- Expand types checked

Author: brettz9

.README/rules/check-types.md

@@ -5,6 +5,8 @@ Reports invalid types.
 Ensures that case of native types is the same as in this list:
 
 ```
+undefined
+null
 boolean
 number
 string
@@ -16,9 +18,9 @@ RegExp
 
 #### Why not capital case everything?
 
-Why are `boolean`, `number` and `string` exempt from starting with a capital letter? Let's take `string` as an example. In Javascript, everything is an object. The string Object has prototypes for string functions such as `.toUpperCase()`. 
+Why are `boolean`, `number` and `string` exempt from starting with a capital letter? Let's take `string` as an example. In Javascript, everything is an object. The string Object has prototypes for string functions such as `.toUpperCase()`.
 
-Fortunately we don't have to write `new String()` everywhere in our code. Javascript will automatically wrap string primitives into string Objects when we're applying a string function to a string primitive. This way the memory footprint is a tiny little bit smaller, and the [GC](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)) has less work to do. 
+Fortunately we don't have to write `new String()` everywhere in our code. Javascript will automatically wrap string primitives into string Objects when we're applying a string function to a string primitive. This way the memory footprint is a tiny little bit smaller, and the [GC](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)) has less work to do.
 
 So in a sense, there two types of strings in Javascript; `{string}` literals, also called primitives and `{String}` Objects. We use the primitives because it's easier to write and uses less memory. `{String}` and `{string}` are technically both valid, but they are not the same.
 

src/rules/checkTypes.js

@@ -31,6 +31,8 @@ const targetTagAliases = [
 targetTags = targetTags.concat(targetTagAliases);
 
 const strictNativeTypes = [
+  'undefined',
+  'null',
   'boolean',
   'number',
   'string',