Reactjs中报错： refs is not defined问题

Question

React组件中不能获取refs的值，页面报错提示：Uncaught TypeError: Cannot read property 'refs' of null，代码如下：

import React from 'react';
import $ from 'jquery'
import '../app.scss';

export default class MyForm extends React.Component {
    submitHandler (event) {
        event.preventDefault();
        console.log(this.refs.helloTo);
        var helloTo = this.refs.helloTo.value;
        alert(helloTo);
    }
    render () {
        return (
            <form onSubmit={this.submitHandler}>
                  <input ref='helloTo' type='text' defaultValue='Hello World! ' />
                  <button type='submit'>Speak</button>
              </form>
        )
    }
}

Answer 1

<form onSubmit={this.submitHandler.bind(this)}>