class Foo extends React.Component{
  constructor( props ){
    super( props );
    // 我们需要这么做，不然 this 会显示 undefined
    this.handleClick = this.handleClick.bind(this);
  }
    
  handleClick(event){
    console.log(this);
  }
    
  render(){
    return (
      <button type="button" onClick={this.handleClick}>
        Click Me
      </button>
    );
  }
}