How to check if an object is an array with vanilla JavaScript

You may have used typeof to check the type of a JavaScript object before. Unfortunately, that doesn’t work for arrays.

Today, I’m going to show you how to check if an item is an array with vanilla JavaScript.

The problem with typeof

For many object types in JavaScript, you can use typeof to determine what kind of object it is.

// Returns 'string'
typeof 'Hello, world!';

var sayHi = function () {
	alert('Hello, world!');
};

// Returns 'function'
typeof sayHi;

However, this doesn’t work with arrays. It returns object, just like objects, dates, and more.

// Returns 'object'
typeof ['tuna', 'chicken', 'pb&j'];

// Also returns 'object'
typeof {sandwich: 'tuna', chips: 'cape code'};

An old-school way to check if something is an array

Todd Motto showed me an interesting way to deal with this: the Object.prototype.toString() method.

This method converts the object type into a string. And in JavaScript, confusingly, everything is an object—not just proper objects ({}).

To make this work, you use call() to call the toString() method on your array.

var arr = ['tuna', 'chicken', 'pb&j'];
var obj = {sandwich: 'tuna', chips: 'cape cod'};

// Returns '[object Array]'
Object.prototype.toString.call(arr);

// Returns '[object Object]'
Object.prototype.toString.call(obj);

The modern way to check if something is an array.

The Array.isArray() method check if a value is an array or not.

var arr = ['tuna', 'chicken', 'pb&j'];
var obj = {sandwich: 'tuna', chips: 'cape cod'};

// Returns true
Array.isArray(arr);

// Return false
Array.isArray(obj);

It works in all modern browsers, and IE9 and up. You can push support back to at least IE6 with a tiny polyfill that actually uses Object.prototype.toString() under the hood.