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Shreshth707 / Web-Bootcamp

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Web Technologies Bootcamp

Banuprakash C

Full Stack Architect,

Co-founder Lucida Technologies Pvt Ltd.,

Corporate Trainer,

Email: banuprakashc@yahoo.co.in

https://www.linkedin.com/in/banu-prakash-50416019/

https://github.com/BanuPrakash/web

=====================================================

Softwares Required:

  1. Chrome Web browser with following extensions 1.1) add React Developer tools [ https://chrome.google.com/webstore/search/react Search : react] 1.2) add Redux DevTools [ https://chrome.google.com/webstore/search/redux Search : redux]
  2. Visual Studio Code [ https://code.visualstudio.com/ ]
  3. NodeJS Latest LTS [ https://nodejs.org/en/download/ ] 16.x.x

=========================================

Web Technologies

HTML + CSS + JS Presentation + styling + dynamic

JavaScript -> Scripting language, loosely typed and dynamically typed language

keyword "var" to declare a variable

var name = "Smith"; // string

var data = 10; // number

data++;

data = "test"; // string data.toUpperCase();

data = true;

if(data) {}

JS ==> JS engine Engines: V8 => Google => Chrome / NodeJS SpiderMonkey ==> Firefox Rhino ==> Mozilla Chakra, Continumm => IE and Edge Nashorn ==> Oracle JavaScriptVM ==> Opera

function add(x, y) { console.log(x + y); }

Execution Context:

var g = 100; // global variable

function doTask() { var a = 10; if(g > a) { var b = 20; c= 30; console.log(g, a, b, c); } console.log(g, a, b, c); }

doTask(); console.log(g, a, b, c);

function add(x, y) { return x + y;

}

var result = add(4,5)

console.log(result); // undefined

semi-colon insertion

function add(x, y) { return; x + y; //unreachable code }

JS is event-driven

myfile.js

console.log("Hello!!!");

setInterval(function timed() { console.log("timed!!!"); }, 100);

console.log("Timer set!!!");

$("#btn").click(function clicked() { console.log("clicked!!!"); });

console.log("End!!!");

=========================

OOP in JS Object can have state and behaviour

  1. var obj = new Object();

obj.x = 10; // state ==> instance variable obj.y = 25; // state ==> instance variable obj.print = function() { console.log(this.x, this.y); }

  1. Functional constructor

function Person(name, age) { this.name = name; this.age = age; }

var p1 = new Person("Tim", 22); // p1.name is set to "Tim" and p1.age is set to 22 // within Person() "this" refers to "p1" ==> Context

var p2 = new Person("Harry", 34); // within Person() "this" refers to "p2"

Person("Anna", 31); // "this" in Person refers to window/ global context

Adding behaviour:

function Person(name, age) { this.name = name; // state this.age = age; // state }

// instance methods Person.prototype.getName = function() { return this.name; }

Person.prototype.setName = function(n) { this.name = n; }

// static methods // class member ==> here "this" refers to by default "window" Person.equals = function(p1, p2) { if(p1.name === p2.name && p1.age === p2.age) { return true; } return false; }

var p1 = new Person("Tim", 22); var p2 = new Person("Tim", 22);

console.log(p1.getName());

if(Person.equals(p1, p2)) { console.log("same") }

==================

  1. JSON ==> JavaScript Object Notation {} Key is string type; value can be undefined, number, string , boolean, array, object, null, function

var obj = {};

Server sends objects to client in JSON notation

{ "id": 1, "name": "Leanne Graham", "username": "Bret", "email": "Sincere@april.biz", "address": { "street": "Kulas Light", "suite": "Apt. 556", "city": "Gwenborough", "zipcode": "92998-3874", "geo": { "lat": "-37.3159", "lng": "81.1496" } }

============

var person = { "id": 1, "name": "George", "getName": function() { return this.name; } }

person.id; person.name; person.getName();

============

JS bind(), apply() and call() functions inherited from Object

All JS functions are inherited from Object

function add(x,y) { console.log(this); return x + y; }

above code is an instance of Function

var add = new Function("x" , "y", "return x + y");

Function extends Object

====================

call() ==> simlarly apply()

function update(name) { this.name = name; }

var product = { name: "iPhone 13" }

function Person(name, age) { this.name = name; // state this.age = age; // state }

var p1 = new Person("Tim", 22);

update.call(product, "Galaxy"); // product.name ----> "Galaxy"

update.call(p1, "Jimmy"); // p1.name --> "Jimmy"

var product = { name: "iPhone 13", getName: function() { console.log(this.name); } }

product.getName();

var ref = product.getName; // get function definition; context is lost

var ref = product.getName.bind(product); // copy product context; inside getName() this should refer to "product"

ref();

High Order Functions ==> HOF

  • Functions which accept function as argument
  • Function which return a function

var data = [6,2,11,5,9,22];

for(i = 0; i < data.length; i++) { console.log(data[i]); }

for(i = 0; i < data.length; i++) { alert(data[i]); }

Task 1:

map HOF

input: var data = [6,2,11,5,9,22];

output: [12, 4, 22, 10, 18, 44]; < == doubled

--

input: var products = [ { "id": 1, "name": "iPhone", "price": 124447.44, "category": "mobile" }, { "id": 2, "name": "Onida", "price": 4444.44, "category": "tv" }, { "id": 3, "name": "OnePlus 6", "price": 98444.44, "category": "mobile" }, { "id": 4, "name": "HDMI connector", "price": 2444.00, "category": "computer" }, { "id": 5, "name": "Samsung", "price": 68000.00, "category": "tv" }];

output: ["iPhone", "Onida", "One Pluse 6", "HDMI Conenctor", "Samsung"]

=== Closure:

HOF ==> Function which return a function

function adder(base) { return function(value) { return base + value; } }

var fiveAdder = adder(5); var tenAdder = adder(10);

console.log(fiveAdder(2)); console.log(tenAdder(2));

fiveAdder.prototype

constructors

scope

========================

Memoization. The main idea behind it is to execute function only once. Follow up calls should not run the logic but return the cached result.

