Who Created This Object?

The instanceof operator checks whether an object was created by a particular class — or by any class in its inheritance chain.

class Animal {}
class Rabbit extends Animal {}

let rabbit = new Rabbit();

console.log(rabbit instanceof Rabbit); // true
console.log(rabbit instanceof Animal); // true — due to inheritance!
console.log(rabbit instanceof Object); // true — everything inherits from Object

How It Works

instanceof walks up the prototype chain looking for a match with the class's prototype property.

Customizing instanceof Behavior

You can override the default behavior using Symbol.hasInstance:

class Edible {
  static [Symbol.hasInstance](obj) {
    return obj.canEat === true; // "Edible" if it has canEat
  }
}

console.log({ canEat: true } instanceof Edible); // true — no class needed!