Add js typecheck array

blog_posts/js-typecheck-array.md

@@ -0,0 +1,32 @@
+---
+title: "Tip: Typechecking arrays with Array.isArray()"
+shortTitle: Array typechecking
+type: tip
+tags: javascript,type,array
+expertise: beginner
+author: chalarangelo
+cover: blog_images/purple-flower-field.jpg
+excerpt: Make sure to use the correct method when checking if a JavaScript object is an array.
+firstSeen: 2022-11-06T05:00:00-04:00
+---
+
+To determine if a JavaScript object is an array, you can either use `Array.isArray()` or the `instanceof` operator. While both methods work for arrays created either using the array literal syntax or the `Array` constructor, there's a key difference. `Array.isArray()` is more reliable, as it works with cross-realm-objects, such as those created in an `iframe`.
+
+```js
+var iframeEl = document.createElement('iframe');
+document.body.appendChild(iframeEl);
+iframeArray = window.frames[window.frames.length - 1].Array;
+
+var array1 = new Array(1,1,1,1);
+var array2 = new iframeArray(1,1,1,1);
+
+console.log(array1 instanceof Array);   // true
+console.log(Array.isArray(array1));     // true
+
+console.log(array2 instanceof Array);   // false
+console.log(Array.isArray(array2));     // true
+```
+
+As illustrated in the previous example, `instanceof` breaks when working with an `iframe`. However, `Array.isArray()` produces the correct result regardless of the way the array was instantiated.
+
+If you are interested in knowing why `instanceof Array` doesn't work across different globals (i.e. `iframe` or `window`), you can read more about it [here](http://web.mit.edu/jwalden/www/isArray.html).