getProduct(3); ==> API call to Spring Boot ==> DB ==> results ==> JSON ==> send JSON to client

getProduct(5); ==> API call to Spring Boot ==> DB ==> results ==> JSON ==> send JSON to client

getProduct(3); ==> get from cache

fibanocci(34) ==> compute heavy CPU intense operation

fibanocci(34) ==> from cache

fibanocci(89) ==> compute heavy CPU intense operation

========================================

ECMAScript 5 / JS 5 version

ES 6 / ES2015 / ECMAScript 6

Most of JS engines are still compatable with ES 5 and not ES 6

https://caniuse.com/

ES 6 ==> Transcompile ==> ES 5 for production code

Babel and Tracuer are 2 most popular Transpiler / Pre-Processor

Features:

  1. Block level scope [ let ]

var g = 100;

function doTask() { var a = 10; if(g > a) { let b = 20; // not hoisted var c = 30; // hoisted to function scope //

}
console.log(c); // c is visible
console.log(b); // not visible

}

if(g > a) { ({ let b = 20; })(); var c = 30; //

}
  1. Constants [ const]

const PI = 3.14159;

constants can also have block level scope

  1. Clone

3.1) var p = {"id":1,"name":"iPhone","price":124447.44,"category" : "mobile"};

var ref = p; // pointer

ref.name = "Samsung";

console.log(p.name); // Samsung

var cpy = {...p}; // clone

cpy.name = "MotoG";

console.log(p.name); // Samsung

3.2)

var data = [5,2,12];

var cpy = [...data];

cpy[0] = 99;

console.log(data[0]) ; // 5

  1. Destructuring assignment

4.1) var data = [5, 2, 12, 44, 11, 67];

old Way:

var a = data[0]; var b = data[1];

with Destructuring:

var [a, b, ...others] = data;

console.log(a); // 5 console.log(b); // 2 console.log(others); // [12, 44, 11, 67]

var elems = ["red", "green", "blue", "pink"];

var [r, g, ...others]= elems;

console.log(r); // red

4.2) var p = {"id":1,"name":"iPhone","price":124447.44,"category" : "mobile"};

old way: console.log( p.name, p.price);

var {name, price} = p;

console.log(name) ; // iPhone

  1. Arrow functions:

function add(x,y) { return x + y; }

var add = (x, y) => x + y;

or

var add = (x,y) => { return x + y; }

  1. Promise API for asynchronous functions

Sync functions:

function doTask() {

}

var result = doTask(); console.log("complete"); // blocked

Async functions

function doTask() { // async }

doTask().then(function(data) {

}, function(err) {

})

console.log("complete"); // not blocked

===

Promise API callback hell

getConnection() .then(con => getProject(con)) .then(prj => getEmployee(prj)) .then(emp => getPhone(emp)) .then(data => console.log(data));

  1. Async and Await functions for Promise API to prevent callback issues / hell Promise code:

fetch("https://jsonplaceholder.typicode.com/users") .then(response => response.json()) .then(data => console.log(data));

convert to async and await: async function getData() { try { let response = await fetch("https://jsonplaceholder.typicode.com/users"); // blocked let data = await response.json(); console.log(data); } catch(ex) { console.log(ex); } }

getData();

  1. Generators are functions with multiple return values over a period of time

    function* saga() { console.log("task1"); console.log("task2");

     yield "first output";
 console.log("task3");
 
 yield 100;
 console.log("task4");
 console.log("task5");
 
 yield {"name": "anitha", age: 21};

    }

    var iter = saga(); iter.next(); // check the return value and decide to proceed or not iter.next();
    iter.next(); iter.next();

  2. class

  3. ES 6 modules

  4. Self Learn ==> Proxy and Reflect

===============================================================

DOM ==> Document Object Model

XML or HTML in the form of Object using Scripting language [ JavaScript / VBScript / JScript/ ...]

  • document

Using DOM

  • create dynamic elements
  • access elements
  • traverse thro elements
  • add event handlers

access elements:

para 1

Some Content

para 2

<div class="card">
</div>

  1. getElementById() document.getElementById("one");

  2. getElementsbyTagName() always returns an array

document.getElementByTagName("p");

querySelector(".card") ==> get by class name querySelectorAll("p") ==> get by tag name querySelector("#one") ==> get by id

document.querySelector(".tmpl-headerNavItem_home").innerHTML document.querySelector(".tmpl-headerNavItem_home").innerText 'Home' document.querySelector(".tmpl-headerNavItem_home").innerText = "Banu"

===================

events ==> click, mousemove, mouseenter, mouseexit, keypress, keydown, keyup, focus, blur, load, unload,..

Button clicked!!! div clicked!!! document clicked!!!

document clicked!!! div clicked!!! Button clicked!!!

==================

Day 1 Recap: JS, HOF, Closure, ES 6 features, OOP, bind(), call() and apply(); DOM & event handling

Day 2

NodeJS

https://github.com/nodejs/node

  • Platform built on V8 javascript engine and Libuv C / C++ cross-platform libraries [ instead of Web Api]

  • libuv is a multi-platform C library that provides support for asynchronous I/O based on event loops

  • event-driven, non-blocking I/O model provided by Libuv

Why NodeJS?

  1. to build APIs [ RESTful web services / GraphQL web services]
  2. build traditional web applications [ server sends rendered HTML pages to client]
  3. Real-time applications ==> ChatBot
  4. Streaming Platform [ Netflix / HotStar / Amazon Prime...]
  5. building web applications for client side

Challanges will building web applications:

  1. we start writing code in ES6 / TypeScript / DART / CoffeeScript / LiveScript use Babel for ES6 -> ES5 use tsc for TypeScript -> ES%

    we need to transpile to ES5

    product.ts

    tsc product.ts --> product.js

  2. we need to do testing [ unit testing / e2e testing]

  3. we need to minify , uglify and bundle our code

    50+ js files

    <script src="a.js"></script> <script src="b.js"></script> <script src="c.js"></script> <script src="d.js"></script>
    • each <script> tag makes a network call
    • ordering of inclusion of <script> tag is important ==> based on dependency
    • each character is a byte ==> payload increases ==> remove all whitespaces ==> minify ==> covert big function/variable names to shorter names ==> uglify

index.

