====== prop-types, For Type Checking ======
prop-types helps us to type check the incoming props for a class in React. If the incoming type does not match with the setting, **the code will still run**, but ''a warning message'' will show up in the JavaScript console like:
<code>
Warning: Failed prop type: Invalid prop `type` of type `string` supplied to `YOUR_CLASS_NAME`, expected `number`.
.
.
.
</code> 

===== Install prop-types =====
<code>npm install --save prop-types</code>
===== Supported Types =====
Taken from the react docs: https://reactjs.org/docs/typechecking-with-proptypes.html
<code>
import PropTypes from 'prop-types';

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

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

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

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

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

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

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

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

  // An object taking on a particular shape
  optionalObjectWithShape: PropTypes.shape({
    color: PropTypes.string,
    fontSize: PropTypes.number
  }),
  
  // An object with warnings on extra properties
  optionalObjectWithStrictShape: PropTypes.exact({
    name: PropTypes.string,
    quantity: 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: PropTypes.func.isRequired,

  // A value of any data type
  requiredAny: 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: PropTypes.arrayOf(function(propValue, key, componentName, location, propFullName) {
    if (!/matchme/.test(propValue[key])) {
      return new Error(
        'Invalid prop `' + propFullName + '` supplied to' +
        ' `' + componentName + '`. Validation failed.'
      );
    }
  })
};
</code>

===== Using prop-types =====
  - Import it in your js <code>import propTypes from 'prop-types';</code>
  - Before exporting your default class, set the type of the incoming prop like<code>
YOURCLASS.propTypes = {
    type: propTypes.number.isRequired,
    otherType: propTypes.string
};
</code> Here ''type'' is a required number type, and ''otherType'' is an optional string type.

===== Example =====
<code>
import React, { Component } from 'react';
import PropTypes from 'prop-types';

class MyComponent extends Component {
    render() {
       <div>whatever, the type is {this.props.type}</div>
    }
}

MyComponent.propTypes = {
    type: PropTypes.number.isRequired,
    otherType: PropTypes.string
};

export default MyComponent;
</code>