ReactConstructor.jsx

class Clicker extends React.Component {
  constructor(props) {
    super(props);
    this.handleClick = this.handleClick.bind(this);
    this.state = {
       clicks: 0
    };
  }
  handleClick() {
    this.setState({ 
      clicks: this.state.clicks + 1
    });
  }
  //...
}