<title>meta</title> <script src="a.js"></script> <script src="b.js"></script> <script src="c.js"></script> <script src="d.js"></script>

client http://server.com --> downloads index.html --> one network call --> DOM parser will evaluate HTML

<script src="a.js"></script> --> network call to download a.js from server will be triggerd by the browser <script src="b.js"></script> --> network call to download b.js from server will be triggerd by the browser

--

Solution is :

bundle.js ==> includes a.js + b.js + c.js + d.js

<script src="bundle.js"></script>

JS Module System: ==> control visiblity like "private" and "public" Different Module System

  1. Plain Vanilla JS module system using IIFE ==> Immediate Invoke Function expression ()();
var ShopModule = (function(){
	var data = 10; // private
	function doTask() {

	}

	function anotherTask() { // private

	}

	return {
		doTask
	}
})();

ShopModule.doTask(); // valid

ShopModule.data; // not visible ==> error

var CustomerModule = (function() {
	var data = 45;
	var name = "Smith";
	function doTask() {

	}
	function getName(){}
	return {
		data,
		doTask,
		getName
	}
})();

CustomerModule.data; // 45
CustomerModule.getName(); // valid
CustomerModule.doTask();
  1. CommonJS module system ==> default supported by NodeJS
lib.js
module.exports.add = (x,y) => x + y;
module.exports.subtract = (x,y) => x - y;
const multiply = (x,y) => x * y; //private to lib.js

other.js
const {add, subtract} = require('./lib');
  1. ES 6 module system
lib.js

export const add = (x,y) => x + y;
export const subtract = (x,y) => x - y;
const multiply = (x,y) => x * y; //private to lib.js

other.js
import {add, subtract} from './lib';
  1. AMD ==> Asynchronous Module Definition
  2. System
  3. UMD

$ node fs1.js --> loads all pre-defined modules like "fs", "http", "url", "crypto", "cluster" ,.. into memory we can import them in our application.

====================================================

NodeJS project

Package Managers:

  1. NPM ==> comes by deault with NodeJS
  2. YARN
  3. PNPM

Node Package Manager ==> Similar to Maven for java developers / similar to PIP for Python developers npm --version

Why?

  • manage 3rd party dependencies into the project npm install react npm i lodash

    or

    yarn add react

    or

    PNPM install react

    this also manages transitive dependencies

    npm i a a depends on b b depends on c

https://www.npmjs.com/

  • Publish modules npm login npm publish

  • Run scripts for "build", "deploy", "test", "start" , ...

==================

each nodejs project has "package.json"

  • scripts are configured
  • dependencies ==> modules which are required in production also
  • devDependencies ==> modules which are required in development mode only
  • peerDependecies ==> reusable modules

nodeprj> npm i -D mocha chai request

nodeprj> npm i lodash

package.json "devDependencies": { "chai": "^4.3.6", "mocha": "^10.0.0", "request": "^2.88.2" }, "dependencies": { "lodash": "^4.17.21" }

Project when pushed onto github will be minus "node_modules" folder

other team members:

nodeprj> npm install

"chai": "4.3.6", ==> look for exact version for repo

"chai": "^4.3.6" ==> download any latest version greater than or eqaul to 4.3.6

"chai": "~4.3.6" => Major version has to be "4"; minor and patch can be greater than or equal to ".3.6"

=====================

JS Unit testing: unit testing frameworks:

  • Jasmine
  • JEST
  • Mocha

https://www.chaijs.com/

Chai is a BDD / TDD assertion library for node and the browser that can be delightfully paired with any javascript testing framework.

AAA ==> Assemble Action Assert

"scripts": { "start": "node app/server.js", "test": "mocha --reporter spec" }

npm start

npm test

https://jestjs.io/docs/asynchronous

============================================

JavaScript Build tools

  • Grunt
  • Gulp
  • Webpack

Grunt is a JavaScript task runner, a tool used to automatically perform frequent tasks such as minification, compilation, unit testing, and linting.

Webpack webpack is a module bundler. Its main purpose is to bundle JavaScript files for usage in a browser, yet it is also capable of transforming, bundling,...

webpackexample> npm init --y

webpackexample> npm i -D webpack webpack-cli webpack-dev-server html-webpack-plugin

"scripts": { "dev": "webpack --mode development", "prod" : "webpack --mode production" }

npm run prod

src/index.js ==> creates dist/main.js

npm run dev

===============

The HtmlWebpackPlugin simplifies creation of HTML files to serve your webpack bundles. This is especially useful for webpack bundles that include a hash in the filename which changes every compilation.

instead of one bundle if many bundles are created ==> code splitting then also HtmlWebpackPlugin will be useful

index.html

<script src="main.Fs2452@5w.js"></script> <script src="main.gse232422#.js"></script> <script src="vendor.js"></script> <script src="product.chunk.dfdfg#@!.js"></script>

webpack-dev-server https://webpack.js.org/configuration/dev-server/

===========================================

webpack ==> transpiler

ES 6 ==> ES 5 => Babel

npm i @babel/core babel-loader @babel/preset-env -D

babel-loader loads files with "js" extension @babel/core has babel compiler to convert higher version of JS to lower version @babel/preset-env has pollyfills

npm i lodash

=======================================

css-loader and style-loader

npm i css-loader style-loader -D

css-loader loads "css" files which are imported like js file import "./styles.css";

style-loader will include loaded "css" into index.html

<style>
	body {
    background-color: antiquewhite;
}

h1 {
    border: 1 solid lightblue;
}
</style>

======================================================

ReactJS

Rendering ==> process of data => presentation [HTML / PDF / Image]

  • Server Side Rendering [ SSR ]
  • Client Side Rendering [ CSR ]
  • Static Site Generation [ SSG ]

Client Side Rendering

  • we need to create DOM dynamically -> document.createElement("table") using plain DOM apis -> jQuery library simplified DOM creation / traversing / event handling $(

    ) same as document.createElement("table")

    $(table) same as document.getElementsByTagName("table")

    $("#first") same as docuemnt.getElementById("first");

    -> Templates

    • Underscore
    • jQueryTemplate
    • Handlebars #
    • Mustache {{
    • Knockout

    Handlebar example: template ==> html files

      {{#each people}}
    • {{this}}
      • {{/each}}

    JSON data from server: { people: [ "Yehuda Katz", "Alan Johnson", "Charles Jolley", ], }

    output:

    • Yehuda Katz
    • Alan Johnson
    • Charles Jolley

    SPA ==> single page application ==> index.html but many views

    • Application has many modules [ customer / product / cart ] and they have dependencies
    • different URLs should show different views

    http://adobe.com/products has to show products in index.html

    http://adobe.com/ has to show landing page

    http://adobe.com/offices has to show offices

    http://adobe.com/products/acrobat has to show only acrobat products

    • Protect URLs

    http://amazon.com/cart

    this URL is accesable only if you have logged in

    • Navigation between views instead of pages

    • Data binding -> single way binding [ data from server place into placeholders {{name}}] -> two way binding [ data from server to placeholders in html ; changes done in view --> server]

