react-stateful-intro.jsx

import {Component} from 'react';

export default class extends Component {
    state = {
        open: false
    };

    toggle = () => {
        this.setState(({open}) => ({open: !open}))
    };

    render() {
        const {open} = this.state;

        return (
            <div>
                <button onClick={this.toggle}>
                    Toggle
                </button>

                {open && (
                    <div>I am open</div>
                )}
            </div>
        );
    }
}