Here are 3 ways to check if a variable is an array in JavaScript:
- Using Array.isArray() method
- Using instanceof operator
- By checking the constructor type
Method 1: Using Array.isArray() method
To check if a Variable is an array in JavaScript, you can use the “array.isArray()” function. The “isArray()” function returns true if an object is an Array; otherwise, it returns false.
Syntax
Array.isArray(obj)
Parameters
obj: It is an object to be tested.
Return Value
It returns a Boolean value. It returns true if the object is an array. Otherwise, it returns false.
Example
console.log(Array.isArray([11, 21, 19]))
console.log(Array.isArray({ data: 1121 }))
console.log(Array.isArray('dataset'))
console.log(Array.isArray(undefined))
Output
true
false
false
false
Method 2: Using JavaScript instanceof operator
An instanceof operator is “used to test whether the prototype property of a constructor appears anywhere in the prototype chain of an object.”
Syntax
variable instanceof Array
Return value
An instance operator returns a true boolean value if the variable is the same as what is specified and a false if it is not.
Example
let arr = [11, 19, 21];
if (arr instanceof Array) {
console.log('arr is an array!');
} else {
console.log('arr is not an array.');
}
Output
arr is an array!
An important caveat is that the instanceof operator doesn’t always work across multiple frames or windows in web browsers. This is because each frame or window has its execution environment and its global object.
If you pass an array from one frame to another, it might not be recognized as an instance of an Array in the receiving frame.
Method 3: By Checking the constructor property of the variable
Another approach to check if a variable is an array in JavaScript is by using the constructor property. Every object in JavaScript has a constructor property that references the constructor function used to create the object. For arrays, this constructor is Array.
Syntax
variable.constructor === Array
Example
let arr = [11, 19, 21];
if (arr.constructor === Array) {
console.log('arr is an array!');
} else {
console.log('arr is not an array.');
}
Output
arr is an array!
If cross-frame compatibility is a concern, it’s still recommended to use Array.isArray().
Conclusion
For more robust array checking, especially when dealing with multiple frames, you can use the Array.isArray() method.
Related posts
How to Check If a Variable is a String
How to Check If a Variable is a String in TypeScript
How to Check If Variable Exists

Krunal Lathiya is a seasoned Computer Science expert with over eight years in the tech industry. He boasts deep knowledge in Data Science and Machine Learning. Versed in Python, JavaScript, PHP, R, and Golang. Skilled in frameworks like Angular and React and platforms such as Node.js. His expertise spans both front-end and back-end development. His proficiency in the Python language stands as a testament to his versatility and commitment to the craft.