Class component example

class App extends React.Component {
    state = {
        number: 0
    };

    add_number = () => {
        this.setState({number: this.state.number + 1})
    };

    render() {
        return (
            <div className="App">
                <h1>The number is: {this.state.number}</h1>
                <button onClick={this.add_number}>Click</button>
            </div>
        );
    }
}