class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
      number: 0
    }
  }
  
  increment() {
    this.setState({
      number: ++this.state.number
    })
  }
  
  render() {
    return (
      <div>
        <div>{this.state.number}</div>
        <button onClick={this.increment.bind(this)}>+1</button>
      </div>
    )
  }
}

ReactDOM.render(<Counter />, document.getElementById("root"))