error on line 5, this.props.onAnswerChange(answer);

notes.jsx

var AnswerClass = React.createClass({
    handleChange: function (e) {
        e.preventDefault();
        var answer = React.findDOMNode(this.refs.answer).value.trim();
        // this.props has an attr of onAnswerChange, however its value is undefined here,
        // causing a Uncaught TypeError: undefined is not a function
        this.props.onAnswerChange(answer); 
    },
    render: function () {
        return (
            <form className="answerClass" onChange={this.handleChange}>
                <input type="text" placeholder="Say something..." ref="answer"/>
            </form>
        );
    }
});

var QuestionClass = React.createClass({
    render: function () {
        return (
            <h3 className="questionClass">{this.props.question}</h3>
        );
    }
});


var QuestionAnswerClass = React.createClass({
    handleSaveAnswer: function (answer) {
        var questions = this.state.questions;
        var answers = [answer];
        this.setState({questions: questions, answers: answers});
    },
    getInitialState: function () {
        return {questions: [], answers: []};
    }, 
    componentDidMount: function () {
        this.setState({questions: ["What can you do in the next hour?"], answers: []});
    },
    render: function () {
        var questionNodes = this.state.questions.map(function (question) {
            return (
                <div className="qaBox" >
                    <QuestionClass question={question} />
                    <AnswerClass onAnswerChange={this.handleSaveAnswer} />
                </div>
            );
        });
        return (
            <div className="questionAnswerClass">
                {questionNodes}
            </div>
        );
    },
});

React.render(
    <QuestionAnswerClass  />,
    document.getElementById('content')
);

With the shift of React from createClass to ES6 classes we need to handle the correct value of this to our methods on our own, as mentioned here: http://www.newmediacampaigns.com/blog/refactoring-react-components-to-es6-classes
Change your code to have the method bounded to correct value of this in constructor:

export default class ComponentClass extends React.Component {
  constructor(props) {
      super(props);
      this._myHandler = this._myHandler.bind(this);
  }

  _myHandler(props) {
    console.log(props);
  }

  render() {
    return (
        <div className="col-xs-6 col-md-4">
            <button type="button" className="btn btn-danger" onClick={this._myHandler}><i className="fa fa-trash"> Delete</i></button>
        </div>
    )
  }
}

The no autobinding was a deliberate step from React guys for ES6 classes. Autobinding to correct context was provided with React.createClass. Details of this can be found here: https://facebook.github.io/react/blog/2015/01/27/react-v0.13.0-beta-1.html#autobinding

So based on this you could also change your code as:

export default class ComponentClass extends React.Component {
  _myHandler = (props) => {
    console.log(props);
  }

  render() {
    return (
        <div className="col-xs-6 col-md-4">
            <button type="button" className="btn btn-danger" onClick={this._myHandler}><i className="fa fa-trash"> Delete</i></button>
        </div>
    )
  }
}