How to check if a variable is an array in JavaScript?

In JavaScript, we can check if a variable is an array by using 3 methods, using the isArray method, using the instanceof operator, and using checking the constructor type if it matches an Array object.

Method 1: Using javascript isArray method checks whether the passed variable is an Array object. 

Syntax:

Array.isArray(variableName)

It returns a true boolean value if the variable is an array and a false if it is not. This is shown in the example below. 

Example 1: In this example, we will check if a given variable is an array or not using the isArray() method in javascript. 

html

<h1 style="color: green">
    GeeksforGeeks
</h1>
<b>
    How to check if a variable
    is an array in JavaScript?
</b>
<p>
    Click on the button to check
    if the variable is an array
</p>
<p>Output for string:
<div class="outputString">
</div>

<p>Output for number:
<div class="outputNumber">
</div>

<p>Output for array:
<div class="outputArray">
</div>


<button onclick="checkArray()">
    Click here
</button>
<script type="text/javascript">
    function checkArray() {
        let str = 'This is a string';
        let num = 25;
        let arr = [10, 20, 30, 40];

        ans = Array.isArray(str);
        document.querySelector(
        '.outputString').textContent = ans;

        ans = Array.isArray(num);
        document.querySelector(
        '.outputNumber').textContent = ans;

        ans = Array.isArray(arr);
        document.querySelector(
        '.outputArray').textContent = ans;
    }
</script>

Output:

Method 2: Using javascript instanceof operator is used to test whether the prototype property of a constructor appears anywhere in the prototype chain of an object. This can be used to evaluate if the given variable has a prototype of ‘Array’.

Syntax:

variable instanceof Array

Return value: The operator returns a true boolean value if the variable is the same as what is specified (here an Array) and a false if it is not. This is shown in the example below. 

Example 2: In this example, we will check if a given variable is an array or not using the instanceof operator in javascript. 

html

<h1 style="color: green">
    GeeksforGeeks
</h1>
<b>
    How to check if a variable is
    an array in JavaScript?
</b>
<p>
    Click on the button to check
    if the variable is an array
</p>
<p>Output for string:
<div class="outputString"></div>

<p>Output for number:
<div class="outputNumber"></div>

<p>Output for array:
<div class="outputArray"></div>


<button onclick="checkArray()">Click here</button>
<script type="text/javascript">
    function checkArray() {
        let str = 'This is a string';
        let num = 25;
        let arr = [10, 20, 30, 40];

        ans = str instanceof Array;
        document.querySelector(
        '.outputString').textContent =
        ans;
        ans = num instanceof Array;
        document.querySelector(
        '.outputNumber').textContent =
        ans;
        ans = arr instanceof Array;
        document.querySelector(
        '.outputArray').textContent =
        ans;
    }
</script>

Output:

Method 3: Checking the constructor property of the variable, another method to check a variable is an array by checking its constructor with Array. 

Syntax:

variable.constructor === Array

This becomes true if the variable is the same as what is specified (here an Array) and false if it is not. This is shown in the example below. 

Example 3: In this example, we will check if a given variable is an array or not by checking the constructor property of the variable.

html

<h1 style="color: green">
    GeeksforGeeks
</h1>
<b>How to check if a variable is
    an array in JavaScript?</b>

<p>Click on the button to check
    if the variable is an array</p>

<p>Output for string:
<div class="outputString"></div>

<p>Output for number:
<div class="outputNumber"></div>

<p>Output for array:
<div class="outputArray"></div>


<button onclick="checkArray()">
    Click here
</button>
<script type="text/javascript">
    function checkArray() {
        let str = 'This is a string';
        let num = 25;
        let arr = [10, 20, 30, 40];

        ans = str.constructor === Array;
        document.querySelector(
        '.outputString').textContent = ans;

        ans = num.constructor === Array;
        document.querySelector(
        '.outputNumber').textContent = ans;

        ans = arr.constructor === Array;
        document.querySelector(
        '.outputArray').textContent = ans;

    }
</script>

Output:

JavaScript is best known for web page development but it is also used in a variety of non-browser environments. You can learn JavaScript from the ground up by following this JavaScript Tutorial and JavaScript Examples.