Add Check If Something Is An Array as a javascript til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_669 TILs and counting..._
+_670 TILs and counting..._
 
 ---
 
@@ -227,6 +227,7 @@ _669 TILs and counting..._
 
 - [Accessing Arguments To A Function](javascript/accessing-arguments-to-a-function.md)
 - [Character Codes from Keyboard Listeners](javascript/character-codes-from-keyboard-listeners.md)
+- [Check If Something Is An Array](javascript/check-if-something-is-an-array.md)
 - [Computed Property Names In ES6](javascript/computed-property-names-in-es6.md)
 - [Create An Array Containing 1 To N](javascript/create-an-array-containing-1-to-n.md)
 - [Create Bootstrapped Apps With Yarn](javascript/create-bootstrapped-apps-with-yarn.md)

javascript/check-if-something-is-an-array.md

@@ -0,0 +1,31 @@
+# Check If Something Is An Array
+
+The `Array` class has a function on it called
+[`isArray()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray)
+which can be used to check if something is an array.
+
+```javascript
+> Array.isArray('Hello, World!');
+// => false
+
+> Array.isArray(['One', 2, [3]]);
+// => true
+
+> Array.isArray({ foo: 'bar' });
+// => false
+
+> Array.isArray([]);
+// => true
+```
+
+The MDN docs provide an [example
+polyfill](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray#Polyfill)
+if it is not natively available.
+
+```javascript
+if (!Array.isArray) {
+  Array.isArray = function(arg) {
+    return Object.prototype.toString.call(arg) === '[object Array]';
+  };
+}
+```