    MVC --> Model View Controller

    Model -> presents the data View -> Presentation Controller -> to control application logic ==> event handling

    Libraries and Frameworks:

    1. Backbone library ==> model and Controller support; view support was not provided -> pick from handlebars/mustache

    2. CanJS / StealJS

    3. AngularJS --> Framework [MVC] --> Google digest() cycle

    4. ReactJS --> View Library --> Performs of view rendering --> 3rd party libraries for Model walmart/ twitter / linked

    5. Angular --> Framework --> Total solution Gmail

    6. Vue

    ReactJS by Facebook

    https://codepen.io/

    Settings: Behaviour: save --> off Preview --> off

    JS: PreProcessor: Babel

    Extenal librarires

    1. react https://cdnjs.cloudflare.com/ajax/libs/react/18.2.0/umd/react.production.min.js

    2. react-dom https://cdnjs.cloudflare.com/ajax/libs/react-dom/18.2.0/umd/react-dom.production.min.js

    html

    JS:

    let Welcome = React.createElement("h1", {style:{'color': 'red'}}, 'Welcome to React!!!');

    // console.log(Welcome);

    ReactDOM.render(Welcome, document.getElementById("root"))

    ==================

    Day 3

    Day 1 & 2: JS , ES 6 , Event Handling and NodeJS {npm, package.json, mocha, chai}

    ReactJS --> View library by Facebook --> CSR

    ReactElement

    1. let Elem = React.createElement("div",{style:{"color":"red"},"Welcome to React!!!");

    tagName = "div" props = style... props.children = "Welcome to React"

    VDOM ==> in memory object [ json] but not visual component ]

    VDOM ==> Visual Representation ==> Renderers

    • react-dom ==> VDOM to DOM elements for web
    • react-native ==> VDOM to mobile elements
    • react-tv ==> VDOM to tv elements

    ReactDOM.render(Elem, document.getElementById("app"));

    1. Reconcillation

    Using React if changes are made ==> will be on VDOM; not directly DOM computes differences between VDOM and VDOM copy==> for every diff DOM is updated

    ============================

    Hello!!!

    React.createElement("div",{style:{"color":"red"}, React.createElement("p", null, "Hello!!!"));

    ===========================================

    • Functional components
    • class components

    props 1:

    function Welcome(props) {
  return <div>
          <h1>
              {props.title}
          </h1>
          <h3>
              {props.location}  
          </h3>
    </div>  
}

ReactDOM.render(<Welcome title="Welcome to React!!!" location="Adobe Virtual"/>, document.getElementById("root"));

    props 2:

    function Welcome({title, location}) {
  return <div>
          <h1>
              {title}
          </h1>
          <h3>
              {location}  
          </h3>
    </div>  
}

    ===================

    var data = [
{"id":1,"name":"iPhone","price":124447.44,"category" : "mobile"},
{"id":2,"name":"Onida","price":4444.44,"category" : "tv"},
{"id":3,"name":"OnePlus 6","price":98444.44,"category" : "mobile"},
{"id":4,"name":"HDMI connector","price":2444.00,"category" : "computer"},
{"id":5,"name":"Samsung","price":68000.00,"category" : "tv"}];

function ProductList({title, products}) {
  return <div>
    <h1>{title}</h1>
    {
      products.map(p => <ProductRow product={p}/>)
    }
  </div>
}

function ProductRow({product}) {
  return <div>{product.name}, {product.price}</div>
}

ReactDOM.render(<ProductList title="Product List" products={data}/>, document.getElementById("root"));

    Class Component: can have state and behaviour

    Button ==> color, caption, width , height, enabled ===> state Button ==> click(), hover(), keyPress(), ===> Behaviour

    class ProductList extends React.Component {
  state =  {
    "products":[
      {"id":1,"name":"iPhone","price":124447.44,"category" : "mobile"},
      {"id":2,"name":"Onida","price":4444.44,"category" : "tv"},
      {"id":3,"name":"OnePlus 6","price":98444.44,"category" : "mobile"},
      {"id":4,"name":"HDMI connector","price":2444.00,"category" : "computer"},
      {"id":5,"name":"Samsung","price":68000.00,"category" : "tv"}]
  }
  
  render() {
     return <div>
       <h1>{this.props.title}</h1>
        {
            this.state.products.map(p => <ProductRow product={p}/>)
        }
    </div>
  }
}
    • npx create-react-app customerapp

    npm ==> node package manager npx ==> execute node package manager npx create-react-app customerapp

    loads create-react-app module into memory and execute

create-react-app ==> to create scaffolding code for react applications.

    If not for create-react-app

    • npm init --y
    • npm i webpack webpack-cli html-webpack-pluging webpack-dev-server
    • npm i react react-dom
    • i need to configure webpack.config.json
    • i need to configure babel.config.json
    • i need to create landing page

    all other components

    "react-scripts" ==> to exceute webpack commands

    Each child in a list should have a unique "key" prop.

    class Example extends React.Component {
  state = {
    count : 10
  }
  render() {
    return <div>
      {this.state.count} <br />
      <button type="button" onClick={this.increment.bind(this)}>Change</button>
    </div>
  }
  
  increment() {
   // this.state.count = this.state.count + 1
     // reconcilation ==> triggers render() call
     this.setState({
       count: this.state.count + 1
     })
    console.log(this.state.count)
  }
  
}

ReactDOM.render(<Example />, document.getElementById("root"));

    Retain enclosing Context using Arrow function: https://www.section.io/engineering-education/how-to-use-javascript-arrow-functions-and-this-keyword/#:~:text=Arrow%20function%20and%20this%20context,to%20a%20global%20object%20window.

    https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

    render() {
    return <div>
      {this.state.count} <br />
      <button type="button" onClick={this.increment}>Change</button>
    </div>
  }
  
  increment = () => {
    //this.state.count = this.state.count + 1
     this.setState({
       count: this.state.count + 1
     })
    console.log(this.state.count)
  }

    Using Arrow to bind event handler:

      <button type="button" onClick={() => this.increment()}>Change</button>
    </div>
  }
  
  increment(){
     this.state.count = this.state.count + 1
     this.setState({
       count: this.state.count + 1
     }, () =>   console.log(this.state.count))
  
