Merge pull request #937 from mkopa/935-isNumber

[FIX] #935 isNumber is wrong for NaN

snippets/isNumber.md

@@ -2,13 +2,15 @@
 
 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. 
+To safeguard against `NaN`, check if `val === val` (as `NaN` has a `typeof` equal to `number` and is the only value not equal to itself).
 
 ```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
 ```