SOURCE

class Toggle extends React.Component {
  constructor(props) {
    super(props);
    this.state = {isToggleOn: true };
    
  }
  

  handleClick= () => {  
  	this.setState( state => ({
      isToggleOn: !state.isToggleOn
    })); 
  }
  
  render() {
    return (
      <button onClick = {this.handleClick}>
      		{this.state.isToggleOn ? 'ON' : 'OFF'}
      </button>
    );
  }
}

ReactDOM.render( <Toggle />, document.getElementById("root"));

<script src='https://unpkg.com/react/umd/react.production.min.js'>
</script>
<script src='https://unpkg.com/react-dom/umd/react-dom.production.min.js'>
</script>

<div id = "root"></div>