Checking if a value is an array with vanilla JS

Today, we’re going to learn how to check if an item is an array with vanilla JS.

Let’s dig in.

The problem

In JavaScript, the typeof operator returns a string indicating the type of an object. Or, at least, it’s supposed to.

But it doesn’t always return the result you would expect for certain types of JS objects.

// returns "number"
// Cool, that's what we'd expect
typeof 42;

// returns "boolean"
// Also expected, cool cool
typeof true;

// returns "object"
// Looking good, keep it up
typeof {};

// returns "object"
// Wait, what?
typeof [];

// returns "object"
// What the... ok, this is just weird
var h1 = document.querySelector('h1');
typeof h1;

The typeof operator says a lot of things that you or I probably wouldn’t consider objects objects.

And yes, (almost) everything is an object in JavaScript. But []? That’s an array, obviously. The h1 element is a Node.

Fortunately, there’s a native method that can help here.

The Array.isArray() method

The Array.isArray() method accepts a single argument: the value you want to check.

If it’s an array, the method returns true. If it’s not, it returns false.

// returns false
Array.isArray({});

// returns true
Array.isArray([]);

Sweet!

Unfortunately, there aren’t currently equivalents for true objects ({}), elements, and so on. For those, I recommend the Object.prototype.toString.call() hack.

Browser compatibility

The Array.isArray() method works in all modern browsers, and back to IE9.