How to Check Instance of Class in JavaScript Using instanceof

In JavaScript, you can check if an object is an instance of a class using the instanceof operator. It returns true if the object inherits from the class prototype, otherwise false.

Syntax

The instanceof operator checks whether an object is an instance of a specific class or constructor function.

• object: The object you want to test.
• ClassName: The class or constructor function to check against.

javascript

object instanceof ClassName

Example

This example shows how to create a class and check if an object is an instance of that class using instanceof.

javascript

class Animal {
  constructor(name) {
    this.name = name;
  }
}

const dog = new Animal('Buddy');

console.log(dog instanceof Animal); // true
console.log(dog instanceof Object); // true

const notAnimal = {};
console.log(notAnimal instanceof Animal); // false

Output

true
true
false

Common Pitfalls

Some common mistakes when using instanceof include:

• Using it with primitive types like strings or numbers, which always return false.
• Checking instances across different execution contexts (like iframes), which can cause unexpected results.
• Confusing instanceof with checking the object's constructor property directly.

javascript

const str = 'hello';
console.log(str instanceof String); // false because str is a primitive

const arr = [];
console.log(arr instanceof Array); // true

// Wrong way:
console.log(arr.constructor === Array); // true but less reliable than instanceof

Output

false
true
true

Quick Reference

Usage	Description
object instanceof ClassName	Returns true if object inherits from ClassName.prototype
primitive instanceof ClassName	Always returns false for primitives like string, number
object.constructor === ClassName	Checks constructor but less reliable than instanceof
instanceof with different frames	May fail if object and ClassName come from different frames

Key Takeaways

Use instanceof to check if an object is created from a specific class.

instanceof works by checking the prototype chain of the object.

Primitive values like strings or numbers are not instances of their wrapper classes.

Be careful when using instanceof across different JavaScript contexts like iframes.

Checking constructor property is less reliable than instanceof.