React bind()なしでcallback呼出

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

    // callback bind
    // this.handleClick = this.handleClick.bind(this);
  }

  // handleClick() {
  //   this.setState(prevState => ({
  //     isToggleOn: !prevState.isToggleOn
  //   }));
  // }

  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')
);