  }

    React JSX needs one root element; can't return multiple elements

    ERROR:

    return (

    df

    dfg

    )

    Correction:

    return (

    df

    dfg

    ); ---

    JSX has to have a root element:

    function Welcome() {
	return <div>
					<h1>Hello</h1>
					<h1>World!!!</h1>
		</div>
}

Solution:

function Welcome() {
	return <React.Fragment>
					<h1>Hello</h1>
					<h1>World!!!</h1>
		</React.Fragment>
}

OR
function Welcome() {
	return <>
					<h1>Hello</h1>
					<h1>World!!!</h1>
		</>
}

    let p = { "name": "Test", "getData": function() { return this.name; } }

    p.getData(); // "Test"

    let ref = p.getData; // when function refernces is taken; it loses the context; only function definition is taken

    ref(); // "name" is not from "context" p

    ===========

    whenever props or state change [ setState] ==> re-render happens

    Testing React

    • Unit Testing React Testing Library React Testing Library builds on top of DOM Testing Library by adding APIs for working with React components.

    Built on top of JEST testing framework [ mocha, jasmine are other testing frameworks]

    Instead of RTL we can use Enzyme

    npm test

    CodeCoverage: npm test -- --coverage --watchAll

    integration testing render()

    Unit Testing components in isolation: render( <CustomerRow delEvent = {(id) => this.deleteCustomer(id)} key={c.id} customer={c} />);

    npm test -- --coverage --watchAll --collectCoverageFrom="**/components/*.js"

    • E2E Testing

    customerapp>npm i -D cypress

    https://www.cypress.io/

    for E2E applicaiton should be running:

    npm start

    package.json scripts: { "e2e": "cypress open" }

    npm run e2e

    cy.get(selector)

    cy.get('button') ==> document.querySelector('button')

    cy.get('.row') ==> document.querySelector('.row') ==> by class name

    instead of

    ===================================================

    create-react-app state and behaviour event handling ==> click and change

    unit testing with RTL E2E with cypress / puppetter / selenium / protractor

    ====================================================

    <link href="https://fonts.googleapis.com/css?family=Oswald|Permanent+Marker&display=swap" rel="stylesheet">
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css" integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">

    Day 3 Recap: Functional components ==> first choice as view component class components ==> extends Component ==> state and behaviour

    whenever state or props changes ==> Reconcillation happens

    changing the state should be done using

    Asynchronous function to mutate the state this.setState({ variable: data });

    state = { x : 25, y : 40 }

    this.setState({ x: 30 });

    above code updates "x", y will still be 40

    class MyComp extends Component { constructor(props) { super(props); // any other init } }

    1. Binding event handlers:

    class MyComp extends Component {

    doTask() {

}

render() {
	<div>
			<button type="button" onClick={this.doTask.bind(this)}> Click </button>
	</div>
}

    }

    1. Binding event handlers:

    class MyComp extends Component { constructor(props) { super(props); this.doTask = this.doTask.bind(this); } doTask() {

    }

render() {
	<div>
			<button type="button" onClick={this.doTask}> Click </button>
	</div>
}

    }

    1. Binding event handlers:

    class MyComp extends Component {

    doTask = () => {
	// "this" will enclosing context
}

render() {
	<div>
			<button type="button" onClick={this.doTask}> Click </button>
	</div>
}

    }

    1. Binding event handlers:

    class MyComp extends Component {

    doTask() {

}

render() {
	<div>
			<button type="button" onClick={() => this.doTask()}> Click </button>
	</div>
}

    }

    Unit Testing RTL render() ==> doesn;t create actual DOM rather in-memory DOM , good enough for testing getBy, findBy, queryBy

    jest.fn(); ==> to create a mock function

    Cypress ==> E2E testing framework

    Browser -> react --> API call to Spring Boot / server --> Database

    functional component returns JSX render() method of class component returns JSX

    Day 4

    npx create-react-app phoneapp

    styles.css

    .card { border: 1px solid red; background-color: lime; color: white }

    import "styles.css"; // css-loader and style-loader

    class CardComponent extends Component { render() { return

    ...
    } }

    • CSS Framework
    1. Bootstrap ==> Twitter
    2. TailWind
    3. foundation / Zurb
    4. Bulma

    Bootstrap CSS Framework

    1. uses Grid System for RWD ==> Responsive Web Design https://getbootstrap.com/docs/4.0/layout/grid/ https://getbootstrap.com/docs/4.0/components/navbar/ https://getbootstrap.com/docs/4.0/components/card/

    google fonts fontawesome for icons

    ES 6 String literal

    var name = "Peter";

    var msg = Welcome to Adobe, Name : ${name};

    https://www.primefaces.org/primereact/datatable/

    =====================

    React Context

    Context provides a way to pass data through the component tree without having to pass props down manually at every level

    Example:

    let PersonContext = React.createContext();

class PersonProvider extends React.Component {
    state = {
      "name": "Smith",
      "email" : ""
    }
    constructor(props) {
      super(props);
      this.state.updateEmail = this.updateEmail
    }
    updateEmail = (email) => {
      console.log(email)
      this.setState({
        email: email
      })
    }
    render() {
      return <PersonContext.Provider value={{...this.state}}>
            {this.props.children}
        </PersonContext.Provider>
    } 
}
        
function App() {
    return <PersonProvider>
          <First />
      </PersonProvider>
}

function First() {
  return <>
    <h1> I am First </h1>
    <Second />
    </>
}
      
class Second extends React.Component {
  render() {
    return <PersonContext.Consumer>
      {
        value => {
          return <>
          Name : {value.name} <br />
          Email : {value.email} <br />
          <button onClick={() => value.updateEmail("me@gmail.com")}>Click</button>
          </>
        }
      }
      </PersonContext.Consumer>
  }
}

ReactDOM.render(<App/>, document.getElementById("root"))

    download share.zip ==> extract it

    • React Router DOM

    • Different URLs has to show different views in SPA index.html

    http://amazon.com/mobiles has to show/render mobiles in index.html

    http://amazon.com/mobiles/iPhone13 has to show/render iPhone13 in index.html

    http://amazon.com/mobiles/Pixel3 has to show/render Pixel3 in index.html

    http://amazon.com/tvs has to show/render tvs in index.html

    http://amazon.com/cart has to show/render cart in index.html

    • Bookmark

    • Navigate between views

    • easy to secure a URL

    • Lazy Loading of components http://amazon.com/cart ==> only on using this URL ==> cart component has to be downloaded

      main.bundle.js ==> should have minimum components to display [ size is small ] other components should be lazily downloaded

      this solves FCP ==> First Contentful Paint

    ==============

    Component Lifecycle methods

    =========

    https://reactrouter.com/en/v6.3.0/getting-started/overview

    phoneapp> npm i bootstrap styled-components react-router-dom

    index.html

    index.js import 'bootstrap/dist/css/bootstrap.min.css';

