# The Object isPrototypeOf() method

> Learn how the JavaScript isPrototypeOf() method checks whether an object appears in the prototype chain of another object, returning true or false.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2019-04-18 | Topics: [JavaScript](https://flaviocopes.com/tags/js/) | Canonical: https://flaviocopes.com/javascript-object-prototype-isprototypeof/

Called on an object instance, accepts an object as argument. If the object you called `isPrototypeOf()` on appears in the prototype chain of the object passed as argument, it returns `true`. Otherwise it returns `false`.

Example:

```js
const Animal = {
  isAnimal: true
}

const Mammal = Object.create(Animal)
Mammal.isMammal = true

Animal.isPrototypeOf(Mammal) //true

const dog = Object.create(Animal)
Object.setPrototypeOf(dog, Mammal)

Animal.isPrototypeOf(dog) //true
Mammal.isPrototypeOf(dog) //true
```