Type checking helps you to validate your component. Consider a scenario where and when a developer wants to use your component, but he/she wasn't sure what needs to be passed as props to your component. The obvious case is to go through code and developer has to figure out what needs to be passed. As the application grows it might be difficult to know exactly what component expects. Type checking validates the structure of the component and ensures developers get sufficient information for using the components. React has some built-in type checking abilities. To run type checking on the props for a component, we need to assign a special property to component call propTypes. It can be used with both functional and class components.

const MyButton = ({color, description}) => {
    return (
        <button style={{color}}>{description}</button>
    )
}

MyButton.propTypes = {
  color: React.PropTypes.string.isRequired,
  description: React.PropTypes.string.isRequired
};

In the above example, we are using propTypes on the component and each property is given a value to what type it belongs to and whether it is required or not. So the developer who wants to use MyButton just needs to go through propTypes to understand what needs to be passed to the component.

React built in propTypes list:

MyComponent.propTypes = {
  // You can declare that a prop is a specific JS primitive. By default, these
  // are all optional.
  optionalArray: React.PropTypes.array,
  optionalBool: React.PropTypes.bool,
  optionalFunc: React.PropTypes.func,
  optionalNumber: React.PropTypes.number,
  optionalObject: React.PropTypes.object,
  optionalString: React.PropTypes.string,
  optionalSymbol: React.PropTypes.symbol,

  // Anything that can be rendered: numbers, strings, elements or an array
  // (or fragment) containing these types.
  optionalNode: React.PropTypes.node,

  // A React element.
  optionalElement: React.PropTypes.element,

  // You can also declare that a prop is an instance of a class. This uses
  // JS's instanceof operator.
  optionalMessage: React.PropTypes.instanceOf(Message),

  // You can ensure that your prop is limited to specific values by treating
  // it as an enum.
  optionalEnum: React.PropTypes.oneOf(['News', 'Photos']),

  // An object that could be one of many types
  optionalUnion: React.PropTypes.oneOfType([
    React.PropTypes.string,
    React.PropTypes.number,
    React.PropTypes.instanceOf(Message)
  ]),

  // An array of a certain type
  optionalArrayOf: React.PropTypes.arrayOf(React.PropTypes.number),

  // An object with property values of a certain type
  optionalObjectOf: React.PropTypes.objectOf(React.PropTypes.number),

  // An object taking on a particular shape
  optionalObjectWithShape: React.PropTypes.shape({
    color: React.PropTypes.string,
    fontSize: React.PropTypes.number
  }),

  // You can chain any of the above with `isRequired` to make sure a warning
  // is shown if the prop isn't provided.
  requiredFunc: React.PropTypes.func.isRequired,

  // A value of any data type
  requiredAny: React.PropTypes.any.isRequired,

  // You can also specify a custom validator. It should return an Error
  // object if the validation fails. Don't `console.warn` or throw, as this
  // won't work inside `oneOfType`.
  customProp: function(props, propName, componentName) {
    if (!/matchme/.test(props[propName])) {
      return new Error(
        'Invalid prop `' + propName + '` supplied to' +
        ' `' + componentName + '`. Validation failed.'
      );
    }
  },

  // You can also supply a custom validator to `arrayOf` and `objectOf`.
  // It should return an Error object if the validation fails. The validator
  // will be called for each key in the array or object. The first two
  // arguments of the validator are the array or object itself, and the
  // current item's key.
  customArrayProp: React.PropTypes.arrayOf(function(propValue, key, componentName, location, propFullName) {
    if (!/matchme/.test(propValue[key])) {
      return new Error(
        'Invalid prop `' + propFullName + '` supplied to' +
        ' `' + componentName + '`. Validation failed.'
      );
    }
  })
};