    ==== Suspense waits for 1500ms for Component to be loaded; else fallback

    Cart ==> this makes a Server side request ==> full page is sent from server [SSR]

    Cart ==> react-router-dom ==> to invoke ==> CSR

    =================== FCP:

    constructor() mock init => render() => componentDidMount() ==> API calls ==> state ==> render()

    share.zip Extract

    1. data.js and Button.js into "components" folder
    2. "img" folder into "public" folder
    3. "App.css" => overwrite existing one

    index.js, Context.js, ProductList.js, Product.js

    Context.js, Product.js

    ================================

    React ==> API call ==> RESTful Web services [ Spring Boot ]

    phoneapp> npm i axios

    JSON Server
    Get a full fake REST API with zero coding in less than 30 seconds

    uses "json" file as persistent store ==> CRUD operations on JSON file

    data.json

    { "movies": [{}, {}], "actors": [{}, {}, {}] }

    "movies" and "actors" are Endpoints

    npx json-server --watch data.json --port 1234

    http://localhost:1234/movies http://localhost:1234/actors

    Get by id http://localhost:1234/movies/2

    GET by query parameter http://localhost:1234/movies?year=2022

    POST, PUT, DELETE operations are also supported

    ===

    window SessionStorage

    on login success: window.sessionStorage.setItem("user", "banu@gmail.com");

    on checkout: let user = window.sessionStorage.getItem("user");

    ===================

    Day 5

    React

    • Functional Components ==> First Choice ==> pure view component ==> return JSX ==> React.ELEMENT
    • Class Components ==> state and behaviour, lifecycle methods ==> render() returns JSX ==. React.ELEMENT

    React.createElement()

    • state & props

    • setState({}); ==> Async in nature ==> Reconcillation ==> trigger re-rendering

    • Avoid props-drill ==> Context ==> Provider; Consumer [ useContext() in functional component]

    • Component LifeCycle methods ==> componentDidMount() and componentDidUpdate() ==> API call

    • Core Web Vitals ==> FCP constructor() do some mock init ==> render() ==> componentDidMount() ==> API call and set state componentDidUpdate() ==> any dependent API calls ==> set State ==> render()

    • axios HTTP client

    React ==> make api calls

    1. fetch() let response = await fetch("http://localhost:8080/api/products"); let data = response.json();

    2. axios 2.1) let response = await axios.get("http://localhost:8080/api/products"); let data = response.data; 2.2) let order = { .. } let response = await axios.post("http://localhost:8080/api/orders", order);

    =============

    Compoenent Lifecycle method: componentDidMount() componentDidUpdate() componentWillUnmount()

    shouldComponentUpdate() shouldComponentUpdate() method is invoked before rendering when new props or state are recieved. This method is not called on initial render and also when forceUpdate() method is called.

    Syntax - shouldComponentUpdate(nextProps,nextState)

    class Child extends React.Component {
	shouldComponentUpdate(nextProps,nextState) {
		if(this.props.name === nextProps.name) {
			return false;
		}
		return true;
	}
	render() {
			console.log("Child renders!!!");
			return <div>
						Name in Child : {this.props.name}
				</div>
	}
}


class Parent extends React.Component {
	state = {
		count : 0,
		name: "Banu"
	}

	increment() {
		this.setState( {
			count : this.state.count + 1
		});
	}

	render() {
		console.log("Parent renders!!!")
		return <div>
				Name : {this.state.name} <br />
				Count: {this.state.count} <br />
				<Child name={this.state.name}/>
				<button onClick={() => this.increment()}>Click </button>
		</div>

	}
}

    ==========

    Option 2:

    class Child extends React.PureComponent {
	render() {
			console.log("Child renders!!!");
			return <div>
						Name in Child : {this.props.name}
				</div>
	}
}

    PureComponent ==> has default implementation of shouldComponentUpdate() use PureComponent if props is primitive type [ string, number, boolean]

    if props is complex ==> object, array, write custom shouldComponentUpdate

    Parent: state = { products = [{}, {}, {}] }

    shouldComponentUpdate uses shallow comparision

    ============

    What if Child Component is Functional Component instead of class Component?

    function Child(props) {
  	console.log("Child renders!!!");
		return <div>
					Name in Child : {props.name}
			</div> 
}

    Solution: use Memoize pattern

    function Child(props) {
  	console.log("Child renders!!!");
		return <div>
					Name in Child : {props.name}
			</div> 
}

let MemoChild = React.memo(Child); // HOC; cache of props; new props comming will be compared to prop-cache
// then return Child(props) or return;

class Parent extends React.Component {
	state = {
		count : 0,
		name: "Banu"
	}

	increment() {
		this.setState( {
			count : this.state.count + 1
		});
	}

	render() {
		console.log("Parent renders!!!")
		return <div>
				Name : {this.state.name} <br />
				Count: {this.state.count} <br />
				<MemoChild name={this.state.name}/>
				<button onClick={() => this.increment()}>Click </button>
		</div>

	}
}

ReactDOM.render(<Parent />, document.getElementById("root"));

    ================ React.memo(Movie); // good for primitive props; ==> Like PureComponent

    Custom Logic for memoize:

    function moviePropsAreEqual(prevMovie, nextMovie) { return prevMovie.title === nextMovie.title && prevMovie.releaseDate === nextMovie.releaseDate; } const MemoizedMovie2 = React.memo(Movie, moviePropsAreEqual); ==> like override shouldComponentUpdate()

    ==================================

    Error Boundaries

    class ErrorBoundary extends React.Component { constructor(props) { super(props); this.state = { hasError: false }; }

    static getDerivedStateFromError(error) { return { hasError: true }; }

    componentDidCatch(error, errorInfo) { console.log(error, errorInfo); }

    render() { if (this.state.hasError) { // You can render any custom fallback UI return

    Something went wrong.

    ; }

    return this.props.children;

    } }

    ==========================================================

    React Hooks

    Hooks are a new addition in React 16.8. They let you use state and other React features without writing a class.

