PropTypes and defaultProps are features in React used to define the expected data types and default values of properties passed to React components.

PropTypes allow developers to enforce the type checking of props being passed to React components, ensuring that the data received by a component is of the expected type. PropTypes can be defined for various data types, including strings, numbers, booleans, objects, and arrays.

For example, if a component expects a prop of type string, but instead receives a number, React will log an error message to the console. This helps prevent bugs and makes it easier to debug code.

DefaultProps, on the other hand, allows developers to set default values for props in a component. If a prop is not passed to the component, the default value will be used instead. This is useful for providing sensible default behavior for a component, and makes the component more resilient to changes in its environment.

For example, if a component expects a prop of type string, but no value is provided, the developer can set a default value using defaultProps to prevent the component from throwing an error.

Overall, both PropTypes and defaultProps are useful tools in making React components more reliable, and ensuring that the component behaves as expected.