React Props

Props stand for "Properties." They are read-only components. It is an object which stores the value of attributes of a tag and work similar

to the HTML attributes. Prop is a way to pass data from one component to other components. It is similar to function arguments. Props are

passed to the component in the same way as arguments passed in a function. Inside the components, we can add an attribute called props. 

Props in functional component

Props in class component

These attributes are available in the component as this.props and can be used to render dynamic data in our render method.

Passing object as props

Since Props are immutable, we cannot modify the props from inside the component. Then how do we maintain component data that may

change over time. We can achieve this using state.

Destructuring Props

It makes it possible to unpack the values from arrays or properties from objects into distinct variables.

Class component

Functional component

React Components

Earlier, the developers write more than thousands of lines of code for developing a single page application. These applications follow the

traditional DOM structure, and making changes in them was a very challenging task. If any mistake is found, it manually searches the

entire application and updates accordingly. The component-based approach was introduced to overcome an issue. In this approach,

the entire application is divided into a small logical group of code, which is known as components.

A Component is considered as the core building blocks of a React application. It makes the task of building UIs much easier. Each

component exists in the same space, but they work independently from one another and merge all in a parent component, which will

be the final UI of your application.

Every React component has their own structure, methods as well as APIs. They can be reusable as per your need. For better

understanding, consider the entire UI as a tree. Here, the root is the starting component, and each of the other pieces becomes

branches, which are further divided into sub-branches.

It is placed in the javascript file. Eg: App component is placed in App.js file.

In ReactJS, we have mainly two types of components. They are,

1. Functional Components

2. Class Components

Function Component

It is just a Javascript function, we can optionaly receive objects of properties which are referred to as props and return html(JSX) which

describe the UI.

It behaves pretty much the same way as a Class component, but Class components have some additions.

It doesn't have their own state. 

It is also known as a stateless component because they do not hold or manage state.

Create a Function component called Car

Here we are exporting the Car function from Car.js and import it in App.js, included in the App component as custom HTML tag.

Using Arrow functionOr you can write as below as well.

Class Components

It is basically a ES6 class similar to functional component, a class component also optionaly receives props as input and returns HTML(JSX).

In addition to that It maintain private internal state. It can maintain some information which is private to that component and use that

information to describe the UI.

When creating a React component, the component's name must start with an uppercase letter.

The component has to include the extends React.Component statement, this statement creates an inheritance to React.Component, and

gives your component access to React.Component's functions.

The component needs to implement a render() method, which returns HTML. 

You can pass data from one class to other class components.

The class component is also known as a stateful component because they can hold or manage local state.

Now your React application has a component called Car, which returns a <h2> element.

To use this component in your application, use similar syntax as custom HTML tag: <Car />

React JSX

JSX(JavaScript Extension), is a React extension which allows writing JavaScript code that looks like HTML.

All of the React components(class component) have a render function. The render function specifies the HTML output of a React component. 

Look at the code from App.jsx where we are returning div.

Nested Elements

If we want to return more elements, we need to wrap it with one container element. Notice how we are using div as a wrapper for h1, h2

and p elements.

Attributes

JSX uses attributes with the HTML elements same as regular HTML. JSX uses camelcase naming convention for attributes rather than

standard naming convention of HTML such as a class in HTML becomes className in JSX because the class is the reserved keyword

in JavaScript.

class → className

for → htmlFor

onclick → onClick

We can also use our own custom attributes in JSX. For custom attributes, we need to use data- prefix. In the below example, we have

used a custom attribute data-demoAttribute as an attribute for the <p> tag.

JavaScript Expressions

JavaScript expressions can be used inside of JSX. We just need to wrap it with curly brackets {}.

As per the below example, we cannot use if else statements inside JSX, instead we can use conditional (ternary) expressions. In the

following example, variable i equals to 1 so the browser will render true. If we change it to some other value, it will render false.

JSX Comments

JSX allows us to use comments that begin with /* and ends with */ and wrapping them in curly braces {} just like in the case of JSX

expressions.

Styling

React recommends using inline styles. When we want to set inline styles, we need to use camelCase syntax. React will also automatically

append px after the number value on specific elements. The following example shows how to add myStyle inline to h1 elements.

React Render HTML

React renders HTML to the web page by using a function called ReactDOM.render(). The ReactDOM.render() function takes two arguments,

HTML code and an HTML element.

The purpose of the function is to display the specified HTML code inside the specified HTML element. 

Display a paragraph inside the "root" element:

ReactDOM.render(<p>Hello</p>, document.getElementById('root'));

The result is displayed in the <div id="root"> element:

<body>

  <div id="root"></div>

</body>

The Root Node

• The root node is the HTML element where you want to display the result.

• It is like a container for content managed by React.

• It does NOT have to be a <div> element and it does NOT have to have the id='root'

React ES6

What is ES6?

ES6 stands for ECMAScript 6. ECMAScript was created to standardize JavaScript, and ES6 is the 6th version of ECMAScript, it was

published in 2015, and is also known as ECMAScript 2015.

Why Should I Learn ES6?

React uses ES6, and you should be familiar with some of the new features like:

• Classes

• Arrow Functions

• Variables (let, const, var)

Classes

Classes are used to define the blueprint for real-world object modeling and organize the code into reusable and logical parts. Before ES6,

it was hard to create a class in JavaScript. But in ES6, we can create the class by using the class keyword.