    99% ==> functional components

    1. useState()
    2. useReducer()
    3. useEffect()
    4. useMemo()
    5. useCallback()
    6. useParams()
    7. useContext()
    1. useState() ==> allows to declare "state" in functional components
    class Parent extends React.Component {
	state = {
		count : 0,
		name: "Banu"
	}

	//
	 this.setState({
	 		count: this.state.count + 1
	})
}

    using Functional component:

    function Parent() { let [count, setCount] = React.useState(0); let [name, setName] = React.useState("Banu");

    return <div>
			Name : {name} <br />
			Count: {count} <br />
			<button onClick={() => setCount(count + 1)}>Click </button>
</div>

    }

    ReactDOM.render(, document.getElementById("root"));

    1. useReducer()

    useState() can be used if state variable is primitive and not conditionally update

    useReducer is usually preferable to useState when you have complex state logic that involves multiple sub-values or when the next state depends on the previous one.

    2.1) Action Object
	{
		type: type_of_action,
		payload
	}

2.2) Reducer function takes (state, action) => newState
	  invoke reducer function using dispatch(action)

	  Action object examples:

    1.    {
   	type: "ADD_TO_CART",
   	product : {...}
   }

    2. { type: "REMOVE_FROM_CART", id: 3 }

    3. { type: "CLEAR" }

    Example:

    let initState = {count: 0, name: "", "age": 32};

let countReducer = (state, action) => {
	switch(action.type) {
		case "INCREMENT": return {
			count : state.count + action.payload
		}
		case "DECREMENT": return {
				count: state.count - 1
		}
		default : return state;
	}
}


function App() {
	let [state, dispatch] = React.useReducer(countReducer, initState);

	return <div>

				Count: {state.count} <br />
				<button onClick={() => dispatch({"type": "INCREMENT", payload: 10})}>
					Increment
				</button>
				<button onClick={() => dispatch({"type": "DECREMENT" })}>
					Decrement
				</button>
	</div>
}


ReactDOM.render(<App />, document.getElementById("root"))

    1. useContext() hook for Context Consumer

      function MyData() { let {cart} = React.useContext(CartContext); let {customer} = React.useContext(CustomerContext); return

      Welcome {customer.email}

      { cart.map(p => ) }
      }

      else in class component

      class MyData extends React.Component { render() { { value => {

      {value.email}

      { value => { value.cart.map(...) } }

       					</CartConsumer>
 				}
 			}
 	</CustomerConsumer>
 	
 }

      }

    1. useEffect() is a hook to simulate component life cycle methods of class component
    function App() {
	let [count, setCount] = React.useState(0);
	let [name, setName] = React.useState("Banu");

	// componentDidUpdate ==> gets called whenever count or name or props changes
	React.useEffect(() => {
		console.log("called effect 1 ==> similar to componentDidUpdate ", count, name)
	});


  // componentDidMount ==> gets called for first render only
	React.useEffect(() => {
		console.log("called effect 2 ==> similar to componentDidMount ", count, name);
		return () => console.log("called when unmount"); // simialr to componentWillUnMount()
	}, []); // [] dependecy list ==> execute if any of these dependecny changes

	 //  gets called whenever count changes
	React.useEffect(() => {
		console.log("called effect 3 ", count, name)
	}, [count]);


	return <div>
				Name : {name} <br />
				Count: {count} <br />
				<button onClick={() => setCount(count + 1)}>Click </button>
	</div>
}

ReactDOM.render(<App />, document.getElementById("root"));
    1. useCallback()

    useCallback will return a memoized version of the callback that only changes if one of the dependencies has changed.

    
function Title() {
  console.log('Rendering Title')
  return (
    <h2>
      Example: Title
    </h2>
  )
};

const MemoTitle = React.memo(Title);

function Button({ handleClick, children }) {
  console.log('Rendering button - ', children)
  return (
    <button onClick={handleClick}>
      {children}
    </button>
  )
}

const MemoButton = React.memo(Button);

function Count({ text, count }) {
	console.log(`Rendering ${text}`)
	return <div>{text} - {count}</div>
}

const MemoCount = React.memo(Count);

function ParentComponent() {
	const [age, setAge] = React.useState(25)
	const [salary, setSalary] = React.useState(50000)

	 const incrementAge = React.useCallback(() => {
		setAge(age + 1);
	}, [age]);

	const incrementSalary = React.useCallback(() => {
   		setSalary(salary + 1000)
	}, [salary]);
  
	return (
		<div>
			<MemoTitle />
			<MemoCount text="Age" count={age} />
			<MemoButton handleClick={incrementAge}>Increment Age</MemoButton>
			<MemoCount text="Salary" count={salary} />
			<MemoButton handleClick={incrementSalary}>Increment Salary</MemoButton>
		</div>
	)
}

ReactDOM.render(<ParentComponent/>, document.getElementById("root"));
    1. useParam() In Product.js
    import React, { useEffect, useState } from 'react'
import { useParams } from 'react-router-dom';
import axios from 'axios';
export default function Details() {
   let [product, setProduct] = useState(null);
   let {id} = useParams();
 
   useEffect( () => {
     console.log(id);
     axios.get(`http://localhost:1234/products/${id}`).then(response => {
       setProduct(response.data);
     })
    }, [id]);
 
    return (
     <div>
       <h1>
           Product Details
       </h1>
       {
         product? <div>
               Title : {product.title} <br />
               Info : {product.info}
           </div> : <p> No Product!!! </p> 
       }
       </div>
   )
 
}
    1. useMemo()

    useMemo will only recompute the memoized value when one of the dependencies has changed. This optimization helps to avoid expensive calculations on every render.

    const memoizedValue = useMemo(() => fibanocci(no), [no]);

    Refer day 1: memoization

    ===========================================

    Custom Hooks:

    hooks built using pre-defined hooks

    should start with

    fuction useXXXXX() {

    }

    https://react-spectrum.adobe.com/react-aria/ https://github.com/adobe/react-spectrum/blob/main/packages/%40react-aria/interactions/src/useHover.ts

    ========================

    
// custom hook

import { useState, useEffect } from "react";

export default function useLocalState(key, defValue) {
    const [count, setCount] = useState(() => {
        let value = parseInt(window.localStorage.getItem(key)) || defValue;
        return value;
    });

    useEffect(() => {
        window.localStorage.setItem(key, count);
    }, [count]);

    return [count, setCount];
}


---

import React from 'react'
import useLocalState from '../hooks/LocalState'

export default function HookComponent() {
const [count,  setCount] = useLocalState("my-app-state", 0);
  return (
    <div>
        Count {count} <br />
        <button onClick={() => setCount(count + 1)}>Click</button>
    </div>
  )
}

    React Reference Refs and the DOM: Refs provide a way to access DOM nodes or React elements created in the render method.

