defined in your propTypes code example

Example 1: use propTypes in react function

import React from 'react';
import { PropTypes } from 'prop-types';

const student = (props) => {
  return (
    <div>
      <p>Student Name: {props.name}</p>
      <p>Age: {props.age}</p>
    </div>
  );
};

student.propTypes = {
  name: PropTypes.string,
  age: PropTypes.number
};

export default student;

Example 2: propTypes

import PropTypes from 'prop-types';

class Greeting extends React.Component {
  render() {
    return (
      <h1>Hello, {this.props.name}</h1>
    );
  }
}

Greeting.propTypes = {
  name: PropTypes.string
};