# Node js

#### What is Nodejs?

* Environment to run javascript out of browser
* Built on top of chrome's V8 JS Engine (fastest and most efficient as of now)
* Full-stack and big community

#### Familiarity required for understaing?

* HTML, CSS, JS, ES6
* Callbacks, Promises, Async-Await

## Nodejs

### Properties

* No DOM
* No Window
* Server Side Apps (pure logic without UI or graphics)
* File System
* Versions (no browser dependence)
* CommonJS

### Global - (No Window in here)

* \_\_dirname - path to current directory
* \_\_filename - file name
* require - function to use modules (NodeJS)
* module - info about current module (file)
* process - info about env where the program is being executed
* console - 
* setTimeout and setInterval

### Modules in node

* Create multiple modules to keep the app clean and seggregated
* module.exports
* Built-in modules
  * OS (for interaction with OS and server) - require('os')
    * os.userInfo()
    * os.uptime()
    * os.type()
    * os.totalmem()
    * os.freemem()
  * PATH - require('path')
    * path.join(a1, a2, a3)
    * path.basename("some path")
    * path.resolve(a1,a2,a3)
  * FS - require('fs')
    * fs.readFileSync("file path", encoding)
    * fs.writeFileSync("file path", "content to write") 
      * if file is not there, create one
      * if file is already there, will overwrite
      * for appending, path one more argument: {flag: 'a'}
    * readFile and writeFile are also possible, with callback functions
  * HTTP - require('http')
    * http.createServer((req, res) => {})

### NPM (Node Package Manager)

* what is it?
  \*
* Commands:
  * npm --version
  * npm i \<package name> (i to install a node package, locally installed)
  * (sudo)? npm i -g \<package name> (to install package globally, use in any project)
  * npm i -D \<package name> (-D add package to devDependencies in package.json file)
  * npm i --save \<package name> (--save add package to dependencies in package.json file)
* package.json - menifest file, stores important information about the project/package
  * npm init -y (to create package.json, y flag set everything by default)

### Important Topics

* **Event Loop**
  * Allows node js to perform non-blocking I/O operations (despite JS being single-threaded)
  * It does so by offloading operations to system kernel whenever required
  * References:
    * a
* **Async Patterns**
  * a
* **Events Emitter**
  * s
* **Streams**
  \*

## HTTP Requests and Response Cycle

### How web works and data transfer on web?

* User / client sends http request, server sends http response
* http messages have following parts:
  * start line
  * optional header (meta information, key value pairs)
  * optional body
  * blank line (to indicate end of data packate)
* Request methode part:
  * method
    * get
    * post
    * delete
    * update
    * create
  * url&#x20;
  * http version
* Response methode part:
  * status code
  * status text
  * headers
  * body (optional)

## References

* <https://www.w3schools.com/nodejs/>
* <https://www.tutorialspoint.com/nodejs/index.htm>
* <https://nodejs.dev/en/learn/introduction-to-nodejs>
* <https://www.javatpoint.com/nodejs-tutorial>