YourCoolComp.js

class YourCoolComponent extends Component {
  constructor(props) {
    super(props);
    this.state = {
      counter: 0,
    };
    
    this.add = this.add.bind(this);
  }
  
  add() {
    this.setState(prevState => ({ counter: prevState.counter + 1 }));
  }
  
  render () {
    return ...
  }
}