class Test extends React.Component {
  constructor(props) {
    super(props)
    this.state = {
      count: 1
    }
  }
  handler = () => {
    this.setState({
      count: ++this.state.count
    })
  }
  render() {
    return (
      <div>
        <button onClick={this.handler}>Add 1</button>
        <p>{this.state.count}</p>
      </div>
    );
  }
}

ReactDOM.render(
  <Test />,
  document.getElementById("test"));