Code the null and undefined values (#3203)

Author: lihz6

content/docs/reference-react-component.md

@@ -274,7 +274,7 @@ Currently, if `shouldComponentUpdate()` returns `false`, then [`UNSAFE_component
 static getDerivedStateFromProps(props, state)
 ```
 
-`getDerivedStateFromProps` is invoked right before calling the render method, both on the initial mount and on subsequent updates. It should return an object to update the state, or null to update nothing.
+`getDerivedStateFromProps` is invoked right before calling the render method, both on the initial mount and on subsequent updates. It should return an object to update the state, or `null` to update nothing.
 
 This method exists for [rare use cases](/blog/2018/06/07/you-probably-dont-need-derived-state.html#when-to-use-derived-state) where the state depends on changes in props over time. For example, it might be handy for implementing a `<Transition>` component that compares its previous and next children to decide which of them to animate in and out.
 
@@ -587,7 +587,7 @@ Normally you should try to avoid all uses of `forceUpdate()` and only read from
 
 ### `defaultProps` {#defaultprops}
 
-`defaultProps` can be defined as a property on the component class itself, to set the default props for the class. This is used for undefined props, but not for null props. For example:
+`defaultProps` can be defined as a property on the component class itself, to set the default props for the class. This is used for `undefined` props, but not for `null` props. For example:
 
 ```js
 class CustomButton extends React.Component {
@@ -607,7 +607,7 @@ If `props.color` is not provided, it will be set by default to `'blue'`:
   }
 ```
 
-If `props.color` is set to null, it will remain null:
+If `props.color` is set to `null`, it will remain `null`:
 
 ```js
   render() {