React is a JavaScript library created by Facebook that allows building of user interfaces through components. Components are independent and reusable bits of code that return HTML via a render function. There are two types of components: class components that require a render method and extend React.Component, and function components that behave similarly but are simpler plain JavaScript functions. Props and state are used to pass data between components - props are immutable while state allows re-rendering on change.

2. What is React?
React is a JavaScript library created by Facebook.
It lets you compose complex UIs from small and isolated pieces of code called “components”.
React allows us to create reusable UI components.

3. What are React Components?
Components are like functions that return HTML elements.
Components are independent and reusable bits of code.
They serve the same purpose as JavaScript functions, but work in isolation and returns HTML
via a render function
 Components come in two types, Class components and Function components.

4. Class Component
When creating a React component, the component's name must start with an upper case
letter.
Class component has to include the extends React.Component statement, this statement
creates an inheritance to React.Component
 component also requires a render() method, this method returns HTML.
Example:
class Greet extends React.Component {
render () {
return <h1> Hi, Good Morning</h1>
}
}

5. Function Component
Function component also returns HTML, and behaves pretty much the same way as a Class
component.
Functional component is just like a plain JavaScript function.
Functional component doesn’t require render() method, and extends React.Component
Functional component are much easier to read and test because they are plain JavaScript
functions without state or lifecycle-hooks
Example:
function Greet () {
return <h1> Hi, Good Morning</h1>
}

6. React Props:
React Props are like function arguments in JavaScript and attributes in HTML.
To send props into a component, use the same syntax as HTML attributes.
Props are also how you pass data from one component to another, as parameters.
If you have a variable to send you just put the variable name inside curly brackets.
React Props are read-only! You will get an error if you try to change their value.
Example:
const myelement = <Car brand = “Ford” />;

7. React State:
React components has a built-in state object.
The state object is where you store property values that belongs to the component.
When the state object changes, the component re-renders.
The state object is initialized in the constructor and it can contain as many properties as you
like.
Refer to the state object anywhere in the component by using the this.state.propertyname.
To change a value in the state object, use the this.setState() method.
Always use setState() method to change the state object, it will ensure that the component
knows its been updated and calls the render() method

8. Pros and Cons of ReactJs:
Pros:
 ReactJS is much easier to learn and use.
 Creating Dynamic Web Applications Becomes Easier, It provides less coding and gives more
functionality.
 The reusable components helps to make your apps easier to develop and maintain.
Cons:
 ReactJS Covers only the UI Layers of the application and nothing else. So need some other
technologies to get a complete tooling set for development in the project.