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
 ```

test/isNumber.test.js

@@ -10,3 +10,6 @@ test('passed argument is a number', () => {
 test('passed argument is not a number', () => {
   expect(isNumber('1')).toBeFalsy();
 });
+test('passed argument is not a number', () => {
+  expect(isNumber(NaN)).toBeFalsy();
+});