es6_proptypes.js

import PropTypes from 'prop-types';

class ItemComponent extends React.Component {
  
  // propTypes can be defined inside the Component definition
  static propTypes = {
    name: PropTypes.string.isRequired,
    price: PropTypes.number.isRequired,
    purchaseHandler: PropTypes.func.isRequired
  };
  
  render() {
    return(
      <div>
        div> Item:{name} </div>
        div> Price:${price} </div>
        <button onClick={purchaseHandler}> Purchase </button>
      </div>
    )
  }
}

// propTypes can also be defined outside the Component
ItemDiv.propTypes = { 
  name: PropTypes.string.isRequired,
  price: PropTypes.number.isRequired,
  purchaseHandler: PropTypes.func.isRequired
}