How do I check if value is an Array?

Monday, August 10, 2020

tldr;

Use the Array.isArray() method to determine if a value is an Array in JavaScript.

But, why?

Not all arrays are of the same Array.prototype to avoid multiple frames from mutating the same prototype. You can read the full details here, but let's look at a quote from the article:

The shared-mutation hazard of having arrays in two coordinating windows be instances of the same Array constructor, sharing the same Array.prototype, is enormous when either page augments Array.prototype (not to mention the security problems when one page is malicious!), so Array and Array.prototype in each window must be different. Therefore, by the semantics of instanceof, o instanceof Array works correctly only if o is an array created by that page's original Array constructor (or, equivalently, by use of an array literal in that page). Pfui.

While we might consider this a bug in the ECMAScript language the intention is to prevent malicious code between frames. Therefore, we cannot reliably expect instanceof to evaluate whether an object is an array, primarily if the array is created by another frame's Array constructor function.

Let's look at an example that will not work:

const i = document.createElement('iframe');
document.body.appendChild(i);
const newIFrameArray = new window.frames[0].Array();

console.log(newIFrameArray instanceof Array); // 🛑 false
console.log(newIFrameArray instanceof window.frames[0].Array); // ✅ true

Let's break this down line by line:

• First, we create a new iframe element.
• Next, we append the new iframe element to the current document's body.
• Then, using the new frame's Array constructor we create a new array instance using the constructor function.
• The first console log will output false because the newIFrameArray is not an instance of the current window's Array prototype.
• The second console log will output true because the newIFrameArray is an instance of the iframe's window Array prototype.

The conclusion is that to deterministically check if a value (object) is an array we should use the Array.isArray() static method.