world-npm": "^1.0.2"
  }
}

Publish a Package

Add the following to your package's package.json

{
  "publishConfig": {
    "registry":"https://npm.pkg.github.com/"
  }
}

Files

• Convert relative path ./file.txt to absolute path /Users/tommy/file.txt

  let absolute_filepath = path.resolve('./file.txt')
  console.log(absolute_filepath)
  

  /Users/tommy/file.txt
  

• Convert local file ./file.txt to file:// URI form:

  let uri = url.pathToFileURL('./file.txt')
  console.log(uri)
  

  URL {
    href: 'file:///Users/austin/hooman/node.html',
    origin: 'null',
    protocol: 'file:',
    username: '',
    password: '',
    host: '',
    hostname: '',
    port: '',
    pathname: '/Users/austin/hooman/node.html',
    search: '',
    searchParams: URLSearchParams {},
    hash: ''
  }
  

Express

The base place to learn Express is the GitHub Learning Lab Introduction to Node with Express

• Example of an Express server

  // Import the express library
  const express = require('express')
  
  // Construct a server object
  const server = express()
  
  // Save the port number as a variable
  const port = 1337
  
  // Support JSON in POST requests
  server.use(express.json())
  
  // Respond to GET requests to /ping with 'pong'
  server.get('/ping', (req, res) => {
      console.log(req.query)
      res.send('pong')
  })
  
  // Respond to GET requests to /dog with `dog` as JSON
  server.get('/dog', (req, res) => {
      let dog = {
          name: 'fido',
          age: 2
      }
      res.status(200).json(dog)
  })
  
  // Respond to POST requests to /whoami by printing the content
  server.post('/whomai', (req, res) => {
      console.log(req.body)
  })
  
  
  // Begin the server, listening on the port declared above
  server.listen(port, () => {
      console.log(`Example app listening on ${port}!`)
  })
  

• An example of an Express server serving files over SSL

  import fs from 'fs';
  import http from 'http';
  import https from 'https';
  import express from 'express';
  
  const app = express();
  
  const options = {
    key: fs.readFileSync('private.key'),
    cert: fs.readFileSync('fullchain.crt')
  };
  
  /* Serve the static website in ./build */
  app.use(express.static('build'));
  
  /* Listen on port 80 for HTTP requests */
  http.createServer(app).listen(80);
  
  /* Listen on port 443 for HTTPS requests */
  https.createServer(options, app).listen(443);
  

And you're done! You're now hosting a website over HTTPS 🥳

MongoDB

• Example of a MongoDB insertion

  const username = 'db_user'
  const password = 'drop_it_like_its_hot'
  const cluster = 'cluster0-asdf.mongodb.net'
  const database = 'students'
  
  const uri = `mongodb+srv://${username}:${password}@${cluster}`
  
  // Import the MongoClient class from the 'mongodb' library
  const {MongoClient} = require('mongodb')
  
  // Construct an instance of a MongoDB Client
  const client = new MongoClient(uri)
  
  
  async function main() {
      try {
          // Establish a connection to the cloud-hosted database cluster
          await client.connect()
          // Use the database 'wiki' if it exists, and create it if it doesn't
          const db = client.db("wiki")
          //create user we will just want to pull the text they enter in text boxes
          let user = {
              username: "ttrojan",
              password: "fighton"
          }
          // Use the collection 'users' if it exists, and create it if it doesn't
          let collection = db.collection("users")
  
          // Insert 'user' into the collection 'users' in the database 'wiki'
          await collection.insertOne(user, (error, response) => {
              if (error) {
                throw error
              }
              console.log("User inserted:")
              console.log(user)
          })
      } catch (e) {
          // If there is an error in connection, print it
          console.error(e)
      } finally {
          // Close the connection to the client when the function ends
          await client.close()
      }
  }
  
  // Execute the main function, printing errors if they occur
  main().catch(console.error)
  

The crypto package

Utility functions for cryptography exist in JavaScript, but they're rather poorly documented. Neither "Learning JavaScript" or "JavaScript: The Definitive Guide" referenced this package even once in the collective 2,000 pages between the two of those books. A bit odd, maybe I'll find out why one day and post an update here.

Hashes

• Getting a list of the available hashes

• Creating a hash of password letmein using SHA-1

  const crypto = require('crypto')
  const hash1 = crypto.createHash('sha1')
  const hash256 = crypto.createHash('sha256')
  
  const password='letmein'
  
  const digest1 = hash1.copy().update(password).digest('hex')
  const digest256 = hash256.copy().update(password).digest('hex')
  
  console.log(digest1)
  console.log(digest256)
  

  b7a875fc1ea228b9061041b7cec4bd3c52ab3ce3 1c8bfe8f801d79745c4631d09fff36c82aa37fc4cce4fc946683d7b336b63032

  shasum -a 1 < =(<<<'letmein')
  shasum -a 256 < =(<<<'letmein')
  

  b7a875fc1ea228b9061041b7cec4bd3c52ab3ce3 1c8bfe8f801d79745c4631d09fff36c82aa37fc4cce4fc946683d7b336b63032

• Getting a list of all available ciphers

In the future, I want to explore how to use the Cipher and ### Verify classes, as well as the methods below:

• crypto.generateKey()
• crypto.generateKeyPair()
• crypto.createPublicKey()
• crypto.createSecretKey()
• crypto.createSign()
• crypto.createVerify()
• crypto.diffieHellman()
• crypto.privateDecrypt()
• crypto.privateEncrypt()
• crypto.publicDecrypt()
• crypto.publicEncrypt()
• crypto.randomInt()

Subcommands

prefix

Print the local prefix

npm prefix

/Users/tommy/Developer/project

• Print the global prefix

  npm prefix -g
  

  /usr/local

You can use npm prefix -g to print the local prefix to standard out.

Source: The official npm documentation