counter example

counter

<div id="root"></div>
<script src="https://unpkg.com/react@16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom@16/umd/react-dom.development.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/babel-standalone/6.26.0/babel.min.js"></script>
<script type="text/babel">
  const rootElement = document.getElementById('root');
  
  class App extends React.Component { 
    state = {
        counter: 0
    }
    
    handleClick = () => {
      this.setState({ counter: ++this.state.counter})
    }
    
    render() {
      return (
        <div>
          <div> {this.state.counter} </div>
          <Button 
            boxName="Click Me" 
            onClick={this.handleClick}
           />
        </div>
      )
      
      
    }
  }
  
  class Button extends React.Component {
    render() {
      return (
        <div onClick={this.props.onClick}> 
          {this.props.boxName}
        </div>
      )
    }
  }
  
  ReactDOM.render(<App />, rootElement)
</script>