class Hello extends React.Component {
 render() {
  return <h1>Hello, {this.props.value}</h1>;
 }
}

class Hello extends React.Component {
 // ...
}
Hello.propTypes = {
 value: React.PropTypes.string
};
Hello.defaultProps = {
 value: 'world'
};

class Hello extends React.Component {
 constructor(props) {
  super(props);
  this.state = {count: props.initialCount};
 }
 // ...
}

class SayHello extends React.Component {
 constructor(props) {
  super(props);
  this.state = {message: 'Hello!'};
  // 这行很重要
  this.handleClick = this.handleClick.bind(this);
 }
 handleClick() {
  alert(this.state.message);
 }
 render() {
  // Because `this.handleClick` is bound, we can use it as an event handler.
  return (
   <button onClick={this.handleClick}>
    Say hello
   </button>
  );
 }
}