Add Check If Something Is An Array as a javascript til

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

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+# 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]';
+  };
+}
+```