They suggest using the prop-types package but I think using TypeScript would be a better option since it is can be used with all javascript and not intended just for react. There are more pros and cons if searched, open to hear all thoughts.
Thanks for the suggestion @dcantatore - we're going to stick with PropTypes for now. The latest version of todos in the react branch is now using the PropTypes package.
https://reactjs.org/blog/2017/04/07/react-v15.5.0.html#migrating-from-reactproptypes
They suggest using the prop-types package but I think using TypeScript would be a better option since it is can be used with all javascript and not intended just for react. There are more pros and cons if searched, open to hear all thoughts.