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:

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