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 the isArray method

The Array.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 false if it is not. This is shown in the example below.

Example-1:

<!DOCTYPE html> 
<html lang="en"> 
  
<head> 
    <title> 
      How to check if a variable 
      is an array in JavaScript? 
  </title> 
</head> 
  
<body> 
    <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> 
</body> 
  
</html> 

Output:
isArray

Method 2: Using the instanceof operator
The 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 an the given variable has a prototype of ‘Array’.

Syntax:

variable instanceof Array

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

Example-2:

<!DOCTYPE html> 
<html lang="en"> 
  
<head> 
    <title> 
      How to check if a variable is 
      an array in JavaScript? 
  </title> 
</head> 
  
<body> 
    <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> 
</body> 
  
</html> 

Output:
instanceof

Method 3: Checking the constructor property of the variable

Another method to check a variable is an array is by checking it’s constructor with Array.

Syntax:

variable.constructor === Array

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

Example-3:

<!DOCTYPE html> 
<html lang="en"> 
  
<head> 
    <title> 
      How to check if a variable is 
      an array in JavaScript? 
  </title> 
</head> 
  
<body> 
    <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> 
</body> 
  
</html> 

Output:

constructor

My Personal Notes

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