export class Counter extends React.Component {
  constructor(props) {
    super(props);
    this.state = {count: props.initialCount};
    this.tick = this.tick.bind(this);
  }
  tick() {
    this.setState({count: this.state.count + 1});
  }
  render() {
    return (
      <div onClick={this.tick}>
        Clicks: {this.state.count}
      </div>
    );
  }
}
Counter.propTypes = { initialCount: React.PropTypes.number };
Counter.defaultProps = { initialCount: 0 };

Methods follow the same semantics as regular ES6 classes, meaning that they don’t automatically bind this to the instance. You’ll have to explicitly use .bind(this) or arrow functions =>:

// You can use bind() to preserve `this`
<div onClick={this.tick.bind(this)}>

// Or you can use arrow functions
<div onClick={() => this.tick()}>

这是react官网上面的例子，也就是说，如果我们使用了es6(这里应该指的是使用的时候采用es6，而不是定义的时候采用es6)的=>，就可以不用bind绑定this，因为es6中this是自动绑定到组件上去的。