How to Check if Variable is Array in JavaScript

Array is one of the most popular and powerful data types in JavaScript. It is used by almost every JS developer. Arrays offer tons of properties and functions. However, if you call them on variables that are not arrays then you may end up getting an error and your code execution might stop. So it is advisable to check if a variable is array before calling any array-specific functions or properties on it.

How to Check if Variable is Array in JavaScript

There are several simple ways to check if a variable is array in JavaScript.

1. Using constructor

Every JS variable has constructor property readily available, which contains its data type. You can check if variable is array or not as shown below. It returns true if the variable is array, else it returns false.

variable.constructor === Array

If your variable is test_data then here is the command to check if it is array or not.

test_data === Array

You can also use it to test if a JS object property is array or not. For example, let us say you have the following JS object.

test_data={'a':[1,2,3],'b':1}

You can use the above method on JS object property as shown below.

test_data.a === Array // returns true
test_data.b === Array // returns false

The above method is the fastest way to check if a variable is array or not, on all modern browsers such as Chrome, Firefox, Edge, Safari, etc. You can be extra cautious and check if the constructor property exists in the first place.

variable.prop && variable.prop.constructor === Array

2. Using isArray

The Array object is available by default in most modern browsers (Chrome, Firefox, Safari, Edge). You can also use the Array object’s isArray function, to check if a variable is array or not.

Array.isArray(variable)

You can also call this function object’s property, not just simple variables. Here is an example to call the function test_data.a property defined above.

test_data={'a':[1,2,3],'b':1}
Array.isArray(test_data.a) //returns true
b=1
Array.isArray(b) //returns false

3. Using Object API

You can also use the Object API available in every modern web browser to get the object type of your variable, as shown below.

Object.prototype.toString.call(variable) === '[object Array]';

This is method is slower than the others but it allows you to check variable against any data type, not just Array. All you need to do is change [object Array] to the data type string of your requirement.

This method can also be used on simple variables as well as JS object properties.

a = [1, 2, 3];
b = 1;
test_data={'a':[1,2,3],'b':1}

Object.prototype.toString.call(a) === '[object Array]' // returns true
Object.prototype.toString.call(b) === '[object Array]' // returns false
Object.prototype.toString.call(test_data.a) === '[object Array]' // returns true

In this article, we have described 3 simple ways to check if variable is array or not. If you want the fastest way to do this, go for the first method. If you want the most flexible way to do this, go for the 3rd method.

How to Check if Variable is Array in JavaScript