In the code editor, take a look at MessageDisplayer‘s render function.

Notice the expression this.props.message. From this expression, you can deduce that MessageDisplayer expects to get passed a prop named message. Somewhere, at some time, this code is expected to execute:

<MessageDisplayer message="something" />

If a component class expects a prop, then you can use propTypes for that component class!

In order to start using propTypes, we need to import the 'prop-types' library.

import PropTypes from 'prop-types';

Then, you can declare propTypes as a static property for your component after the component has been defined. See the example of a propTypes property on lines 11-13. Notice that the value of propTypes is an object, not a function!

The second step is to add properties to the propTypes object. For each prop that your component class expects to receive, there can be one property on your propTypes object.

MessageDisplayer only expects one prop: message. Therefore, its propTypes object only has one property.