A class definition can include constructors and methods. The classes contain constructors that allocate the memory to the objects of a

class. Classes contain methods that are responsible for performing the actions to the objects.

Example

A simple class constructor:

Now you can create objects using the Car class:

Create an object called "ford" based on the Car class. The constructor function is called automatically when the object is initialized.

Note : The keyword 'this' refers to the current instance of the class.

Method in Classes

You can add your own methods in a class. Create a method named "getBrand":

As you can see in the example above, you call the method by referring to the object's method name followed by parentheses

(parameters would go inside the parentheses).

Class Inheritance

Look at the following Animal, Rabbit classes. There are common data members and methods in those classes. So it will lead to code duplication.

We can solve this problem using class inheritance as shown below.

To create a class inheritance, use the extends keyword.

A class created with a class inheritance inherits all the methods from another class.

Create a class named "Rabbit" which will inherit the methods from the "Animal" class.

Calling parent class from child class

The super() method refers to the parent class. By calling the super() method in the constructor method, we call the parent's constructor method and get access to the parent's properties and methods.

Arrow Functions

Arrow functions were introduced in ES6. Arrow functions allow us to write shorter function syntax:

Before:

With Arrow Function:

It gets shorter. If the function has only one statement, and the statement returns a value, you can remove the brackets and the return keyword:

Arrow Functions Return Value by Default: This works only if the function has only one statement.

hello = () => "Hello World!";

Arrow Function With Parameters: If you have parameters, you pass them inside the parentheses.

hello = (val) => "Hello " + val;

Arrow Function Without Parentheses: In fact, if you have only one parameter, you can skip the parentheses as well.

hello = val => "Hello " + val;

Variables

Before ES6, there was only one way of defining your variables with the var keyword. Now, with ES6, there are three ways of defining your variables: var, let, and const.

var

var x = 5.6;

let

let x = 5.6;

const

const x = 5.6;

const is a variable that once it has been created, its value can never change.

const has a block scope.

Introduction to React JS

You can refer to this video to learn React js Introduction in Tamil.

React is a JavaScript open-source library created by Facebook. It is not a framework. 
React is only responsible for building rich user interfaces. 
React is used to build single-page applications.

React has component-based architecture. This lets you break down your application into small encapsulated parts which can then be

        composed to make a more complex UI as shown in the below image. Components make it possible to write reusable code.

React is declarative - We just need to tell React what we want to do. React with its dom library will build the actual UI.

Why learn ReactJS?

Today, many JavaScript frameworks/libraries are available in the market(like angular, node), but still, React came into the market and gained

popularity amongst them. The previous frameworks follow the traditional data flow structure, which uses the DOM (Document Object Model).

DOM is an object which is created by the browser each time a web page is loaded. It dynamically adds or removes the data at the back-end

and when any modifications are done, then each time a new DOM is created for the same page. This repeated creation of DOM makes

unnecessary memory wastage and reduces the performance of the application.

Therefore, a new technology ReactJS library was invented which removes this drawback. ReactJS allows you to divide your entire application

into various components. ReactJS still used the same traditional data flow, but it is not directly operating on the browser's Document Object

Model (DOM) immediately. instead, it operates on a virtual DOM. It means rather than manipulating the document in a browser after changes

to our data, it resolves changes on a DOM built and run entirely in memory. After the virtual DOM has been updated, React determines what

changes made to the actual browser's DOM. The React Virtual DOM exists entirely in memory and is a representation of the web browser's

DOM. Due to this, when we write a React component, we did not write directly to the DOM; instead, we are writing virtual components that

react and will turn into the DOM.

Create react App

Install NodJs and npm

We can use this to quickly create the react application. We can simply run this cmd and the entire project will be created for you.
npx is an npm package runner which gets installed when you install node. Since then we are able to run and create react apps without

having to install it. npx takes care of that.

When you start the application, It will open the browser on localhost 3000

Folder structure

package.json - Contains the dependencies and the scripts required for the project.

    Eg: React version listed as dependency. There are some scripts to run, build, test the application.

package-lock.json - It ensures the consistent installation of your dependencies.

node-modules - This is the folder in which all the dependencies are installed. It is generated when running the create-react-app cmd.

public - 

        index.html - Only one html file we are going to have in the application. We are building a single page application. The view might

dynamically change in the browser. But this html file gets served up. We are not going to add any code here. Here,

the react will control the UI. At runtime react will take this div tag and it will ultimately be responsible for the UI.

src - 

        index.js - Here we’ll specify the root component which is the <App/> component and DOM element which will be controlled

                        by react. Here the DOM element is the id with ‘root’. The <App/> component will be rendered in the root element.

        App.js - App component is present in the App.js. It is responsible for the html displayed in the browser. App components

                      represent the view we see in the browser.

        App.css - For styling which applied in the App.js.

        App.test - For unit test.

        index.css - For body tag styling.

        Components - It represents the part of the UI. As in the previous image we have 5 components. Header, Footer, Side navigation,

                                 Main content, other to contain the above components that is the root(App) component. Each of the 4 nested

                                 components describes only the part of the UI. However all the components come together to make up the entire

                                 application. Components are also reusable. The same component can be used with different properties. It displays

                                 different information. Eg: The side nav component can be left side/right side nav. It can contain other components.

When npx starts executing, the index.html will be saved in the browser. Which contains the root DOM node. Next controls enter to the

index.js, react DOM render the App component into the root DOM node. The App component contains the html which is ultimately

displayed in the browser.