How to Check If Variable is Array in JavaScript

To check the data type of any value in JavaScript, use the typeof operator. There are some use cases like, let’s say, we are fetching the json data from the server, and then we convert it into an object, and then if we wanted to make sure that the data object contains the array values or not, use the isArray() function.

Check If Variable is Array in JavaScript

To check if a variable is an array in JavaScript, use the array.isArray() method.The array isArray() is a built-in function that checks whether the passed value is an array or not.

If the value is an array, true is returned; otherwise, false is returned. When checking for the Array instance, the isArray() function is preferred over the instanceof because it works through iframes.

Syntax

Array.isArray(value)

Parameters

The value is to be checked.

Example

See the following example.

// app.js

let arr = ['Millie', 'Bobby', 'Brown'];
console.log(Array.isArray(arr))

let data = {
  realname: 'Millie Bobby Brown',
  character: 'Eleven',
  series: 'Stranger Things'
};
console.log(Array.isArray(data));

In the above example, we have defined an array of Strings and an array of Objects.

See the following output.

➜  es git:(master) ✗ node app
true
false
➜  es git:(master) ✗

Javascript isArray behavior with different types

TYPE	VALUE AS JSON	ARRAY.ISARRAY
Empty Array	[]	true
An array of Number literal	[1,2,3]	true
An array of String literal	[“a” “b”, “c”]	true
Array (using constructor.from)	[null,null,null,null,null,null,null,null,null,null]	true
Array with empty values using constructor	[null,null,null,null,null,null,null,null,null,null]	true
Array with values using constructor	[1,2,3]	true
Array prototype	[]	true
Array from another window	[1,2,3]	true
Empty Object	{}	false
Object with `.length` property	{“length”:10}	false
Object whose `__prototype__` is Array	{}	false
Object	{“a”: “2”, “b”: “c”}	false
Object with nested Array	{“nested”:[“2”, “c”]}	false
Empty Set	{}	false
Set of String	{}	false
Set of Number	{}	false
Empty Map	{}	false
Map with different types	{}	false
String literal	“hello world.”	false
String Object	“hello world”	false
Boolean true literal	true	false
Boolean false literal	false	false
Explicitly undefined	undefined	false
Implicit undefined	undefined	false
null	null	false
Number (integer)	123	false
Number (float)	1.23	false

An array can be checked if it is empty by using the array.length property. This property returns several elements in the array.

If the number is greater than 0, it evaluates to true.

This method and property can be used together with the AND(&&) operator to determine whether the array exists and is not empty.

Array isArray() Polyfill

Running the following code before any other code will create Array.isArray() if it’s not natively available in your browser.

if (!Array.isArray) {
  Array.isArray = function(arg) {
    return Object.prototype.toString.call(arg) === '[object Array]';
  };
}

That’s it for this tutorial.