add getUnhandledProps util

[levithomason]

Fixes #69

This util gets all props for a component that are not defined in propTypes nor in defaultProps. These extra props are considered 'unhandled` by the component at hand.

The immediate usage of this util will be for spreading props the user passed in that we aren't already explicitly handling.

[eanplatter]

This is an awesome utility? Could it be extended to work with extra props on classes? I guess it would be hard to tell if the classes are being used or not. Either way 🍂

[athurman]

Just a quick question here as to why you chose to make this variable a const as opposed to a let? Is it solely because this variable is meant to be read only? Or another, more specific reason?

😃 Just curious

[levithomason]

Good question! We've updated the linter. If you make a variable whose value never changes, it will tell you to make it a const instead. The only use case for a let now is for a variable whose value changes. Example:

let hasCertainChild;
React.Children.forEach(child => {
  if (someCondition(child) {
    hasSomeChild = true;
  }
})

In this case, the var is initialized as undefined and then set to true if the condition is met.

Going a bit further, a const wouldn't work here for both of those reasons. It cannot be initialized with an empty value nor can it's value change.

[athurman]

🌊 🐙 Looks super handy

[levithomason]

@eanplatter yes this util works with classes. Classes are just functions with a new name. The important thing is that the util looks for static properties SomeClass.propTypes and SomeClass.defaultProps to do it's magic. It finds these via this.constructor.<foo>.

These are equivalent and all produce objects like the above:

// ES5
function SomeClass() {}

// ES6
class SomeClass {}
SomeClass.propTypes = {};
SomeClass.defaultProps = {};

// ES7
class SomeClass {
  static propTypes = {};
  static defaultProps = {};
}

Finally, whether or not the class extends anything is irrelevant. Just so long as it has the right static props.