clarify existance checking

README.md

@@ -411,32 +411,42 @@ See: [Compound State](#compound-state) pattern
 
 ## Existence Checking
 
-Do not check existence of `prop` objects. Use `defaultProps`.
+Do not check existence of props at the root of a component.
+Componenst should not have two possible return types.
 
 ```javascript
 // bad
-render () {
- if (this.props.person) {
- return <div>{this.props.person.firstName}</div>;
- } else {
- return null;
- }
+const Person = props => {
+ if (this.props.firstName)
+ return <div>{this.props.firstName}</div>
+ else
+ return null
 }
 ```
 
+Componenents should *always* render. Consider adding `defaultProps`, where a sensible default is appropriate.
+
 ```javascript
-// good
-class MyComponent extends React.Component {
- render() {
- return <div>{this.props.person.firstName}</div>;
- }
+// better
+const Person = props =>
+ <div>{this.props.firstName}</div>
+
+Person.defaultProps = {
+ firstName: "Guest"
 }
+```
 
-MyComponent.defaultProps = {
- person: {
- firstName: 'Guest'
- }
-};
+If a component should be conditionally rendered, handle that in the owner component.
+
+```javascript
+// best
+const TheOwnerComponent = props =>
+ <div>
+ {props.person
+ ? <Person {...props.person} />
+ : null
+ }
+ </div>
 ```
 
 This is only where objects or arrays are used. Use PropTypes.shape to clarify