Instanceof operator in JavaScript

The instanceof operator in JavaScript is used to check whether an object is an instance of a particular class or constructor function. It helps determine if an object is created from a specific prototype or constructor. The syntax for using the instanceof operator is:

object instanceof constructorFunction

Checking Instance of a Constructor Function

function Person(name) { = name; } var person1 = new Person("Alice"); console.log(person1 instanceof Person); // Output: true console.log(person1 instanceof Object); // Output: true (all objects are instances of Object)

In this example, person1 is an instance of the Person constructor function, and it's also considered an instance of the base Object constructor.

Checking Instance of Custom Objects

function Car(make, model) { this.make = make; this.model = model; } var car1 = new Car("Toyota", "Camry"); console.log(car1 instanceof Car); // Output: true console.log(car1 instanceof Object); // Output: true console.log(car1 instanceof Array); // Output: false

Here, car1 is an instance of the Car constructor but not of the Array constructor.

Inheritance and Instanceof

function Animal() {} function Dog() {} Dog.prototype = Object.create(Animal.prototype); var dog1 = new Dog(); console.log(dog1 instanceof Dog); // Output: true console.log(dog1 instanceof Animal); // Output: true (due to prototype chain)

In this example, dog1 is an instance of both Dog and Animal constructors because of the prototype chain.

Checking Inheritance with Primitives

var num = 42; console.log(num instanceof Number); // Output: false console.log(num instanceof Object); // Output: false

For primitives like numbers, strings, and booleans, the instanceof operator generally returns false because they are not objects, despite having corresponding wrapper objects like Number, String, and Boolean.

It's important to note that the instanceof operator only works with objects and constructor functions, not with primitive values. Additionally, it relies on the prototype chain, so if you're dealing with complex inheritance structures, make sure to consider how prototypes are set up.


The instanceof operator in JavaScript is used to determine whether an object is an instance of a specific constructor function or a class by checking its prototype chain. It helps verify relationships between objects and their constructors, aiding in object type identification and inheritance validation.