Fix isNumber snippet and add test for NaN value

snippets/isNumber.md

@@ -2,13 +2,14 @@
 
 Checks if the given argument is a number.
 
-Use `typeof` to check if a value is classified as a number primitive.
+Use `typeof` to check if a value is classified as a number primitive. Because `typeof NaN` is equal to `number`, the `val === val` is for protection. NaN compares unequal (via ==, !=, ===, and !==) to any other value, including to another NaN value.
 
 ```js
-const isNumber = val => typeof val === 'number';
+const isNumber = val => typeof val === 'number' && val === val;
 ```
 
 ```js
-isNumber('1'); // false
 isNumber(1); // true
+isNumber('1'); // false
+isNumber(NaN); // false
 ```