State in React is a JavaScript object that contains private data for a component. The state data can be used to render a component and is initialized in the constructor method. For example, a Hello component defines a msg property in its state that is rendered in an h1 tag, displaying the message "Hello, RTDL - Tutorials!" on the page.

1. ReactJs : Tutorial-05-State in ReactJs
What is State in React JS?
A state is a JavaScript object similar to props that have data to be used with the reactjs render.
The state data is a private object and is used within components inside a class.
Components as functions
test.jsx
import React from 'react';
import ReactDOM from 'react-dom';
class Hello extends React.Component {
constructor(props) {
super(props);
this.state = {
msg: "Hello, RTDL - Tutorials!"
}
}
render() {
return <h1>{this.state.msg}</h1>;
}
}
export default Hello;
index.jsx
import React from 'react';
import ReactDOM from 'react-dom';
import Hello from './test.jsx';
ReactDOM.render(
<Hello />,
document.getElementById('root')
);