ES2015 React JS Component classes can pass React's TestUtils tests (using Jest in my case) while not working in a practical implementation. Here's an example of what to do and what not to do.

CheckboxWithLabel-bad.jsx

import React from 'react'

/*
This will happily pass your tests (via TestUtils), but will not work when
implemented in a real-world scenario (via browserify & babelify with es2015
and react presets, for example)
*/
export class CheckboxWithLabel extends React.Component {
  constructor (props) {
    super(props)
    this.state = {isChecked: false}
  }

  onChange () {
    this.setState({isChecked: !this.state.isChecked})
  }

  render () {
    return (
        <label>
          <input type='checkbox' checked={this.state.isChecked} onChange={this.onChange.bind(this)} />
          {this.state.isChecked ? this.props.labelOn : this.props.labelOff}
        </label>
      )
  }
}
CheckboxWithLabel.propTypes = {
  labelOn: React.PropTypes.string,
  labelOff: React.PropTypes.string
}
CheckboxWithLabel.defaultProps = {
  labelOn: 'On',
  labelOff: 'Off'
}

CheckboxWithLabel-good.jsx

import React from 'react'

/*
This will pass your tests (via TestUtils) and will also work when implemented
in a real-world scenario (via browserify & babelify with es2015 and react
presets, for example)
*/
class CheckboxWithLabel extends React.Component {
  constructor (props) {
    super(props)
    this.state = {isChecked: false}
  }

  onChange () {
    this.setState({isChecked: !this.state.isChecked})
  }

  render () {
    return (
        <label>
          <input type='checkbox' checked={this.state.isChecked} onChange={this.onChange.bind(this)} />
          {this.state.isChecked ? this.props.labelOn : this.props.labelOff}
        </label>
      )
  }
}
CheckboxWithLabel.propTypes = {
  labelOn: React.PropTypes.string,
  labelOff: React.PropTypes.string
}
CheckboxWithLabel.defaultProps = {
  labelOn: 'On',
  labelOff: 'Off'
}

/*
This is the important bit: export default <ClassName> instead of
export class <ClassName>
*/
export default CheckboxWithLabel