This code helps to declare state variable inside and outside the constructor. You can use the state variable anywhere inside the using using this keyword.

state-inside-constructor.js

class App extends Component {
	/**
	 * Read more about creating constructor here:
	 * https://reactjs.org/docs/react-component.html#constructor
	 */
	constructor(props) {
		super(props);
		this.state = {
			key: "value"
		};
	}
	render() {
		/**
		 * Destructuring state object
		 * Learn more about it here:
		 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#object_destructuring
		 */
		const { key } = this.state;
		return <span>{key}</span>;
	}
}

state-outside-constructor.js

class App extends Component {
	state = {
		key: "value"
	};
	render() {
		/**
		 * Destructuring state object
		 *
		 * Learn more about it here:
		 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#object_destructuring
		 */
		const { key } = this.state;
		return <span>{key}</span>;
	}
}