State in React is similar to a private property that is managed by React. Every change in state calls the render method, displaying the new changes in the user interface.

You can define a state in this way:

class Clock extends React.Component {
  constructor(props) {
    super(props); // calls parent contructor with props
    this.state = {date: new Date()}; // define state with initial value
  }

  render() {
    return (
      <div>
        <h1>Hello, world!</h1>
        <h2>It is {this.state.date.toLocaleTimeString()}.</h2>
      </div>
    );
  }
}