NodeJS: Express JS

Express JS (MVC)

• framework/architecture for nodejs (web)
• express js as Rest API (get, post, put, delete - req. http method).
• Htpp interaction communication (http req -> request response cycle)
• middleware
• input data, process, meaningful output (Web App)
• npm install express -- save

Note:

- An object is equivalent to Javascript.

- NodeJS is equivalent to the callback.

- ExpressJS is equivalent to Middleware.

Creating Server in NodeJS and http module (Client Server Communication)
1. mkdir firstapp
2. cd firstapp
3. npm init
4. create app.js

Explanation (app.js):

- http module injection

- createServer method (callback function use -> req, res)

- After client and server connection, server will listen and client will request.

(req is always http req.object and res or 2nd argyment is always http response object.)

- req.end (end the request and listen the server (1st arg-> port, 2nd arg-> host and 3rd arg-> callback function.))

- If err -> server listening failed and if not server listening at port 4040 (to acknowledge the server status)


# Express Auto Folder Generator

1. npm install -g express-generator (one time run)
2. express folder_name (always run this code to build app)
3. npm install (to install package mention in package.json)
4. npm start (or edit "start": "nodemon app.js"- need to install nodemon)
5. localhost: 3000 (run in browser)

Express 

- Express is routing and middleware web framework.
HTTP Method

1. Get (default): req.data from a specified resource (create)
2. Post: send data to the server to create/update (read)
3. Put: send data to a server to create/ update resource (rename)
4. Delete: delete specified resource (delete)

#Postman
- Postman contains a powerful runtime based on Node. js that allows you to add dynamic behavior to requests and collections. 
- API Development Environment.
- save, import, export, env., link share
- unique, username, email (null value -> unique)

#Middleware
- ExpressJS is a lightweight, efficient middleware and routing framework.
- Middleware functions are functions that have access to the request object (req) and the response object (res) and the next function in the applications req-response cycle.
- Everything in the express is middleware.
Example:
var a = function (req, res, next){
}
app.use(a);
app.get(a);

Types
1. Application Level middleware

var app.express()

app.use(function(req, res, next){

     console.log('hi');

     next();

});

2. Routing Level middleware
Example: 
app.use('/auth', function (req, res, next){
});

3. Inbuilt Level middleware

Program -> inactive state
Process -> active state (thread- execution sequence of process eg: save, delete, edit).
- NodeJS is single threaded programming language. Process is keyword that holds running process instance. 0-> false, 1-> true
Example:
var path = require('path);
app.use('images', express.staet(path.join __dirname, 'images'))

4. Third Part Level middleware (available in nodejs.com)

Example: Morgan (req. logger middleware for node js)

5. Error Handling Level middleware
Example:
app.use(function(err, req, res, next){
     console.log('Err is', err);
     res.json({
             status: err.status || 400,
             msg: err.message || "Wrong"
})
})

Description:
- it doesnt self invoked in http request cycle
- err handling middleware needs to be called execute
- call garne method next with argument
- next is middleware function
- next without arg calls another successive control
- next with arg le sidai error handling middle ware call garxa

#Templating Engine
- HTML doesn't bind data so we use template engine.
- Example: Pug, Handlebars, Ejs, Jade, Mustache


# JWT(Json Web Token)
- is open industry standard RFC 7519 method for representing claims securely between 2 parts. JWT allows you to decode, verify and generate
Parts:
1. headers (algorithms and token type)
2. Signature (secret key)
3. data (payload)

# Error Handling
- Garbage collection and error handling (memory usage release). memory usage handle
Example:
- Var A (locate memory), if its usage is over, we can make null value in A variable, this is called garbage.