    React.createRef()

    class App extends React.Component {
  state = {
    name : "",
    email : ""
  }
  nameRef = React.createRef();
  emailRef = React.createRef();
  updateState() {
    this.setState({
      name: this.nameRef.current.value,
      email : this.emailRef.current.value
    });
    this.nameRef.current.focus();
  }
  render() {
    return <>
     Name: {this.state.name} , Email {this.state.email} <br />
     <form>
      Enter Name : <input type="text" ref={this.nameRef} /> <br />
      Enter Email : <input type="text" ref= {this.emailRef} /> <br />
    <button type="button" onClick={this.updateState.bind(this)}>Change </button>
    </form>
   </>
  }
}
ReactDOM.render(<App/>, document.getElementById("root"));

    Alternate way:

    class App extends React.Component { state = { name : "", email : "" }

    update(evt) { let name = evt.target.name; this.setState({ [name]:evt.target.value }); }

    render() { return <> Name: {this.state.name} , Email {this.state.email}
    Enter Name : <input type="text" name="name" onChange={(evt) => this.update(evt)}/>
    Enter Email : <input type="text" name="email" onChange={(evt) => this.update(evt)}/>
    </> } } ReactDOM.render(, document.getElementById("root"));

    ================

    1. useRef()
    function TextInputWithFocusButton() {
  const inputEl = useRef(null);
  const onButtonClick = () => {
    // `current` points to the mounted text input element
    console.log(inputEl.current.value);
    inputEl.current.focus();
  };
  return (
    <>
      <input ref={inputEl} type="text" />
      <button onClick={onButtonClick}>Focus the input</button>
    </>
  );
}

    Day 6

    React recap:

    1. React.createElement() ==> VDOM
    2. ReactDOM.render() ==> VDOM to DOM
    3. Functional Components ==> JSX; props
    4. Class components ==> [render() returns JSX, can have state, lifecycle methods and behaviour]
    5. change state using this.setState({}) ==> Async trigers reconcillation
    6. componentDidMount() and componentDidUpdate() is place where API calls are done ==> to speed up FCP
    7. shouldComponentUpdate() can be used to avoid re-rendering of a component / PureComponent / React.memo()
    8. whehever state or props change component re-renders
    9. Context: React.createContext() ==> prevents props drill [ Provider places data into Context; Consumer gets data from context directly]
    10. Event handling onEvent; Arrow, bind, ...
    11. RTL for unit testing and Cypress for E2E
    12. React Hooks: a) useState() b) useEffect() c) useContext() d) useCallback() ==> memoize callback functions e) useReducer() ==> conditionally mutate the state based on action type

    <Route path="/products/:pid" element={} /> <Route path="/products" element={} />

    f) react-router-dom useParams() to read pathparam [http://localhost:3000/products/3] and queryparam [http://localhost:3000/products?category=mobile http://localhost:3000/products?page=1&size=10] let {category} = useParams(); let {page, size} = useParams() Refs and DOM: React.createRef() g) useRef()

    Redux: Predictable State Management

    A Predictable State Container for JS Apps, Redux helps you write applications that behave consistently. Centralizing your application's state and logic enables powerful capabilities like undo/redo, state persistence, and much more. Redux's architecture lets you log changes, use "time-travel debugging"

    Context ==> use to to avoid props-drill; avoid mutation with context

    ProductContext ==> Make API call get products from RESTful aPI and place it into ProductContext; consumer can get products

    Cart ==> Prefer Redux==> ADD_TO_CART, REMOVE_TO_CART, INCREMENT, DECREMENT Checkout ==> Redux ==> to Server

    Multi-stage Form submit ==> Registration pages / Survey pages / feedback every page update state in Redux; Submit ==> Redux state => send to Server

    MVC --> Model View Controller --> Architectural pattern

    Flux pattern: The Flux pattern solves the scalability issue by using unidirectional data flow to pass data among the different component views in the app.

    React-Redux:

    1. Store: one store per application/ place where data recides
    2. Reducer: functions which take (state, action) => return new state based on action type
    3. Action Object and Action Creator
    4. React: View component

    add contacts, remove contacts $ npx create-react-app reduxexample reduxexample> npm i redux react-redux

    ===============

    Redux Middleware for async actions

    • Thunk
    • Saga
    • Redux Observable
    • Redux Promise ...

    Redux is Sync in nature: dispatch(action); ==> store ==> reducer ==> updates the state

    Async Logic and Data Fetching https://redux.js.org/tutorials/fundamentals/part-6-async-logic

    UI:fetchProducts() 1) dispatch({type:FETCH_PRODUCTS}) ==> state is updated to { loading:true, products:[] } I can start displaying HourGlass, Loading...

    2) Middlewre: Make RESTful or GraphQL API call.
based on STATUS code
2.1) SC 200
	{
		loading:false,
		products:[{....}]
	}

    2.2) SC 404 or 500 { loading: false, products:[], error: {...} }

    $ npx create-react-app thunkexample

    thunkexample> npm i redux react-redux axios redux-thunk

    https://jsonplaceholder.typicode.com/users

    Redux Hooks:

    connect(mapStateToProps, mapDispatchToProps)(App);

    function mapStateToProps(state) { users: state.users }

    function mapDispatchToProps(dispatch) { fetchUsers: () => dispatch({type:...}) }

    useSelector() The selector is approximately equivalent to the mapStateToProps argument to connect conceptually.

    useDispatch() is approximately equivalent to the mapDispatchToProps argument to connect conceptually.

    Mini Project:

    1. Fetching JSON from Server componentDidMount() or useEffect(()=>{}, []) update state in ReactContext ==> useContext() to consume in functional component

    2. whenever state is managed by application [survey pages/ feedback pages / multipage registration/ cart] use Redux final step from Redux ==> event handling ==> Send to Server

    ====

    useParams hook The useParams hook is used to read the dynamic params from the current URL that were matched by

    useSearchParams hook The useSearchParams hook is used to Read and Modify the query string in the URL for the current location

    ===============================

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