Javascript：测试表单提交与sinon间谍和柴

Question

您好我试图使用sinon spy和chai测试表单提交，但是我不确定我是否正在做正确的事情。每当我运行测试时，秒数会以未定义的形式返回。不知道为什么会这样。柴，我是新来的。这是代码

这是测试文件：

describe('CountdownForm', () => {
  it('should exist', () => {
    expect(CountdownForm).to.exist;
  });
  it('should call onSetCountdown if valid seconds entered', () => {
    const spy = sinon.spy();
    const countDownForm = shallow(<CountdownForm onSetCountdown={spy}/>)

    countDownForm.refs.seconds.value = '109';
    countDownForm.find('form').simulate('submit', { preventDefault(){} })

    expect(spy.called).to.be.true;
  });
});

这是jsx文件：

class CountdownForm extends Component {

  onSubmit = (e) => {
  e.preventDefault();
  const stringSeconds = this.refs.seconds.value;

  if (stringSeconds.match(/^[0-9]*$/)) {
    this.refs.seconds.value = '';
    this.props.onSetCountdown(parseInt(stringSeconds, 10));
    }
  }
  render() {
    return (
      <div>
        <form ref="form" onSubmit={this.onSubmit} className='countdown-form'>
          <input type="text" ref="seconds" placeholder="Enter time in seconds"/>
          <button className="button expanded">Start</button>
        </form>
      </div>
    );
  }
}