import React from 'react';
class MyComponent extends React.Component {

  state = {
    data: 0
  }

  handleClick() {
    const data = this.state.data;
    this.setState({
      data: data + 1
    });
  }

  render() {
    return (
      <button onClick={ this.handleClick }>Click me!</button>
    );
  }
}